NMOS as a capacitor, capacitance calculations

Thread Starter


Joined Jun 10, 2020
I am using the nmos as a capacitor by shorting the drain, substrate and the source together in ltspice. After this I tried to calculate the Effective capacitance of the device via cut off frequency of the device. The cutoff frequency of a first order device is given as 1/2*pi*R*C.

So I used a couple of .measure statements to calculate Cutoff frequency and then the capacitance, which I got as -274.97dBHz(about 17.84femto farads).


Now my concern is that, the dc operating parameters I see in the above error log, does not seem to add up to my effective capacitance and I have no idea why gate to source voltage is zero in the above logs.

Also is the calculation correct? is there anyway to verify it using the model file parameters?

Thank you and any kind of help, suggestions or feedback is appreciated.



Joined Nov 6, 2012
Why are You trying to do this ?
Most simulation programs use a Mathematical-Model of a particular part,
that Model will not always act like the real-part, in the real-World.
And, a MOSFET Gate isn't a very desirable Capacitor for any application that I know of.