transistors: so confusing!!
andrew_h
thing I'm learning. I did do Physics in school, but like alot of people
- didnt pay a HUGE amount of attention to things I learnt. Alot of it
has actually come back - now I wish I paid a bit more attention!
Something that has been confusing me no-end, and I just cant seem to
grasp, is how a TRANSISTOR works!!!
I have read many explanations, but they are confusing and vauge. People
have explained like a tap, that a small change in the base current
allows a much larger amount of current to flow through the
collector/emitter.
I cant grasp WHY they are so extremely important - probably because I'm
finding it hard to understand their basic operation !
Any help with this would be greatly appreciated ... this is really
proving to be a stumbling block ...
Thanks..
John Popelish
There are two PN junctions in a transistor, one is the emitter to base
junction and one is the base to collector junction. Normally, the
base to collector junction is reverse biased, to produce an insulating
layer between the base and collector with no movable charges.
Lets pick a polarity... NPN.
So the collector has a positive voltage with respect to the base, so
the doped in electrons in the collector N material are attracted away
from the base and the holes in the base are attracted away from the
collector, leaving just insulating silicon between them.
When the base emitter junction is slightly forward biased (emitter
relatively more negative and base relative more positive), the doped
in electrons in the emitter are repelled toward the base, and the
holes doped into the base are repelled toward the emitter. At about a
half volt forward bias, the holes and electrons begin to find each
other and the electrons tend to jump into the holes and both
effectively dissappear. However, a well made transistor has the
emitter much more highly doped than the base, so more electrons get
pushed into the base than holes get pushed into the emitter.
So the holes that get pushed into the emitter are anihilated very
quickly, but the electrons that get pushed into the base have to hunt
around a while beforo they dissappear.
The small positive base voltage causes these electrons to wander
toward the base lead (the most positive voltage around them). But the
base layer is very thin, and the electrons drift rather slowly in that
direction. If the temperature was very low, this is about all that
would happen, and the forward biased base emitter junction would have
almost no effect on the collector curret.
But at normal ambient temperatures (well above absolute zero) the
movement of the electrons is randomized by the thermal energy in the
silicon, so they stagger quite randomly, with only a little progress
toward the base lead. And since the base layer is so thin, most of
them will never make it to the base lead. They will fall off the
cliff into the highly stressed charge-empty reverse biased base
collactor junction. There, instead of wandering in a drunken stagger
through a very small electric field (volts per meter) they will whoosh
out of the reverse biased junction, because it is much more highy
stressed with e-field. They become collector current.
The more strongly you forward bias the base emitter junction, the more
electrons are pushed into the base layer, and the more stagger over
the cliff into the collector, though there will also be more that make
it out the base lead. Over a wide range of collector current, the
collector be a fairly fixed multiple of the base current. This is the
transistor's current gain or beta.
So the electrons are drunks being encouraged with a slightly tilted
sidewalk onto a slightly down hill, vibrating curb, next to street
that tilts away from the curb, very steeply. Most never make it to
the end of the curb, but fall onto the street where they slide into
traffic.
This is the drunken bum on a crazy street transistor analogy.
Abstract Dissonance
"andrew_h" <ah...@heyntech.com.au> wrote in message
news:1139458553.7...@g43g2000cwa.googlegroups.com...
I have the same problem to some degree.
I have two suggestions that might help though.
Learn the 3 common mode configurations and play around with them to get a
good understanding of how to use them. You don't necessarily need to know
the inner workings of a transistor if you want to use them(although it can't
hurt). This will help you in recognizing the basic building blocks of larger
circuits that use transistors.
Learn the basic models and apply them to pratical, but small circuits. This
will/should help you in analyzing transistor circuits and also to make your
own.
For me, the fundamental problem comes from not understanding completely the
inner workings of a diode. A transistor, after all, is basicaly a diode that
can be controlled(like a vacuum tude) by a current. So if you can't
understand a diode then you can't really understand a transistor(atleast on
the fundamental atomic level).
I do understand the motion of electron flow and holes in a semiconductor and
also the junction as these are basic physics... but theres something more
that I can't seem to grasp.
My problem is that I insist on viewing it in terms of electron flow instead
of electrons and holes... I feel the hole idea is just a "trick"(even though
its equivilent it should work without it). But when I try to think about it
with just electrons I get the fact that electrons can move either way so the
diode has no problems conducting one way or the other(which is true but its
not symmetric). (its also true that the electrons will flow easily in both
directions at some point).
I can't figure out why for low voltages the current flow is more in one
direction than the other except that it has something to do with the
junction and electrons on one side not having enough energy to get over the
junction barrier(but they are able to get over it in the reverse direction
easily for some reason).
Anyways, to use a diode you don't have to understand its inner workings
either.
The idea with a transistor is that its like a pipe:
|
--------| | |---------
| | |
--------| | |---------
|
where the line inbetween is some "value" that controlls the flow from one
side to the other(could be water, light, etc...).
the left and right sides are the N junctions and the "value" is the P
junction(for NPN)... by attaching a voltage to the P junction you can remove
its effects(because after all it will resist flow but if you can remove it
then electrons will flow very easily).
If you think about it somewhat you can see that the P junction acts like a
barrier(just like a diode in a diode) but that since you have an N on the
other side the electrons have to flow through the P to get to it... by
controlling the effective "size" of the P region you can control how many
electrons flow(its like a variable resister to some degree).
Obviously if you had no P region between the two N's then electrons would
flow unimpeaded. The larger the region(corresponding to a larger distance
in a vacuum tube) makes it harder and harder for the electrons to get
across(meaning you have to have a larger voltage). By biasing the P-N
region properly you can weaken its effects and make it much easier for the
electrons to get across it. Hence you can use it as an "amplifier" by
putting a small varying single on the "valve" and having a large current
flowing through it.
There are other ways to use the transistors too but thats the basic idea for
the transistor amplifier. I think it mainly rests on understanding a diode
as its just two diodes(but different dopings) stuck back to back.
It would be nice if someone who really understood this stuff(and not just
that they think they do but actually do) would make some animations(or even
some simulations) of the inner workings. (I've seen some on the net but they
just tend to be crap and expect you to assume a lot of stuff without telling
you why).
Jon
David L. Jones
Try this animated description:
http://www.lucent.com/minds/transistor/tech.html
More info here too:
http://en.wikipedia.org/wiki/Transistor
Keep trying until you find a description that clicks!
Dave :)
Jonathan Kirwan
<Abstract.Dissonance.hotmail.com> wrote:
><snip>
>It would be nice if someone who really understood this stuff(and not just
>that they think they do but actually do) would make some animations(or even
>some simulations) of the inner workings. (I've seen some on the net but they
>just tend to be crap and expect you to assume a lot of stuff without telling
>you why).
The animations I've seen are 'crap'.
By the way, a place where I'm unsure is the idea of mobility of holes
as compared to the mobility of electrons in an N-doped material, for
example. (Not a diode, not a transistor.)
I believe I understand enough of valence for both the lattice atoms as
well as the dopant atoms, the lattice conduction band, the proximity
of the dopant's valance to the lattice's conduction band and the ease
with which room temperature thermal agitation can move n-dopant
electrons into the lattice conduction band, etc. All this seems
sensible.
But I see the mobility of both holes and electrons described as
different in the same material -- for example, in lower dopant
situations where the mobility is limited by lattice collisions, the
holes are about 3 times "slower" than electrons in a silicon lattice.
This is where I am unsure about the exact explanation.
Almost all the holes are dopant holes, where the thermal agitation is
enough to overcome the very slight eV required (30-50 milli-eV?) When
an electron in the conduction band does collide after being drift
accelerated by an E-field, it will most likely collide with a lattice
atom. The collision energy may be absorbed via an energy band
transition of one of that atom's electrons or else converted into
phonon energy in the lattice and energy in the electron as it leaves
again in a random direction. I don't suppose this necessarily often
creates another conduction band electron and a lattice hole atom,
though. And the only way a dopant hole "moves" is with recombination
events, where a conduction band electron is recaptured for a moment?
So is it correct to see that the reason for the difference between the
speed of electron motion and hole motion is due to the difference in
these mechanisms -- that of the mean free path and the E-field
acceleration for electrons and that of the frequency of recombination
events for hole motion?
Or?
Jon
Joe McElvenney
>Something that has been confusing me no-end, and I just cant seem to
>grasp, is how a TRANSISTOR works!!!
You are not alone!
Try this, it is much closer to the truth although, as with most
explanations, an over-simplification.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
The base and emitter form a diode.
Put a forward voltage across that diode and electrons will flow
out of the emitter into the base (i.e. in a NPN transistor).
Now the base region is narrow and the base/collector junction is
biased so as to attract electrons to it.
So, on the way to the base terminal, more than 90% of them are
kidnapped by the collector and never get there. It acts like a
narrow pipe with a big hole in it.
More voltage on the base, more emitter current and so more of it
available to be diverted.
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
There are two hang-ups that can cause confusion -
1). Current flow in the collector is caused by a smaller current
flow in the base.
But what causes that base current in the first place? It is of
course the base to emitter voltage.
Essentially then, a transistor is a voltage (not a current)
operated device although when working out biasing, it is
convenient to stay with the 'current' model.
2). The collector-base region is reverse-biased.
Yes it is, but only for electrons trying to flow in from the
collector. For the electrons in the base trying to get into the
collector it is forward-biased.
Cheers - Joe
cabra...@msn.com
Dear friends,
Once again, the "VICE" issue has reared its head
("voltage-current/chicken-egg" or "voltage-current/cause-effect" to
some folks). When motors, generators, transformers, LED's incandescent
light bulbs, transistors, etc. are under examination, the VICE monster
often shows up.
In a bjt, the base current Ib, and the base-emitter voltage Vbe,
**mutually coexist**. They are interdependent, interactive,
concurrent, mutual, simultaneous, inclusive, joined at the hip, etc.
In other words one cannot exist without the other, but at the same
time, one is not "caused" by the other. Vbe does NOT "cause" Ib, and
vice-versa. Examine the I-V curve of any p-n junction, whether it be a
juction diode, gate-cathode terminals of an SCR, base-emitter junction
of a bjt, LED, etc., and it is immediately obvious that the curve
passes through the origin and does not touch either axis elsewhere. In
other words, diode current Id is zero only when diode voltage Vd is
zero, and vice-versa. If one is non-zero, so is the other.
In spite of this, some have insisted, since day one, that current is
"caused" by voltage. If that were the case (it isn't), then every
electrical device in the universe would be "voltage operated" and
nothing would be "current operated". We wouldn't even have to bother
with these terms.
A bjt is a *charge* operated device. Charge must be injected into
the forward biased b-e jcn, and the reverse biased c-b jcn. It takes
energy, or work in order to move the charge. Voltage doesn't move the
charge, energy does. Likewise with FETs. It takes energy, or work, to
charge the gate to source terminals.
With FETs, this charge and energy can be provided by either a
low-impedance constant-voltage source, or a high impedance
constant-current source. To keep it brief, a low-Z constant voltage
source is better suited for driving a FET gate due to the high-Z of the
gate. The v-source provides both Vgs and Ig, both necessary for FET
operation.
With a bjt, Ib and Vbe are also both needed for operation. A low-Z
constant voltage source, or a high-Z constant-current source could
provide both Vbe and Ib. Again, for brevity, a high-Z constant-current
source is better suited for driving the b-e junction due to its low-Z
characteristic and temperature dependency of current. The constant
i-source provides both Ib and Vbe, both of which are absolutely
indispensable.
All electrical devices require BOTH I and V working together in
tandem in order to function. Some devices, depending on their terminal
characteristics regarding impedance, temperature, etc., are more
amenable to being driven by a constant I source vs. a constant V
source, or vice versa. The constant I source provides BOTH I and V, as
does a constant V source. Have I explained myself? Best regards.
Claude
Abstract Dissonance
"Jonathan Kirwan" <jki...@easystreet.com> wrote in message
news:7e1mu11t4on2acr80...@4ax.com...
> On Thu, 9 Feb 2006 02:28:30 -0600, "Abstract Dissonance"
> <Abstract.Dissonance.hotmail.com> wrote:
>
>><snip>
>>It would be nice if someone who really understood this stuff(and not just
>>that they think they do but actually do) would make some animations(or
>>even
>>some simulations) of the inner workings. (I've seen some on the net but
>>they
>>just tend to be crap and expect you to assume a lot of stuff without
>>telling
>>you why).
>
> The animations I've seen are 'crap'.
>
> By the way, a place where I'm unsure is the idea of mobility of holes
> as compared to the mobility of electrons in an N-doped material, for
> example. (Not a diode, not a transistor.)
>
> I believe I understand enough of valence for both the lattice atoms as
> well as the dopant atoms, the lattice conduction band, the proximity
> of the dopant's valance to the lattice's conduction band and the ease
> with which room temperature thermal agitation can move n-dopant
> electrons into the lattice conduction band, etc. All this seems
> sensible.
>
yeah. To me too... I think ;)
> But I see the mobility of both holes and electrons described as
> different in the same material -- for example, in lower dopant
> situations where the mobility is limited by lattice collisions, the
> holes are about 3 times "slower" than electrons in a silicon lattice.
> This is where I am unsure about the exact explanation.
>
Do you mean holes in a p doped material as compared to electrons in a n
doped material?
> Almost all the holes are dopant holes, where the thermal agitation is
> enough to overcome the very slight eV required (30-50 milli-eV?) When
> an electron in the conduction band does collide after being drift
> accelerated by an E-field, it will most likely collide with a lattice
> atom. The collision energy may be absorbed via an energy band
> transition of one of that atom's electrons or else converted into
> phonon energy in the lattice and energy in the electron as it leaves
> again in a random direction. I don't suppose this necessarily often
> creates another conduction band electron and a lattice hole atom,
> though. And the only way a dopant hole "moves" is with recombination
> events, where a conduction band electron is recaptured for a moment?
>
I'm not entirely sure but what I think is the free(conduction/valences)
electrons have enough energy to "ride" the lattice in the sense that they
will not collide with any atom(in the sense as far as electrons and atoms
"collide").
If, say, one free electron "colides" with a bound electron then atleast one
must stay and the other must leave. I suppose the electron either doesn't
have enough energy to dislodge a bound electron, has enough only to exchange
places(which, as far as were concerned doesn't do much), or has enough to
dislodege a bound electron and not take its place(i.e. the avalanche
effect).
The hole concept though is really just a mathematical analogy of how bound
electrons move through the lattice. I would expect that conduction electrons
and holes are different but I have nothing to back it up. I suppose these
are the things you learn in an advanced class in semiconductor theory.
My guess is though that conduction electrons don't get captured in any real
size... even if they do it is a very smooth transition and for all pratical
purposes we can consider them as "free"(i.e., they don't take part in any
interactions on the lattice). I'm not sure how true this is or how well it
models the actual problem. I suppose it is determined by how strongly bound
the valence electrons are to the atom and the probability of an electron
getting "close" enough to an atom to be associated with it.
Now hole movement may actually be the exact same thing in the sense that in
a p doped material an "absense" of an electron(which basicaly means a
potential to bind the electron into its atom). The reason is we have to know
how strong the "hole" is. When an electron moves through a p doped material
depending on how stronge the holes affinity for the electron is it may be
completely captured by the atom(ofcourse this doesn't happen completely
but). If the holes affinity to capture the electron is very small then
obviously the electron acts like a free electron similar to in n doped
material.
> So is it correct to see that the reason for the difference between the
> speed of electron motion and hole motion is due to the difference in
> these mechanisms -- that of the mean free path and the E-field
> acceleration for electrons and that of the frequency of recombination
> events for hole motion?
>
> Or?
heh, I have no idea. I doubt anyone really does and these are just models
we use that seems to work. Its surely a much more complex situation than
think about electrons floating around in a well defined lattice, etc...
I suppose its up to you to dig deep enough until you are satisfied with it.
I think what I said above basicaly boils down to saying that the difference
in speed due to hole flow and that due to free electron flow is due to the
hole affinity for the electrons. applying a potential across both types of
materials should result in the same mean free velocity if the atoms were
exactly the same(obviously). Hence the difference must come about from the
doping. N doping, AFAIK, is in effect lowering the free electrons
attraction from the lattice atoms... p doping will have the opposite effect
though.
You might think about it like and see if it makes any sense. Think of an
electron in an n-doped material moving along through space. As it
approaches an impurity there is a small repulsion due to the electron cloud
in the atom. We could graph the repulsion with time/distance as sorta a
convex parabola with maximum corresponding to the atom's nucleus(basicaly).
The "holes" are opposite as when the electron approaches an impurity in a
p-doped material there is an attraction and it gets stronger as the electron
gets closer corresponding to a similar, but concave, parabola(ofcourse not a
real parabola but just an approximation).
As the electrons bounce around in the p doped material there will be
attractions toward the valence shells of the impurity atoms.... but ofcourse
since the valence shell is full(or empty) this electron either cannot be
captured or will have to boot out another electron.
I have no idea how to tell though as it is impossible to fine what an atom
is and what it means to be "captured"(one would probably try and define in
terms of distance but thats only a mathematical concept).
I don't know and don't remember enough about nuclear physics to know whats
really going on though. You could bet that its you'll never arrive at the
answer though ;/ It sounds like you got the basic idea as far as I'm
concerned ;)
For me, the real issue, and the more problematic is at the junction of two
oppositely doped materials. There seems to be some "strange" things
happening there. There has to be some non-symmetric aspect of this junction
IMO to give rise to its non-symmetric behavior. Most will use the hole
analogy to get the result but if you replace the holes with its
"corresponding" electron flow then it gets much more difficult as electrons
can "easily" flow in both directions.
What I have come up with in trying to understand it is that one gets two
charge density functions for the two biasing types.
One looks like this
/\ /\ |
/ \---/ \| --- /
|\ / \ /
| \/ \/
/\ | /\
---/ \| ---/ \
\ / \ /
\/ \/
where the | represents the junction and the "bumps" are the
potential/charage distribution.
The first one corresponds to a P-N that is reversed biased and the second to
one that is forward biased. As "pressure"(i.e., potential) is applied to
the second the far edge potentials get pushed closer to the center and
eventually will cancel out leaving a constant potential(approximately) which
means electrons flow easily through.
The first will just tend to increase the overall potential as more and more
voltage is put across it.
I'm not sure if this is the right interpretation but I basicaly got it as
thinking about how electrons would distribute themselfs in the two different
materials and by considering the junction potential.
The problem is that you can view it in two ways in that if you apply enough
"reverse" bias then electron flow will become easy too(you end up pulling
the two potentials apart so much that they become negligible...) this
correseponds to breakdown of the diode.
So the problem is not so much about symmetric as it is symmetric to some
degree but about magnituide. If you look at a diodes IV curn it is actually
pretty damn symmetric except the reverse current is scaled differently.
The reason is that it might just be harder for "hole" flow than conduction
flow and hence this makes it easier for electrons to flow in one way than
the other... but this isn't right either as it breaks physical symmetry
laws(flipping the material shouldn't change its behavior).
I'm still searching for what is really going on too ;/ Maybe one day it will
make sense.
>
> Jon
Jon
ehsjr
Why are transistors so extremely important? Because
there are millions upon millions upon millions upon
millions etc in use. The number is so huge its beyond
counting. They are used in almost every electronic
device most people encounter on a daily basis, throughout
the day.
> Something that has been confusing me no-end, and I just cant seem to
> grasp, is how a TRANSISTOR works!!!
Regrading the above, the confusion may come from pondering
"how it works" versus thinking about "what it does".
How it works gets into physics - how material behaves
at the atomic level. It's fascinating and interesting etc -
but not needed to understand *what* a transistor does.
So, look at what a transistor does.
It can be used as a switch that turns things on or off.
It can be used as an amplifier.
In both cases a small electrical source controls
the transistor at its input. And in both cases, the
transistor output controls a separate and much stronger
electrical source.
Damn clever engineers create a wide variety of circuits
that use these two functions of a transistor to do miraculous
things - display your heartbeat on an EKG machine, guide
rockets to the moon, control your microwave oven, create
an ABS system to prevent brake lockup on your car, allow
you to play games or surf the internet on your PC and
on and on and on.
All based on a device that is controlled by a very weak
source, which in turn controls a separate and much stronger
source. Yes, it is important!
Ed
Abstract Dissonance
<cabra...@msn.com> wrote in message
news:1139498176.6...@g47g2000cwa.googlegroups.com...
So your saying without a voltage(difference) the charge would still move?
Um.. Voltage is defined as the change in potential energy between two points
needed to move a "test" charge in an electric field between two points.
Hence it has everything to do with motion. One can argue that there is no
potential difference between two points unless an electron is moving.
If an electron is moving in an electric field then it is doing so because
there is a potential difference.
there are three explicit points involved in the definition: Distance,
charge, and the electric field. Since current measures the rate of change
of w.r.t to time past some point and since the speed of charge in a
conductor is approximately constant then current approximately measures the
amount of charge passing through a point at any given time.
If we consider a uniform electric field, say between two points a distance d
apart and electrons are flowing between the two points with a constant
velocity then
V = -q*E*d
and
I = a*q
hence
V = I*E*d/a
or if since E,d,a are all constants in this problem we see that V is
proportional to I.
i.e., ohms law in an ideal resistor.
in a transistor its the same thing except E and q are changing due(and they
are related).
But defintely there is a relationship and you can't have one without the
other. Voltage and energy are synonymous in the sense that you can't really
have a potential difference without moving a charge. When you measure a
voltage with your voltmeter you are actually measuring current flow on an
extremly small scale and for all pratical purposes you assume there is no
current flow.
Its true though that you can't have voltage without current flow but the
opposite is true too.
> With FETs, this charge and energy can be provided by either a
> low-impedance constant-voltage source, or a high impedance
> constant-current source. To keep it brief, a low-Z constant voltage
> source is better suited for driving a FET gate due to the high-Z of the
> gate. The v-source provides both Vgs and Ig, both necessary for FET
> operation.
> With a bjt, Ib and Vbe are also both needed for operation. A low-Z
> constant voltage source, or a high-Z constant-current source could
> provide both Vbe and Ib. Again, for brevity, a high-Z constant-current
> source is better suited for driving the b-e junction due to its low-Z
> characteristic and temperature dependency of current. The constant
> i-source provides both Ib and Vbe, both of which are absolutely
> indispensable.
> All electrical devices require BOTH I and V working together in
> tandem in order to function. Some devices, depending on their terminal
> characteristics regarding impedance, temperature, etc., are more
> amenable to being driven by a constant I source vs. a constant V
> source, or vice versa. The constant I source provides BOTH I and V, as
> does a constant V source. Have I explained myself? Best regards.
Yeah. I think you just need to understand that ultimately voltage and energy
related. They are exactly related by
V = U/q where U is the change in potential energy and q is charge.
Otherwise everything else seems to make sense(which you happen to point).
Just seems that you are implying in some cases that you can have current
without a voltage.
I think ultimately current and voltage are one in the *same* but we view
them from different perspectives. You can't have one without the other and
there is a relationship between them. This relationship is sorta a
transformation that depends on the physical constraints we impose on the
electrons... they are related by an electric field and in some sense they
the same manifestations of some singular phenomena. (sorta like the wave
particle duality idea or special theory of relativity or even how the
electric and magnetic fields are manifestations of the same thing(and are
equivilent in some sense depending on your "perspective"))
Anyways,
> Claude
>
Jon
redbelly
> Here is the not exactly right approximate run through.
(snip)
> This is the drunken bum on a crazy street transistor analogy.
John,
I've seen quite a few descriptions of transistors, but this one has
given me a better feel for what's going on than any of the others.
(And I'm no slouch when it comes to math & physics.) Thank you.
Mark
redbelly
ehsjr wrote:
> andrew_h wrote:
> > Something that has been confusing me no-end, and I just cant seem to
> > grasp, is how a TRANSISTOR works!!!
>
> Regrading the above, the confusion may come from pondering
> "how it works" versus thinking about "what it does".
> How it works gets into physics - how material behaves
> at the atomic level. It's fascinating and interesting etc -
> but not needed to understand *what* a transistor does.
>
> So, look at what a transistor does.
(snip)
> Ed
That's the kind of thinking that finally allowed me to understand
photodiodes. Forget about electrons, holes, and bandgaps, and just
think in terms of current and voltage characteristics. (Still working
on my understanding of transistors.)
Mark
John Popelish
I'm glad it helped you. That makes the effort worthwhile. I like
this story, because it can be extended to cover so many different cases.
It covers the case of collector to base low reverse and even forward
bias. The street is only slightly tilted, so some of the bums stagger
back up on the curb, or the street is a little higher than the curb,
so bums from the other side show up on the curb.
It also covers base thinning (curb narrowing) from increased collector
to base reverse voltage (Early effect). A narrower curb decreases the
chance that any bum will make it to the end of the curb, so the
current gain goes up slightly.
But mainly, the first part of the description illustrates that
junction transistors are thermal devices. Without the random drift
produced by heat, there is no collector current or current gain.
Dorian McIntire
one concept you took issue with; the idea of a charge moving with no applied
voltage. It is indeed possible for a charge to move with no voltage.
Superconductivity is one situation where charges move with no applied
voltage. Granted, a voltage is initially required to get the charges moving
but the voltage is not required to keep charges moving at a constant rate in
a superconducting material.
One special case of this is an electron moving at some constant velocity in
a vacuum. Voltage is initially required to accelerate the electron but
momentum will carry it on at a constant velocity if no extraneous fields are
allowed to act on the charge.
This may seem to be minor point to some, but physical concepts such as
momentum, inertia and energy are just as important in electronics as they
are in mechanical systems.
Dorian
"Abstract Dissonance" <Abstract.Dissonance.hotmail.com> wrote in message
news:11umqmq...@corp.supernews.com...
Jonathan Kirwan
<Abstract.Dissonance.hotmail.com> wrote:
><snip>
>V = U/q where U is the change in potential energy and q is charge.
><snip>
I suspect this is not so good phrasing, as it might imply a dU divided
by some finite q. In which case, the result would be a dV, not a V.
I prefer to think of V, especially when using the word "change" in
phrasing, as the infinitesimal form: V = dU/dq (joules/coulomb) or
else just dU = V*dq = dV*q. Otherwise, if U and q are both finite,
they represent some meaningful average finite value or the differences
between two meaningful finite values, which is too often less "exact"
in my mind.
Jon
Joe McElvenney
> All electrical devices require BOTH I and V working together in
>tandem in order to function. Some devices, depending on their terminal
>characteristics regarding impedance, temperature, etc., are more
>amenable to being driven by a constant I source vs. a constant V
>source, or vice versa. The constant I source provides BOTH I and V, as
>does a constant V source. Have I explained myself? Best regards.
>
>Claude
I trust you feel better now :-)
Cheers - Joe
Abstract Dissonance
"Dorian McIntire" <dori...@bellsouth.net> wrote in message
news:T_KGf.10427$697....@bignews3.bellsouth.net...
> While I don't completely agree with Claude's post I can shed some light on
> one concept you took issue with; the idea of a charge moving with no
> applied voltage. It is indeed possible for a charge to move with no
> voltage. Superconductivity is one situation where charges move with no
> applied voltage. Granted, a voltage is initially required to get the
> charges moving but the voltage is not required to keep charges moving at a
> constant rate in a superconducting material.
>
>
Technically if there is an electric field then there is charge moving then
there is work being done. Obviously there are electrons "moving" all the
time without a "field"(well, theres always a field though) but we don't
consider a voltage moving them. The reason, Ig uess, is that there is no
mean free movement and/or the voltages are extremely small as to be
inconsequential.
About the SC material. The problem with that is that one cannot do any work
with it(hence no voltage) else one has a perpetual motion device. If you
try to measure the current flowing the electrons will stop flowing(or
decrease depending on how much work was used to get them to move). Its
similar to the uncertainty principle. So we can only talk about it in a
theoretical way.
>
> One special case of this is an electron moving at some constant velocity
> in a vacuum. Voltage is initially required to accelerate the electron but
> momentum will carry it on at a constant velocity if no extraneous fields
> are allowed to act on the charge.
>
Yes, but you are forgetting that voltage is basicaly the work/energy
supplied or given up by the moving charge. The electron has energy(you said
momentum) so technically it has a voltage. Voltage is an abstract definition
as is an electric field. As far as nature is concerned theres no such thing
as a E field or voltage. So you say that when we apply a voltage to
accelerate an electron and then remove that voltage it is gone... but it is
not. We have just transfered it to the electron(as far as you can transfer
voltage(really transfering energy)).
I think the problem is that you are thinking of voltage in macroscopic terms
then applying it to the microscopic. (sorta like wave-particle duality).
I.e., if you could just look at one electron at the microscopic level then
you wouldn't be able to see a "voltage" directly. If you were watching the
electron and you saw it move then you could hypothesize that there was a
voltage applied to it by some electric field. But you wouldn't be able to
really "see" the voltage.
Basicaly these are mathematical concepts applied to natural so we have to be
careful what we really been by these ideas. Although I can see what you
mean when we "apply" a voltage then stop "applying" it but you have to ask
yourself "What does it really mean to "apply" a voltage?". Then you might
realize that a voltage is really looking at the boundry conditions of some
even... i.e., measuring the energy at one point in time then at another
point in time and if they are different then something must have
happened(i.e., a "voltage").
For an analogy I could create a "field" that discribes how people move. We
could think of people as point charges and then plot there velecity vectors.
We would see areas in the field that look like there are "paths"(like a
highway with all the cars going in one direction or in an elevator,
etc...)... We could then hypothesize there is some force that makes the
"people" go in that common direction. In some locations it would be
disorganized or random and we could say there is no field there. We could
come up with some way to measure this "field" and create a whole theory
around it. In actuality there is probably not a field though and it is just
a "tool"/"concept" used to help us understand why the people seem to be
moving as they do. We could say "These people over here are experiencing a
force that makes them move along this path" and "These people here have some
forces that are holding them together"(such as at a football game), etc...
Hell, who knows ;) Basicaly the definition of voltage is independent of
what happens between two measurements so we can't really talk about what is
going between them(just like a particle as voltage is a measurement on
particles)... If we do talk about them we end up arriving at the problems we
do.
>
>
> This may seem to be minor point to some, but physical concepts such as
> momentum, inertia and energy are just as important in electronics as they
> are in mechanical systems.
>
True, but they are not well defined physical terms. They are mathematical
concepts applied to reality. Any time you start to dive into this stuff all
kinds of little problems start creeping up and then you have to start using
the quantum mechanical definition of these terms to get around these
problems... but then even stranger things start happening. I do agree that
it helps and is usually a good idea but you gotta be careful in how you use
them too. Our knowledge about reality is very limited and is only decent
approximation. We could actually be way off and just by happenstance happen
to have a model that approximates it well. That is, our model's axioms could
be completely wrong in that the "real" model has totally different axioms
but it turns out that the end results(large theorems) happen to agree with
each other. I have a feeling this is what is going on and our concepts of
reality is way off(but in some sense parallel to reality). This is why we
have so many little problems in the sciences today that don't make much
sense and it seems that we keep on having to modify them to get it to work
and introduce things that make less and less sense. I doubt thats how
reality works but it could be. My evidence is it seems everyone we find a
problem in nuclear physics it seems we have to create/find a new particle to
make everything right again... It could be this way or we are just "forcing"
it to work.
Anyways,
>
> Dorian
>
>
> <snip>
Jon
Abstract Dissonance
> On Thu, 9 Feb 2006 10:16:30 -0600, "Abstract Dissonance"
> <Abstract.Dissonance.hotmail.com> wrote:
>
>><snip>
>>V = U/q where U is the change in potential energy and q is charge.
>><snip>
>
> I suspect this is not so good phrasing, as it might imply a dU divided
> by some finite q. In which case, the result would be a dV, not a V.
>
It doesn't matter to much if you use a dV or V as long as you are
consistant.
Why? in either case you arrive at the same result and also V is really a
difference in itself(but on a global scale). They are really relative terms
and we tend to use concepts like dV -> ^v(delta V) -> V -> limit V->oo to
move through the different "resolutions".
I do see what you mean. Normally it should be written xV = xU/q where x
represents the "resolution". But since we normally take V to have 0
reference at infinity it doesn't matter in the end result.
Cause with your logic one could even say that q should be dq and we must
write dV = dU/dq but it depends on how we are treaing q. i.e., if q =
int(dq). if dq is not changing with position or time then we would normally
write q but if we are setting up an equation to handle varying q we have to
write dq and integrate).
I guess what I mean is that dV = dU/dq implies that we are dealing with
infintesimal quantities that will be used as part of a larger picture(i.e.
to integrate over). But if we are only dealing with "point charges" then we
can think of dq as = q and write a "simplified" notion of dV = dU/q. Now
since U and V does not depend on the path taken we can write it as Va - Vb =
(Ua - Ub)/q which in the standard notation delta(V) = delta(U)/q. But if our
reference point for V is taken to be Vb = 0 then we have either Va =
delta(U)/q or Va = U/q depending on how we look at it. delta(U) means change
in energy and U means change in energy w.r.t to an infinit point.
So you end up getting the same result since Va = Va - Voo. i.e., the
potential difference ALWAYS represents a "change" as does the potential
energy. So while notationally it might be a bit confusioning in general we
take a "short" cut to have to write one or two less characters as long as we
know that we mean that they represent differences else they make no sense.
so you can write it as dV = dU/q, V = dU/q delta(V) = delta(U)/q or V =
delta(U)/q. If you want to be precise depending on the problem then ofcourse
you have to choose the appropriate notation. (i.e., if you are dealing
writing a differential equation for the problem to find out is macroscopic
characteristics then you need to use the dV = dU/dq version).
heh. I guess what I'm saying is that its true if we use dU then we need to
use dV. In actuallity the real difference is simply that its always dV (or
delta(U) and delta(V)) because there is no such thing as V... but we
interpret V to be V - V_oo and make V_oo = 0 so we are left with just V....
so even though if we use delta(V) it is always equal to V with the above
criteria.
So mathematically you are correct but since we have that little criterion it
makes it *ok* for basic use.
i.e., when you measure voltage you are actually measuring between two
points... but we almost always use gnd as an implicit reference so we can
talk about the potential at a point(w.r.t to ground implied). I don't see
it as being technically wrong if everyone is in agreement about the implicit
reference used.
> I prefer to think of V, especially when using the word "change" in
> phrasing, as the infinitesimal form: V = dU/dq (joules/coulomb) or
> else just dU = V*dq = dV*q. Otherwise, if U and q are both finite,
> they represent some meaningful average finite value or the differences
> between two meaningful finite values, which is too often less "exact"
> in my mind.
>
yeah, but if V = dU/dq then mathematically it makes no sense if V is implied
to be a real number. dU/dq are not real numbers(or even complex) so we can't
get a real number from there ratio. Hell, differentials themselfs are not
really mathematically sound anyways and really are just short cut(as most
mathematics is anyways) to writing difference equations and then taking the
limit. As long as you are consistant in what you mean then it should be
ok... after all, they are just symbols anyways and they could mean anything
you want them to. (Ofcourse mathematicians have defined standard meanings
for them so everyone can communicate easily)
> Jon
Jon
Jonathan Kirwan
<Abstract.Dissonance.hotmail.com> wrote:
>Cause with your logic one could even say that q should be dq and we must
>write dV = dU/dq
No, I couldn't logically say that for a physical system of finite
functions of U and q.
Jon
Abstract Dissonance
What do you mean by finite functions? Discrete functions? bounded functions?
If you mean that q can only take on discrete values(and even U) then you can
say it pretty easy. Differentials exist for all for generalized functions
which can be used to sorta make discrete functions act like continuous
functions(I mean that mathematically you can use the same notation and
theorems).
i.e, say you have 10 point charges you can make this function into a
"normal" function(I mean one that behaves in mathematically similar way to
normal functions that you see every day) by using dirac delta functions. You
can then integrate and differentiate this function just like "normal"
functions and arrive at valid results just like normal functions.
You have to remember that this is mainly all notation and depending on what
concept you attach to the notation could make it "mean" something completely
different.
If you want to know what I mean just look up stieltjes integrals and it has
a pretty clear idea of what I mean. Basicaly lets you integrate over a
discontinuous
"differential"(notationally anyways).
Ultimately in the standard definition derivatives you are right but
physicists always try and bend the rules to make things easier to work with
so one has all kinds of extentions of notations and theorems that ordinarly
would seem odd to most people.
for example, check out fractional differentiation/calculus. You will seed a
notation that looks like an ordinary derivative but instead of the "nth"
derivative its something like the "rth" derivative where r is not an
integer. You could call it an abuse of notation but really its just an
extension made by someone that fits logically into the frame work in most
cases and sometimes can even be helpful.
Jon
> Jon
Jonathan Kirwan
<Abstract.Dissonance.hotmail.com> wrote:
Let's just say that my own imagination is limited, then, and that this
limits my ability to comment on physical systems. And I'd prefer for
now to keep it that way, given that all of the physics I've yet been
exposed to in my life remains congruent with my limited ability to
imagine here and doesn't require any new stretching on this. Abraham
Robinson's putting on solid mathematical ground, what had been little
more than physicists' intuition of infinitesimals until then, is
entirely enough for my needs.
Not that I don't enjoy learning more. But I'm right now stuffed on
Lie groups and algebras (I've always enjoyed finite and infinite group
theory) and catastrophe theory (which I'm struggling with), so that's
where I'm likely to be at.
Jon
Ryan
> grasp, is how a TRANSISTOR works!!!
I thought this would be the time I finally understood it, but after
reading John's answer, and my head exploding, I think I am going to
give up again for another year. (Not a bad answer I just couldn't
understand it.)
My feeble understanding of a transistor is like this:
Take a wire, cut it in half and leave a small gap. Current won't flow
across this gap. In the magical gap where you cut it, insert another
wire. As you add voltage to this new wire, it makes the gap between
the old wires conductive. It's a tiny relay?
The other version is a "normally on" relay. Current does flow acrosst
he gap. Add voltage to the extra wire and the gap becomes non
conductive.
I think in a regular gap there is always some current leaking between
the extra wire and the ground, but in a MOSFET there isn't?
I have no idea the actual why of it.
John Popelish
>> Something that has been confusing me no-end, and I just cant seem to
>> grasp, is how a TRANSISTOR works!!!
>
>
>
> I thought this would be the time I finally understood it, but after
> reading John's answer, and my head exploding, I think I am going to give
> up again for another year. (Not a bad answer I just couldn't understand
> it.)
Please, for my benefit, if not your own, post a copy of my
explanation, up to the point where your head exploded, so I can take a
crack at improving the explanation. By the way, the part at the end
about the sidewalk, the curb and the slope down to the street was
inspired by the famous photo of the cross section of that paving being
explained by William Shockley:
http://www.thocp.net/biographies/pictures/shockley_junction.jpg
More paving cross sections here:
http://www.mtmi.vu.lt/pfk/funkc_dariniai/transistor/bipolar_transistor.htm
Ryan
> Please, for my benefit, if not your own, post a copy of my explanation,
> up to the point where your head exploded, so I can take a crack at
> improving the explanation.
I think a good explanation of terms is the first area I need to
understand. I don't mean just definitions, but the whole concept.
(Analogy: More than a piston goes up&down and a crankshaft spins,
but that a piston is pushed downward with great force from a
controlled explosion and this linear force is converted into a
spinning force which is also used to reset the system...)
I've read about PN junctions before and tried to grasp them (diodes).
I've read about "holes" and electrons but these things don't really
mean anything to me.
I'm not certain that I understand why the emitter, base, and other
wire are called those names. I think I always confuse the two. The
digrams for transistors as used in circuit layouts don't make sense to
me. The arrows going in or out, etc. In those diagrams it isn't
intuitive for me which is the emitter, which is the base, etc.
I wonder if understanding a vacuum tube would help, but I've not quite
got that one either I don't think.
I've read over this stuff maybe half a dozen times over the past 10
years and at times thought I understood it, but I decide later that I
do not.
What does reverse biased mean? What is a doped electron? What does
it mean that holes find each other? Holes are pushed? I thought
they were a stationary place for electrons to gather? Are these made
of silicon or what?
>But at normal ambient temperatures (well above absolute zero) the
>movement of the electrons is randomized by the thermal energy
I think the general lack of understanding is why the behavior at a
different temperature is further beyond me.
John Popelish
> I think a good explanation of terms is the first area I need to
> understand. I don't mean just definitions, but the whole concept.
> (Analogy: More than a piston goes up&down and a crankshaft spins, but
> that a piston is pushed downward with great force from a controlled
> explosion and this linear force is converted into a spinning force which
> is also used to reset the system...)
Okay, my explanation was not tailored for your particular mind. I'll
try again.
> I've read about PN junctions before and tried to grasp them (diodes).
> I've read about "holes" and electrons but these things don't really
> mean anything to me.
(snip)
Then the rest of your questions must be put on hold till we get past
basic semiconductor physics. Maybe we will get back to the
transistor, later.
(a little ramble on solid state electronics)
A solid material is conductive only if there are movable charge
carriers in the material. In metals, these movable charge carriers
are electrons (usually, one per atom) that do not take part in holding
the material together as a crystalline form (electrons that are shared
between two neighbors, and locked into that bond) nor are buried in
the electron cloud of each atom, (and are not shared with any other
atom) but are shared fairly equally with several, neighboring atoms.
The total negative charge of this sea of shared electrons is exactly
balanced by an equal number of protons in the nuclei of the atoms
sharing these electrons. If a very slight electric voltage is applied
across a chunk of such material, the whole sea of conduction electrons
drifts toward the more positive end of this voltage, with electrons
leaving the metal at the point of positive application, and new ones
entering the metal at the point of negative connection. There is no
forward bias or reverse bias, only bias in some direction, or none at
all. The ratio of the voltage across the chunk to the current passing
through the chunk (volts per ampere) is called the resistance of the
chunk (ohms).
Insulating materials, in contrast use all the electrons of each atom
to hold the material together, except for the electrons that are
trapped around their individual nuclei. When a voltage is applied
across a chunk of insulator, almost no electrons jump from atom to
atom to contribute to any current, so the volts per ampere
(resistance) is very high.
Semiconductors are made of materials that, if very pure, are
essentially insulators. But if you jam in a few atoms, here and there
that have one extra electron in the outer interacting layers, compared
to the semiconductor (dope the semiconductor) all but one of their
external electrons will link into the semiconductor crystal structure,
leaving one dangling electron that is easily dislodged, because it is
not part of any bond. The semiconductor atoms can be encouraged to
accept one of these extra (in the sense of bonding requirements, but
not in the sense of not being balanced by a nuclear proton) with only
a little coaxing. The doping atoms have to be close enough to each
other that their protons don't miss their wondering electrons so much
that they pull them back, because someone else's electron is getting
closer at the same time theirs is wondering away.
So these doped in electrons act a lot like the way shared conduction
electrons act in metals. This combination of semiconductor and
electron donor doping creates N type semiconductor.
If, instead of doping with electron donor atoms, you jam into the
semiconductor, atoms with one less electron in their outer shell than
the semiconductor, you get a crystal that is electrically neutral
(equal number of protons and electrons, so no net charge imbalance),
but semiconductor atoms with electrons that are in a position to share
with a neighbor to form a bond, but a neighbor who has no
electrostatic need for that electron, because it has one less proton
in its nucleus. This unfilled crystal bond position is called a hole.
This hole acts a lot like a particle with a positive charge, even
though, it is really the absence of a particle with a negative charge.
With very little encouragement (additional energy), a semiconductor
electron can be nudged into a crystal bond with that doping atom
neighbor, to take the place of the electron the doping atom does not
provide, in spite of the net electrical imbalance it produces, because
being in a bond association is a low energy state for an electron.
This ability of the doping atom to hold on to an extra electron, by
virtue of it being part of a crystal structure that makes such bonds a
low energy state for an electron, is the reason we call these kind of
atoms acceptors.
However, once the semiconductor atom gives up one of its bonding
electrons for this purpose, it is missing one of its electron bonds
with some other neighbor. In effect,the hole has changed atoms. But
just as with donor atoms, as long as the acceptors are close enough to
each other that a hole moving away from it is compensated for by some
other hole moving toward it, it takes relatively little energy to make
this sea of holes drift in response to an applied voltage. And since
the mathematics of movements of missing negative charged particles in
one direction is is equivalent to the mathematics of positively
charged particles moving in the other direction, we refer to the holes
as positive charge carriers, as if they were particles.
Semiconductors that have the ability to conduct current by the
movement of holes are called P type. If we connect a voltage across a
chunk of P type semiconductor with metal contacts, the holes drift
toward the more negative connection, where they are filled with
electrons, as they arrive. At the positive connection, electrons are
pulled out of the semiconductor crystal, creating new holes that head
away as they form.
If we put chunk of N type semiconductor in intimate contact with a
chunk of P type semiconductor, we have an unsymmetrical arrangement,
so we can describe two possible relationships of applied voltage to
this pair. If negative voltage is applied to the N-type material and
positive voltage is applied to the P type material, we say the
junction is forward biased (each side of the junction is being
injected with the kind of charge carriers it normally would use in
order to conduct current). Electrons are being pushed into the N
type, repelling the inherent doped in electrons toward the junction,
and electrons are being sucked out of the P type material, creating
holes that repel the doped in holes toward the junction. At the
junction, electrons from the N type side are pouring into the P type
side, filling holes in short order, and holes are being pushed into
the N side, where they are swallowing up electrons in short order.
Continuous conduction of current (partly made up of holes moving, and
partly made up of electrons moving) ensues.
If the voltage is connected up the other way, we say the junction
reverse biased. That is, the more positive voltage is connected to
the N type material, attracting its electrons toward that terminal
(and removing some of them), and the more negative voltage is
attracting the holes in the P type material (and filing some of them
with electrons). At the junction, electrons are being pulled away on
the N side, and holes are being pulled away on the P side, leaving
semiconductor that acts as if it is made of pure silicon with neither
donor electrons or acceptor holes, but like an insulating crystal.
Once the pull of the external voltage is balanced by the internal pull
of the separated electrons and holes across the insulating junction,
all current ceases and a stand off is created.
These two bias cases (forward and reversed) define the conducting and
nonconducting states of a junction diode.
Has your head exploded, yet?
Ryan
> in the material. In metals, these movable charge carriers are electrons
> (usually, one per atom) that do not take part in holding the material
> together as a crystalline form (electrons that are shared between two
> neighbors, and locked into that bond) nor are buried in the electron
So does this disqualify all elements or compounds that when bonded
precisely "use up" all of the residual electrons in their
respective orbits? My chemistry skills are not as sharp, but I assume
that hydrogen(2) compounds would insulate as would
carbon(4) since the outer orbit is saturated? I presume carbon would
since it lies in the same column as silicon and I
assume silicon's outer orbit has the same limit of 8 electrons as does
the 2nd orbit of carbon?
Does is matter the type of bond? For example, I remember covalent
bonds the most from high school, but recall there are
other types.
Tell me if I am mistaken. I see on the periodic table that copper has
a "need" for two more electrons. Is this why it
makes a good carrier medium for electricity?
> The total negative charge of this sea of shared electrons is
> exactly balanced by an equal number of protons in the nuclei
> of the atoms sharing these electrons.
Still with you.
> If a very slight electric voltage is applied across a chunk
> of such material, the whole sea of conduction electrons drifts toward
> the more positive end of this voltage, with electrons leaving the metal
> at the point of positive application, and new ones entering the metal at
> the point of negative connection.
So now we've made a circuit, right?
If I cram two extra electrons per atom into this mass of atoms, does
that comprise voltage X? If I cram 4 times more
electrons than this into this mass, does that comprise a voltage higher
than X? Do these electrons become less stable in
that mass and more anxious to "jump away" because the atom is that much
less stable? Are these electrons will to jump a
greater distance, or arc, because of the opposite positive force of the
protons against them? Does the light from the arc
occur because there is a quantum leap taking place, or am I way off here?
> There is no forward bias or reverse
> bias, only bias in some direction, or none at all.
I'm not sure I understand. Is this another way of saying that a
negatively charged sea doesn't have any natural propensity
to go one way versus another? It will just exit to whatever positive
hole is created in the same way a sea of water doesn't
care where it goes so long as it is downward and will take the
opportunity to go down into any hole introduced?
> The ratio of the voltage across the chunk to the current
> passing through the chunk (volts per ampere) is called
> the resistance of the chunk (ohms).
I've long understood resistance as an unwillingness to permit current to
flow, however, I don't understand this mathematical
relationship. It would see that if I increase voltage, the numerator
goes up and I have changed the resistance of the
circuit. I thought resistance was a static thing that existed by
nature and we didn't have the ability to change this
figure.
> Semiconductors are made of materials that, if very pure, are essentially
> insulators.
If I recall correctly, H2O is a good insulator?
> But if you jam in a few atoms, here and there that have one
> extra electron in the outer interacting layers, compared to the
> semiconductor (dope the semiconductor) all but one of their external
> electrons will link into the semiconductor crystal structure, leaving
> one dangling electron that is easily dislodged, because it is not part
> of any bond.
If I follow correctly, you occasionally disobey the cystalization
(vocabulary?) rule within this compound and intentionally
leave one of the crystal structures incomplete. This incompleteness has
to occur with exactly the surplus of 1 satisfied
electron? This imperfect crystal is a "doped" crystal?
Would this be like an arrangement of 3 silicon atoms and 1 phosphorus
atom? If not, can you give an example of the normal
formula and the doped one?
> The semiconductor atoms can be encouraged to accept one of
> these extra (in the sense of bonding requirements, but not in the sense
> of not being balanced by a nuclear proton) with only a little coaxing.
If I follow, then you have created an electron highway?
> The doping atoms have to be close enough to each other that their
> protons don't miss their wondering electrons so much that they pull them
> back, because someone else's electron is getting closer at the same time
> theirs is wondering away.
Hmmm. So any lone doped atom won't work, but if you put a whole bunch
of them in a row, the charges interact enough that
your electron can hover over all of the atoms? (Like those
superconductor levitation examples?)
> So these doped in electrons act a lot like the way shared conduction
> electrons act in metals. This combination of semiconductor and electron
> donor doping creates N type semiconductor.
Do you not mean the doped atoms rather than doped electrons? (Or maybe
doped compounds? Doped crystals?) I think what you
mean is that by forcing in an atom with an extra electron than what is
needed for the bond (higher mass, not an anion), you
have created a material that would typically have insulated if done
properly, but instead now can conduct?
I want to be sure I understand this one.
> If, instead of doping with electron donor atoms, you jam into the
> semiconductor, atoms with one less electron in their outer shell than
> the semiconductor, you get a crystal that is electrically neutral (equal
> number of protons and electrons, so no net charge imbalance), but
> semiconductor atoms with electrons that are in a position to share with
> a neighbor to form a bond, but a neighbor who has no electrostatic need
> for that electron, because it has one less proton in its nucleus. This
> unfilled crystal bond position is called a hole. This hole acts a lot
> like a particle with a positive charge, even though, it is really the
> absence of a particle with a negative charge.
I think I am following along. We've made a crystal but done the
opposite of before. I assume this time we used something
like aluminum which is 1 unit lighter. I'm guessing it acts like a
cation (+) not because of charge but because it won't
complain too much about being host to an extra electron. If not
aluminum, then what is it?
> With very little encouragement (additional energy), a semiconductor
> electron can be nudged into a crystal bond with that doping atom
> neighbor, to take the place of the electron the doping atom does not
> provide, in spite of the net electrical imbalance it produces, because
> being in a bond association is a low energy state for an electron. This
> ability of the doping atom to hold on to an extra electron, by virtue of
> it being part of a crystal structure that makes such bonds a low energy
> state for an electron, is the reason we call these kind of atoms
acceptors.
I think I follow.
> However, once the semiconductor atom gives up one of its bonding
> electrons for this purpose, it is missing one of its electron bonds with
> some other neighbor. In effect,the hole has changed atoms. But just as
> with donor atoms, as long as the acceptors are close enough to each
> other that a hole moving away from it is compensated for by some other
> hole moving toward it, it takes relatively little energy to make this
> sea of holes drift in response to an applied voltage. And since the
> mathematics of movements of missing negative charged particles in one
> direction is is equivalent to the mathematics of positively charged
> particles moving in the other direction, we refer to the holes as
> positive charge carriers, as if they were particles.
So the new electron in town has really shaken up the dynamics of the
group? But they are fickle enough to "network" and move
around and be content since the environment is so condusive? By way of
quantum "osmosis" everything balances out?
> Semiconductors that
> have the ability to conduct current by the movement of holes are called
> P type. If we connect a voltage across a chunk of P type semiconductor
> with metal contacts, the holes drift toward the more negative
> connection, where they are filled with electrons, as they arrive. At
> the positive connection, electrons are pulled out of the semiconductor
> crystal, creating new holes that head away as they form.
Whoa, this way of thinking seems all backwards to me. I guess it makes
sense. In my mind, the first scenario was like a
stage diver being passed around overhead of a crowd. This example, in
my mind is more like water pails being passed along an
old fire line swinging from one person to the next.
I think I see how neither of these are conductive if there is no voltage
present to them. If there is voltage applied, it
takes some activation energy to get things started, but once it does,
you have a sort of chain reaction. Is this break down
voltage? Is this why diodes don't turn on until 0.7 volts?
So an N type and a P type would take the opposite state from each other
if given the exact same voltage? Is this right, or
do they both become conductive but by different internal mechanisms as
already described?
> If we put chunk of N type semiconductor in intimate contact with a chunk
> of P type semiconductor, we have an unsymmetrical arrangement, so we can
> describe two possible relationships of applied voltage to this pair. If
> negative voltage is applied to the N-type material and positive voltage
> is applied to the P type material, we say the junction is forward biased
> (each side of the junction is being injected with the kind of charge
> carriers it normally would use in order to conduct current). Electrons
> are being pushed into the N type, repelling the inherent doped in
> electrons toward the junction, and electrons are being sucked out of the
> P type material, creating holes that repel the doped in holes toward the
> junction. At the junction, electrons from the N type side are pouring
> into the P type side, filling holes in short order, and holes are being
> pushed into the N side, where they are swallowing up electrons in short
> order. Continuous conduction of current (partly made up of holes moving,
> and partly made up of electrons moving) ensues.
Hold up. Does this mean "forward bias" means that the two semicondutor
types will trade electrons all day long?
In order for this to happen, we had to put negative voltage to the N
type... I may be lost here. Where is the circuit? Is
there a wire on each end of the N, and then two other wires of a
separate circuit on both ends of the P type? Perhaps the
two are connected in series as part of the same circuit? I am not sure
what it looks like.
What is the negative voltage going to cause to happen to the N? If
voltage is a difference, what does it matter if it is
"postive" or "negative" it's relative right?
> If the voltage is connected up the other way, we say the junction
> reverse biased. That is, the more positive voltage is connected to the
> N type material, attracting its electrons toward that terminal (and
> removing some of them), and the more negative voltage is attracting the
> holes in the P type material (and filing some of them with electrons).
> At the junction, electrons are being pulled away on the N side, and
> holes are being pulled away on the P side, leaving semiconductor that
> acts as if it is made of pure silicon with neither donor electrons or
> acceptor holes, but like an insulating crystal.
> Once the pull of the external voltage is balanced by the internal pull
> of the separated electrons and holes across the insulating junction, all
> current ceases and a stand off is created.
>
> These two bias cases (forward and reversed) define the conducting and
> nonconducting states of a junction diode.
So "forward" is like they are looking towared each other for electrons
(eye-to-eye), and "reversed" is like they are looking
"away" from each other for electrons (back-to-back)?
> Has your head exploded, yet?
It's about to, but this time it is due to a relapse of a sinus infection.
Ryan
Rich Webb
[snip...snip...]
>I'm not certain that I understand why the emitter, base, and other
>wire are called those names. I think I always confuse the two. The
>digrams for transistors as used in circuit layouts don't make sense to
>me. The arrows going in or out, etc. In those diagrams it isn't
>intuitive for me which is the emitter, which is the base, etc.
To a first approximation (maybe 0.1-th approximation):
Electricity flows from positive to negative. Electrons and holes don't
exist, there's just this "stuff" that goes from a place where there's a
lot of it (positive) to where there's not much of it (negative). Stay
with me, it works, and I blame Franklin.
Most of the things that this stuff interacts with are either metals,
where the stuff flows freely, or non-metals, where the stuff doesn't
flow at all.
Some guys found a material where the electricity sort of half-way wants
to flow and also discovered that they could fabricate it in such a way
(doesn't matter how) where electricity prefers to flow in just one
direction through the material. Pretty neat trick.
When one side of the material with the special fabrication is connected
to the positive electrical source and the other to the negative side,
then electricity will flow through it from positive to negative; flip it
around and there is no flow. Doesn't matter why, really, it just works
that way.
For convenience, call the side that would connect to the positive
electrical source when there is a flow of current 'p' and the side next
to the negative one 'n'. That's because they were engineers and liked to
keep things simple; if they had been scientists they'd probably have
used some Hittite script or something. Be thankful for little things.
So, to review, if the 'p' side of the material is more positive than the
'n' side, there is a current flow. If the 'n' side is more positive than
the 'p' side, it's backwards and there is no current flow.
Okay, these guys had a new toy and wanted to fool around with it. Turns
out that they noticed that you can stack three layers in the order n-p-n
and, pretty much as expected, if the 'p' bit is more positive than the
'n' bit there is a current flow into the 'p' and out the 'n'. Ho hum.
Then, when they stuck the positive side onto the top 'n' bit and stuck
the other 'n' bit onto the negative side, so that the current would have
to go from 'n' to 'p' and then to 'n', well ... oops, no current flow.
No surprise, either, since the top bits are backwards n-p, of course.
But remember that the middle 'p' bit can be connected on the outside so
that when it's enough more positive than the bottom 'n' bit then there
*is* a current flow between those two. The really cool thing is that if
the top 'n' bit is the most positive of all, then the small current that
flows in the p-n direction (middle to bottom) "picks up and carries" a
much greater current from the top, past the middle, and out the bottom.
The effect is kind of like grains of sand in a funnel (okay; big grains,
small funnel, stay with me here) where the grains bunch up so tightly
that they plug the funnel and nothing gets out. But if you ran a small
thread down through the funnel and kept pulling the thread out, the
relatively tiny "flow" of the thread would keep a much larger flow of
sand moving. Stop pulling the thread, the funnel clogs. Pull it, you get
a pile of sand.
Pretty much, that's all that's happening: a small current flowing into
the base (the middle 'p') and out the emitter (the bottom 'n') controls
a much larger current that flows from the collector (the top 'n'), past
the base, and then out the emitter.
An aside: For some transistor types (called bipolar; the ones drawn with
the outside lines at an angle) the collector and emitter (always the one
with the arrow) are optimized for their roles and can't usefully have
their roles reversed. There are other types (called MOS; drawn with the
outside lines parallel like the sides of a box) where it's sometimes
possible ignore which is the collector and emitter and use either one
in either role; the base is always the base for both bipolar and MOS
transistors. Note that in MOS transistors the terms collector, base, and
emitter are replaced by drain, gate, and source. Don't let it worry you,
life is perverse.
Okay, back to the transistor / funnel. The ratio of the amount of
current that you get to flow from the top (collector) out the bottom
(emitter) versus the amount of current going into the middle (base) and
out the emitter varies from the 10's to the 100's depending on how the
transistor is made, how much current is flowing through it, how hot it
is, etc., etc. By themselves, there's so much variation that transistors
aren't of much use in applications where precision is needed.
But resistors can be used to control the amount of current into the
base, or into the collector, or out the emitter. They can be selected so
that the gain of the "stage" (i.e., the ratio of output to input for the
transistor and all of its hangers-on) depends more on the ratios of the
resistors and less on the gain of the individual transistor. Now the
stage gain is predictable. This is technically known as A Good Thing.
Resistors are mundane and not very exciting. They have the virtue of
being relatively stable and can be selected for specific values and
tolerances. Transistors are, by comparison, exotic creatures with widely
varying characteristics. When a transistor stage is anchored by
resistors, the effects of item-to-item variation are much reduced and
circuit behavior is much more predictable. Lets us build a thousand
widgets that all work the same, instead of having to hand-tune every one
of them (and worry about what happens if the widget ends up in Saskatoon
or in Salt Lake City).
The different ways of hooking-up transistors and resistors result in
stages that function as switches, voltage amplifiers, or current
amplifiers. Which you use depends on what you want.
Remember: The one with the arrow (irrespective of the direction of the
arrow) is the emitter. The middle one is the base; and the other one is
the collector. Current flows from positive to negative (the p to the n)
so if the arrow points from the middle to the outside then the middle
must be the one that's more 'p' and the outside one more 'n' and so you
have an n-p-n transistor.
There are p-n-p transistors, also, where the arrow on the symbol (always
the emitter) points from the outside to the inside. They are used in the
same way, by setting up a small current in the p-to-n direction between
the base and emitter to control a larger current between the emitter and
the collector.
The next question "Where to put the resistors to 'bias' a transistor?"
is answered: What do you want the transistor to do?
--
Rich Webb Norfolk, VA
John Popelish
> > A solid material is conductive only if there are movable charge carriers
> > in the material. In metals, these movable charge carriers are electrons
> > (usually, one per atom) that do not take part in holding the material
> > together as a crystalline form (electrons that are shared between two
> > neighbors, and locked into that bond) nor are buried in the electron
>
>
> So does this disqualify all elements or compounds that when bonded
> precisely "use up" all of the residual electrons in their
> respective orbits?
I think it does. The exception might be ionic solutions and plasmas,
where whole atoms (minus an electron) move, as well as electrons. But
those aren't solids. The electrons under discussion are the outside
ones, normally referred to as valence electrons. The rest are so
close to the nucleus that they never leave that atom, under normal
solid state situations.
> My chemistry skills are not as sharp, but I assume
> that hydrogen(2) compounds would insulate as would
> carbon(4) since the outer orbit is saturated?
Frozen hydrogen is an insulator, but if you compress it enough, it
switches to a metal.
> I presume carbon would
> since it lies in the same column as silicon and I
> assume silicon's outer orbit has the same limit of 8 electrons as does
> the 2nd orbit of carbon?
>
Carbon in some crystalline forms is fairly metallic (graphite) and on
others is a semiconductor (diamond). It depends on what those
electrons are busy doing and what the energy levels of those tasks are.
> Does is matter the type of bond? For example, I remember covalent
> bonds the most from high school, but recall there are
Yes it matters. I think all semiconductors are held together with
covalent bonds, but this is getting out of the area of knowledge I use
often.
> other types.
>
> Tell me if I am mistaken. I see on the periodic table that copper has
> a "need" for two more electrons. Is this why it
>
> makes a good carrier medium for electricity?
I think copper is a metal that contributes two electrons to the
conductive cloud. But don't quote me on that. This is the kind of
thing I have to look up when I need it. My explanation was not
intended to be exactly right in all respects, as it was to be a fly
over of the basic effects involved in conductivity.
> > The total negative charge of this sea of shared electrons is
> > exactly balanced by an equal number of protons in the nuclei
> > of the atoms sharing these electrons.
>
> Still with you.
>
> > If a very slight electric voltage is applied across a chunk
> > of such material, the whole sea of conduction electrons drifts toward
> > the more positive end of this voltage, with electrons leaving the metal
> > at the point of positive application, and new ones entering the metal at
> > the point of negative connection.
>
> So now we've made a circuit, right?
Right.
> If I cram two extra electrons per atom into this mass of atoms, does
> that comprise voltage X? If I cram 4 times more
> electrons than this into this mass, does that comprise a voltage higher
> than X? Do these electrons become less stable in
If you cram even one extra electron per atom into a crystal of metal,
it explodes with great violence from the incredible repulsion of all
that negative electric charge. Al conductors except diffuse plasma
are almost perfectly neutral, meaning that in order to jam an electron
in somewhere, you have to pull one out, somewhere else.
> that mass and more anxious to "jump away" because the atom is that much
> less stable? Are these electrons will to jump a
>
> greater distance, or arc, because of the opposite positive force of the
> protons against them? Does the light from the arc
>
> occur because there is a quantum leap taking place, or am I way off here?
You are not talking about electro statically balanced matter any more,
but a collection of ions.
> > There is no forward bias or reverse
> > bias, only bias in some direction, or none at all.
>
> I'm not sure I understand. Is this another way of saying that a
> negatively charged sea doesn't have any natural propensity
> to go one way versus another?
Right. Isotropic. The same in any direction. No inherent sides or
grain.
> It will just exit to whatever positive
> hole is created in the same way a sea of water doesn't
> care where it goes so long as it is downward and will take the
> opportunity to go down into any hole introduced?
Right. Water flows downhill.
Same with electrons in metals.
> > The ratio of the voltage across the chunk to the current
> > passing through the chunk (volts per ampere) is called
> > the resistance of the chunk (ohms).
>
> I've long understood resistance as an unwillingness to permit current to
> flow, however, I don't understand this mathematical
> relationship. It would see that if I increase voltage, the numerator
> goes up and I have changed the resistance of the
> circuit. I thought resistance was a static thing that existed by
> nature and we didn't have the ability to change this
> figure.
Many materials, metals included, hold a quite constant ratio of
voltage versus current for a wide range of both. This property is
what makes the production of linear resistors possible. But an ohm is
still a volt per ampere. A hundred ohms requires 100 volts across it
to push an ampere through it.
> > Semiconductors are made of materials that, if very pure, are essentially
> > insulators.
>
> If I recall correctly, H2O is a good insulator?
So is ice.
> > But if you jam in a few atoms, here and there that have one
> > extra electron in the outer interacting layers, compared to the
> > semiconductor (dope the semiconductor) all but one of their external
> > electrons will link into the semiconductor crystal structure, leaving
> > one dangling electron that is easily dislodged, because it is not part
> > of any bond.
>
>
> If I follow correctly, you occasionally disobey the cystalization
> (vocabulary?) rule within this compound and intentionally
> leave one of the crystal structures incomplete. This incompleteness has
> to occur with exactly the surplus of 1 satisfied
> electron? This imperfect crystal is a "doped" crystal?
You got it.
> Would this be like an arrangement of 3 silicon atoms and 1 phosphorus
> atom? If not, can you give an example of the normal
> formula and the doped one?
That would be a very highly doped crystal. 1 phosphorus atom per
10,000 atoms of silicon might be a more normally doped semiconductor.
> > The semiconductor atoms can be encouraged to accept one of
> > these extra (in the sense of bonding requirements, but not in the sense
> > of not being balanced by a nuclear proton) with only a little coaxing.
>
> If I follow, then you have created an electron highway?
>
> > The doping atoms have to be close enough to each other that their
> > protons don't miss their wondering electrons so much that they pull them
> > back, because someone else's electron is getting closer at the same time
> > theirs is wondering away.
>
> Hmmm. So any lone doped atom won't work, but if you put a whole bunch
> of them in a row, the charges interact enough that>
> your electron can hover over all of the atoms? (Like those
> superconductor levitation examples?)
They don't have to be that close. Their effect reaches out over quite
a few atomic diameters.
> > So these doped in electrons act a lot like the way shared conduction
> > electrons act in metals. This combination of semiconductor and electron
> > donor doping creates N type semiconductor.
> Do you not mean the doped atoms rather than doped electrons?
I mean electrons provided by the doping atoms. But I get tired typing
that long phrase. I appreciate your consideration.
> (Or maybe doped compounds? Doped crystals?) I think what you
> mean is that by forcing in an atom with an extra electron than what is
> needed for the bond (higher mass, not an anion), you
> have created a material that would typically have insulated if done
> properly, but instead now can conduct?
Yes.
> I want to be sure I understand this one.
Me too. It is hard to picture.
> > If, instead of doping with electron donor atoms, you jam into the
> > semiconductor, atoms with one less electron in their outer shell than
> > the semiconductor, you get a crystal that is electrically neutral (equal
> > number of protons and electrons, so no net charge imbalance), but
> > semiconductor atoms with electrons that are in a position to share with
> > a neighbor to form a bond, but a neighbor who has no electrostatic need
> > for that electron, because it has one less proton in its nucleus. This
> > unfilled crystal bond position is called a hole. This hole acts a lot
> > like a particle with a positive charge, even though, it is really the
> > absence of a particle with a negative charge.
>
>
> I think I am following along. We've made a crystal but done the
> opposite of before. I assume this time we used something
>
> like aluminum which is 1 unit lighter. I'm guessing it acts like a
> cation (+) not because of charge but because it won't
> complain too much about being host to an extra electron. If not
> aluminum, then what is it?
Boron, aluminum, gallium, or indium. ( I Googled [acceptor atoms].
Sounds good to me.
> > Semiconductors that
> > have the ability to conduct current by the movement of holes are called
> > P type. If we connect a voltage across a chunk of P type semiconductor
> > with metal contacts, the holes drift toward the more negative
> > connection, where they are filled with electrons, as they arrive. At
> > the positive connection, electrons are pulled out of the semiconductor
> > crystal, creating new holes that head away as they form.
>
>
> Whoa, this way of thinking seems all backwards to me. I guess it makes
> sense. In my mind, the first scenario was like a
> stage diver being passed around overhead of a crowd. This example, in
> my mind is more like water pails being passed along an
> old fire line swinging from one person to the next.
>
> I think I see how neither of these are conductive if there is no voltage
> present to them. If there is voltage applied, it
> takes some activation energy to get things started, but once it does,
> you have a sort of chain reaction. Is this break down
> voltage? Is this why diodes don't turn on until 0.7 volts?
You will have to ask someone else about that. But I think you are
very close.
> So an N type and a P type would take the opposite state from each other
> if given the exact same voltage? Is this right, or
> do they both become conductive but by different internal mechanisms as
> already described?
The second, I think. They conduct effectively with opposite polarity
of charge carrier. Remember that holes (spots that can be encouraged
to hold an electron) get passed around exactly as if they were
particles with positive charge. They move at a different speed and
have a different effective mass than electrons, so P type conductivity
has a unique description from an electrical perspective than N type
conductivity. The general concept is the same, but they are somewhat
different.
>
> > If we put chunk of N type semiconductor in intimate contact with a chunk
> > of P type semiconductor, we have an unsymmetrical arrangement, so we can
> > describe two possible relationships of applied voltage to this pair. If
> > negative voltage is applied to the N-type material and positive voltage
> > is applied to the P type material, we say the junction is forward biased
> > (each side of the junction is being injected with the kind of charge
> > carriers it normally would use in order to conduct current). Electrons
> > are being pushed into the N type, repelling the inherent doped in
> > electrons toward the junction, and electrons are being sucked out of the
> > P type material, creating holes that repel the doped in holes toward the
> > junction. At the junction, electrons from the N type side are pouring
> > into the P type side, filling holes in short order, and holes are being
> > pushed into the N side, where they are swallowing up electrons in short
> > order. Continuous conduction of current (partly made up of holes moving,
> > and partly made up of electrons moving) ensues.
>
> Hold up. Does this mean "forward bias" means that the two semicondutor
> types will trade electrons all day long?
...will spill oppositely charged carriers into each other all day.
> In order for this to happen, we had to put negative voltage to the N
> type... I may be lost here. Where is the circuit? Is>
> there a wire on each end of the N, and then two other wires of a
> separate circuit on both ends of the P type?
There is a wire from the negative terminal of a voltage source
connected to the N type material and a wire from the positive side of
the voltage source connected to the P type material, and the block of
N type material in in intimate contact (continuous crystal structure)
with the block of P type material. A diode with a battery connected
across it.
> Perhaps the two are connected in series as part of the same circuit?
That's it.
> I am not sure
> what it looks like.
>
> What is the negative voltage going to cause to happen to the N?
It will pour electrons into it, forcing electrons out the other side.
Total charge balance is maintained, or very nearly so.
> If voltage is a difference, what does it matter if it is
> "postive" or "negative" it's relative right?
Right. voltage difference across the chunk of material is all that
matters. Inside the block, the atoms have no way of knowing what the
average potential of the rest of the universe is.
> > If the voltage is connected up the other way, we say the junction
> > reverse biased. That is, the more positive voltage is connected to the
> > N type material, attracting its electrons toward that terminal (and
> > removing some of them), and the more negative voltage is attracting the
> > holes in the P type material (and filing some of them with electrons).
> > At the junction, electrons are being pulled away on the N side, and
> > holes are being pulled away on the P side, leaving semiconductor that
> > acts as if it is made of pure silicon with neither donor electrons or
> > acceptor holes, but like an insulating crystal.
> > Once the pull of the external voltage is balanced by the internal pull
> > of the separated electrons and holes across the insulating junction, all
> > current ceases and a stand off is created.
> >
> > These two bias cases (forward and reversed) define the conducting and
> > nonconducting states of a junction diode.
>
>
> So "forward" is like they are looking towared each other for electrons
> (eye-to-eye), and "reversed" is like they are looking
Make that electrons and holes.
> "away" from each other for electrons (back-to-back)?
Works for me.
> > Has your head exploded, yet?
>
> It's about to, but this time it is due to a relapse of a sinus infection.
I hate it when that happens. )-;
jwe...@ccwf.cc.utexas.edu
: Something that has been confusing me no-end, and I just cant seem to
: grasp, is how a TRANSISTOR works!!!
<snip>
: I cant grasp WHY they are so extremely important - probably because I'm
: finding it hard to understand their basic operation !
Other people have given you explanations that are way too
complicated (There might've been some in there that were to the point, but
I might've missed them.)
My explanation/answer to your two questions above will focus on
MOS transistors, but can be applied to BJTs as well.
A simple explanation of HOW a transistor works is that it is a
device that can implement 2 elementary functions:
1. A switch
2. A constant current source
Breaking this down a little bit more, a transistor can be in one
of three modes of operation, each of which can be modeled with a basic
circuit element:
1. An open switch
2. A closed (but non-ideal) switch
3. A constant current source.
Which mode of operation depeds upon the relationships between the
voltages at each of the transistor's 3 terminals (actually, a MOSFET has 4
terminals, but let's assume that the Bulk is connected to the Source so
that Vbs = 0 all the time, to make things easier.)
When I say non-ideal switch, I mean that the switch has some
resistance between it's terminals. A non-ideal switch behaves like an
open circuit when it is open, and a resistor (NOT a short-circuit) when it
is closed.
Furthermore, let's also assume that, for the moment, we are only
interested in examining how the transistor behaves for DC voltages.
Therefore, NO CURRENT flows into the gate.
Let's look at an NMOS transistor.
Vgs = Gate Voltage - Source Voltage
Vds = Drain Voltage - Source Voltage
When Vgs < Vt (The threshold voltage -- an intrinsic property of
the transistor, usually equal to about 0.5V - 1.0V in modern processes)
the transistor is in the CUTOFF region of operation, and behaves like an
OPEN SWITCH. No current flows from drain to source.
When (Vgs > Vt), AND (Vds < Vgs - Vt) the transistor is said to be
in the LINEAR (or TRIODE, or NON-SATURATED) region of operation, and
behaves like a CLOSED, NON-IDEAL SWITCH. A current will flow from the
drain to source. The amount of current that flows is DIRECTLY
PROPORTIONAL TO BOTH Vgs AND Vds.
When (Vgs > Vt) AND (Vds > Vgs - Vt) the transistor is said to be
in the SATURATION region of operation and behaves like a CONSTANT CURRENT
SOURCE. Current will flow from the drain to source. This current is
DIRECTLY PROPORTIONAL to Vgs, but INDEPENDENT of Vds (so long as Vds > Vgs
- Vt).
That's the HOW. Now, the WHY. Any digital circuit is essentially
a collection of voltage-dependent switches. A binary voltage represents a
boolean value. Therefore, transistors can implement any boolean function,
which enables the enormous microprocessors and other marvels of digital
logic that are commonplace today. On the flip-side, most useful analog
circuits require constant current sources. Amplifiers are essentially 2
constant current sources that fight each other. Sure, vacuum tubes can
also implement these functions, but transistors can implement them and are
MICROSCOPIC. Therefore, transistors are important because they allow for
the building of both digital and analog INTEGRATED CIRCUITS of enormous
complexity that are still barely visible to the naked eye.
Hope that helps in answering your questions....
Joe
ehsjr
You are being deluged with information.
Start *really* simple, then build on it.
A bipolar transistor has 3 leads: emitter, base and collector.
The schematic diagram always has the base in the middle, between
the emitter and the collector. The emitter is always drawn with
an arrow. The arrow points toward the base for PNP transistors,
and away from the base for NPN transistors. Base-emitter current
controls current across the emitter-collector. When the arrow
points toward the base (PNP) the base must be more negative
than the emitter (by roughly .6 volts) to cause current across
the emitter-collector. When the arrow points away from the base
(NPN) the base needs to be more positive than the emitter (again,
by roughly .6 volts) to cause emitter-collector current.
That's the basics to learn for now. Once you learn that to
where it is second nature to you, you can build on it. There
are other types of transistors, and other materials (which affect
that "roughly .6 volts" figure) but hold off on worrying about
them. Hold off on holes, and electrons, and valence rings etc
until you understand the symbol and the concept of controlling
the c-e (collector-emitter) current via the b-e (base-emitter)
current.
Ed