BJT Symmetric Electrical Characteristics
Jon Slaughter
voltage polarity? At least those that are not explicitly mentioned in the
datasheet. BVceo = BCeco? Obviously this is not always the case but I'm
wondering about those that are not shown in the datasheet in general.
Tim Wescott
In general I think the answer is no. Certainly the base-emitter
breakdown is usually small (on the order of 6V); I think that would
dominate with the collector junction forward biased.
Jim Thompson
wrote:
True!
Only the ancient "INCH" chopper devices had symmetrical
characteristics (they were _lateral_ NPN's).
Vertical devices are quite asymmetric due to the progressively higher
doping densities as you move from substrate to the surface.
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
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I love to cook with wine. Sometimes I even put it in the food.
bob.jo...@gmail.com
But he said the ones that are not mentioned in the datasheet. VEBO and
VBEsat are mentioned.
Tim Wescott
> On Thu, 29 Oct 2009 18:22:37 -0500, Tim Wescott <t...@seemywebsite.com>
> wrote:
>
>>On Thu, 29 Oct 2009 18:08:50 -0500, Jon Slaughter wrote:
>>
>>> In general are the bjt electrical charateristics symmetric with
>>> respect to voltage polarity? At least those that are not explicitly
>>> mentioned in the datasheet. BVceo = BCeco? Obviously this is not
>>> always the case but I'm wondering about those that are not shown in
>>> the datasheet in general.
>>
>>In general I think the answer is no. Certainly the base-emitter
>>breakdown is usually small (on the order of 6V); I think that would
>>dominate with the collector junction forward biased.
>
> True!
>
> Only the ancient "INCH" chopper devices had symmetrical characteristics
> (they were _lateral_ NPN's).
>
> Vertical devices are quite asymmetric due to the progressively higher
> doping densities as you move from substrate to the surface.
I always assumed that they were asymmetric for performance reasons, i.e.
that you'd want a different doping profile, geometry, etc., for each bit
of the thing to make it work best. In fact, I distinctly remember being
told that the collector of a transistor was bigger than the emitter
because then "it would collect carriers better".
So in a perfect world where it didn't cost anything to have the part put
together atom-by-atom, would a high-performance BJT be, in general,
symmetric?
Jim Thompson
wrote:
If you don't mind low beta. If you examine the beta equations, beta
is _proportional_ to....
((emitter-doping)/(base-doping))
and
((base-doping)/(collector-doping))
There's also a "constant" in there which I can't remember without
getting out some old text books ;-)
mi...@sushi.com
If you are making BJTs in a vanilla CMOS process, they can be
symmetric, with lateral current flow. There is also a vertical current
flow to the substrate, but I think the lateral device would break down
first due to higher doping levels. Generally these devices were only
used where you needed low noise (better than your MOS devices).
Tim Wescott
So it isn't _just_ because of the way they happen to go together -- it's
really intentional, to get performance.
Hey! A guy with a PhD was right about something!
Jim Thompson
wrote:
They're good at deriving and writing equations, but not much good at
building things that work consistently.
...Jim Thompson
--
| James E.Thompson, CTO | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC's and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
In Memoriam...
Duane Lee Thompson
October 31, 1972 - April 20,2006
4th Child, 2nd Son, of Jim & Naomi Thompson
Victim of Colon Cancer
Would Have been 37 Today
Tim Williams
> So in a perfect world where it didn't cost anything to have the part put
> together atom-by-atom, would a high-performance BJT be, in general,
> symmetric?
No need to be so precise, they're perfectly possible -- just not often
seen. Check out the datasheet for 2SC2878. It exists, I pulled a few
from junk before.
Lots of old germanium transistors measure with identical breakdown
voltages (typically 30V), probably constructed symmetrically (probably
partly because they didn't know or couldn't make better at the time).
I have a TO-3 power germanium that's good for about 10A either way...
Tim