aaron_do wrote on Feb 6th, 2007, 10:53pm:i'm trying to calculate the gate drain capacitance in the subthreshold region for a nmos. I'm using the BSIM3.2 model, and i figured for a good approximation I only need the Gate Drain overlap capacitance. This was mentioned in the book by T.H. Lee. Is this correct?
In subthreshold, Qd for the channel should be zero, so yes, that's a good approximation.
Quote:Ok part 2. In the manual it says Cij=dQi/dVj. i.e. Cgd=dQd/dVg and Cgd is reciprocal. However, the equation given is Qd versus Vgd. Can I say Cgd=dQd/dVgd?
You mixed up your formula; Cgd = dQg/dVd. Cgd for the channel is not reciprocal. Is Cgdov only a function of Vgd? Is the equation for Qd for the channel charge or only the overlap charge?