# How to use BSIM3v3 capacitance for hand calculation?

As you all probably know, capacitances in BSIM3v3 are just the partial derivatives of the terminal charges with respect to the terminal voltages. They are not physical two terminal capacitors.

Cij = dQi/dVj.

For a MOSFET, we have Cgs, Cgd, Csg, Cgg,...

Because they are not physical two plates capacitor, how do you convert these capacitances to physical value for hand calculation?

For example, Cgs as formula above is not the total equivalent capacitance between Gate and Source. How do you calculate total real Cgs? Also, how do you use these partial derivatives for hand calculation?

Use the method described in the link to do the hand calculation:

Approximate models for hand calculations

But do not expect very accurate results using hand calculations, especially for short channel lengths (L < 2u)