Hi Riad, thanks for your kind reply!
As a matter of fact, my interest in obtaining these capacitances
directly from the simulator is that I want to compare those values
with the ones I get from my hand calculations, using the theoretical
expressions. Indeed I was using the theoretical expressions for
estimating the parasitic capacitances, but the results I get seem to
be very different from the ones calculated by the simulator, because
the predicted behavior of the circuit is not too much in agreement
with the simulated response. Thus it would be really helpful if I
could see what are the parasitic capacitances calculated by Spectre
and compare them with my theoretical calculations to see were my
mistakes are...
Furthermore, I am a little confused about how to correctly apply some
of the expressions found in books. For instance, Allen's book says the
gate-source overlap capacitance is
Cgs(overlap) = Cox*Ld*Weff
where Cox is the oxide capacitance, Ld is the overlap between gate and
source, and Weff is the effective channel width. When I try to use
this expression, I find the following issues...
1. Cox is not found in the (BSIM3v3) model file. I only have dox
(oxide thickness), which along with the SiO2 permitivity I use to
calculate Cox=eox/dox. The parameter Ld is also not found in the model
file. I only have the Lint parameter, and since the BSIM manual says
Leff=Ldrawn-2Ld, I suspect they are equivalent. However, in my model
file this parameter... is negative!!!!! I was told (see
groups.google.com/group/comp.cad.cadence/browse_thread/thread/
35bbed8ae25e846a/a7ed4316a8f3c027) that this is due to model fitting,
but unfortunately this doesn't resolve the problem of how to calculate
Cgs(overlap) using the theoretical expression.
2. The book also says, if you have spice models you can forget about
the previous expression and just use Cgs(overlap) = CGDS0*Weff. Now
CGDS0 does appear in the model file, but then I get different values
when using the two expressions! What value is "more correct"? I would
like to know by "seeing" what is Spectre calculating...
To summarize, I think having the possibility to see what are the
parasitic capacitance used by the simulator is the only way in which I
could spot my hand-calculation mistakes and, most im****tantly, get the
required insight about what approximations should/can be done when
estimating the values of those parasitic elements.
It would be really interesting to hear how other designers estimate
the parasitic elements when performing their hand calculations.
Unfortunately I am currently in the academia and not in the industry,
and there are no practical designers around here at the moment :(
....Any further advice about this issue is welcome. Thanks again for
any help!
Cheers,
Jorge.
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