Dear Jorge
I do concur with St=E9phane. It is too ambitious trying simulator
parameters for hand calculation. a spectre -h command shows that
BSIM3V3 makes use of 358 model parameters against 864 for BSIM4.
besides, unlike surface potential models, BSIM is less physical based
and full of curve fitting equations. It's not the best model suitable
for hand made calculations.
As other chaps mentioned earlier, the MOS G/D/S capacitance are
dependent of the voltage at the given node. For more details, Please
give a look at the following slides:
http://bwrc.eecs.berkeley.edu/IcBook/Slides/chapter3.ppt
As explained in the SR you've mentioned above, The gate capacitance
computed by BSIM are not physical. Cgs and Cgd are not two-terminal
capacitance. Drain voltage has actually an effect on Cgs, even though
the drain is not directly connected to that "capacitor". That's the
way BSIM works and it has nothing to do with Spectre. BSIM computes
the MOS capacitance using a charge-based approach and this needs the
charge equations for all four terminals.
Another difficulty to compute the D/S capacitances based on the Qd/
Qs charges is the fact that only the total channel charge QINV =3DQD +QS
is really known. BSIM uses a partition factor (XPART) to compute both
Qd/Qs. BSIM has 3 partition schemes: 0/100, 50/50 and 40/60. Only the
latter is physical-based. Please refer to the BSIM manual for more
information aboy this.
My advice is to keep your equations very simple for hand calculation.
BSIM will rather give you some head-eachs ;-)
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