Cgs and Cgd from the DC simulation

Hi, all:

I am trying to get the Cgs and Cgd of a transistor (the voltages of G
D S and Bulk are known.) I ran the DC simulation and printed the
transistor working point. I got Cgs, Csg, Cdg, Cgd, .... Some values
are even negative.

I am confused. Can you tell which one is the gate to source
capacitance?

Thanks.


Best Regards,

Xiaofeng

 

Xiaofeng:
There should be 16 transcapacitances in total, dQi/dVj where i,j can
be S, G, D or B. 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
delta V, dQi should be zero too). Therefore, by knowing 9 of 16
transcapacitances, you can always figure out the rest. This
transcapacitance model is kind of messy for hand calculation and it
makes more sense to think of Cij = Cji for hand calculation.