Maryam

May I give a hint that you may use to solve the problem.

When you want to calculate or measure some physical parameter you must stick to its definition relation. Specifically here, you have

cgd= d QGD/dVGD

In small signal notation

cgd= qgd/ vgd

So accordingly:

You have to set certain DC operating point

apply a small signal voltage vgd in series with the DC bias,

Then measure or calculate qgd.By dividing qgd by vgd you get cgd.

You can also calculate the current igd

Then you you can get igd/vgd= jwcgd which is the susceptance of the cgd.

Best wishes

Prof Abdelhalim abdelnaby Zekry , thank you for your thorough response. I will investigate this case.

Best regards

you can use this command :

Solve name=gate vgate=0 vstep=** vfinal=** ac freq=1e6

you can also use "direct" at the end of the command line ,if you had a convergence problem.