MOS operating point capacitance parameters...
R. Wilcock
in a circuit.
I do a dc simulation using analog artist and spectre, and save the dc
operating point then go to results->print->dc operating points and
click on the relevant transistor in the circuit. The results window
pops up and lists a whole load of parameters including about 19
capacitances... Which one do it need? I know there are actually
hundreds but for simple calculations i just need the usual Cgs, Cdiff
and perhaps overlap capacitances... So which of the following mean
what?
cbb, cbd, cbg, cbs, cdb, cdd, cdg, cds, cgb, cgd, cgg, cgs, cjd, cjs,
csb, csd, csg, css
Oh and why are a number of them negative?
Many Thanks
Reuben
steve
wrote:
>I need to find the total gate capacitance of various MOS transistors
>in a circuit.
You want to look at cgg, most likely.
<snip>
> So which of the following mean what?
>cbb, cbd, cbg, cbs, cdb, cdd, cdg, cds, cgb, cgd, cgg, cgs, cjd, cjs,
>csb, csd, csg, css
Depends on how they're defined. You didn't mention which model these
are from, but... cxy is typically dQx/dVy, although sometimes they're
defined with different references. And this can vary from model to
model, depending on how the person who wrote it felt that day.
Basically, four terminal charges, but charge neutrality means three
are unique; 16 partial capacitances, but only nine are unique. etc...
The cj's are the drain/source junction capacitances.
>Oh and why are a number of them negative?
This can again be an artifact of how they're defined, or just bad
physics. Or both. Go ahead and ask an AE-- the answer should be
amusing -- unless they have source code and a lot of time, they
probably won't know either :)
--Steve
Erik Wanta
There should be 16 transcapacitances in total, dQi/dVj where i,j can be
S, G, D or B. However, if you write these transcapacitances in matrix
form. The sum over i of dQi/dVj is zero since there's no change in the
total charge in a device. Also, the sum of j of dQi/dVj is zero since the
capacitance depends only on voltage difference (if all dVj change by ,
says, delta V, dQi should be zero too). Therefore, by knowing 9 of 16
transcapacitances, you can always figure out the rest.
Most of the time, you need to be concerned with Cgs, Cgd, Cjs, Cjd.
Sometimes, Csg or Cdg is also of interest. Anyhow, this transcapacitance
model is kindof messy for hand calculation and it makes more sense to
think of Cij = Cji for hand calculation.
reuben...@hotmail.com (R. Wilcock) wrote in message news:<3224903c.0210...@posting.google.com>...
Andrew Beckett
Here's a solution we have on sourcelink that a colleague wrote based
on a note I'd put together (a few years back now). When this question
first came up I had indeed checked the source code (and then got
it confirmed by R&D just to make sure). Anyway, hope you
find this useful: (Note, the references to 4.4.1 apply to any version
of spectre later than that).
Description -
-----------
You are simulating a circuit using the bsim3v3 model in spectre.
You look at the operating point information and notice that some
of your capacitances such as cgd, cgs, etc are negative. Why?
Resolution -
----------
The values you are seeing are correct because the values of these
parameters are the partial derivatives of the terminal charges with
respect to the terminal voltages, so they can be negative. They are NOT
physical two terminal capacitors. See the UCB bsim3v3 documentation,
equations 4.3.28 - 4.3.31.
An earlier spectre implementation only gave the values of three
capacitors which were closer to the intuitive value of what the
capacitance should be, but many customers asked for the complete set of
partial derivatives, so that is what spectre currently provides.
Here is a summary of the differences in the capacitance operating point
figures between 4.4 and 4.4.1 BSIM3V3 models in spectre.
---------------------------------------------------------------
In the following section Cxx refers to the parameter calculated by
the bsim evaluator inside the code. The lower case cxx refers to the
value reported by the operating point.
In 4.4:
cgs = -Csg + pModel->OverlapCgs * pInst->Width * pInst->MFactor
cgd = -Cdg + pModel->OverlapCgd * pInst->Width * pInst->MFactor
cgb = -Cbg + pModel->OverlapCgb * pInst->Length * pInst->MFactor
cbd = Cjd
cbs = Cjs
In 4.4.1:
Cgsovl = pInst->pSDModel->OverlapCgs * pInst->MFactor;
Cgdovl = pInst->pSDModel->OverlapCgd * pInst->MFactor;
Cgbovl = pInst->pSDModel->OverlapCgb * pInst->MFactor;
cgs = Cgs - Cgsovl
csg = Csg - Cgsovl
cgd = Cgd - Cgdovl
cdg = Cdg - Cgdovl
cbd = Cbd
cbs = Cbs
cjd = Cjd
cjs = Cjs
Here's a mapping between the parameter name in 4.4, and what it
corresponds to in 4.4.1:
4.4 4.4.1
------------
cgs = -csg
cgd = -cdg
cgb = -cbd
cbd = cjd
cbs = cjs
The parameter names are those reported by the operating point in the two
versions.
Regards,
Andrew.
--
Andrew Beckett
Senior Technical Leader
Custom IC Solutions
Cadence Design Systems Ltd
Vladimir
If you want just estimate total gate capacitance use AC analysis and
calculator functions. Plot deriv((IF/VF)/twopi), where IF is AC current
of gate terminal and VF is AC voltage of gate terminal.
To make sure it works you can verify this way with simple capacitor first.
Best regards,
Vladimir Fomin
> Reuben