Is Cgs= -(Cgg+Cgb+Cgd) or Cgs=-(Cgg+Cgb)?

[Alex Liao]

Hi,

I found these two equations in MOS Capacitance Model section from Virtuoso Simulator Components and Device Models Reference.

Triode: Cgs= -(Cgg+Cgb+Cgd)
Saturation: Cgs=-(Cgg+Cgb)

In triode region, the first equations works and the second one works for saturation region.

But in my Print DC table, I copied the values of Cgg, Cgb and Cgd. I find the Cgs always has a value very closer to the result got from triode region equation verification even I am sure it is working in saturation region.

Is there anybody who knows why?

Thanks,

 

[dick_freebird]

Cgg is the total gate charge, sum of Cgs, Cgd and Cgb.

In Spectre the OP values seem to make no consistent
sense, and I don't pay much attention to them, other
than as a rough guide.

 

[Alex Liao]

Cgg is the total gate charge, sum of Cgs, Cgd and Cgb.

In Spectre the OP values seem to make no consistent
sense, and I don't pay much attention to them, other
than as a rough guide.

Hi Dick_freebird,

Got it. Thanks.