Question

To calculate the cutoff frequency of an ideal MOSFET with a constant mobility. Assume that the electron mobility in an n-channel device is mu(n) = 400cm2/V-s and that the channel length is L = 4 micro m. Also assume that Vt = I V and let Vcs = 3 V.

Answer 1

For an  n-channel MOSFET, for example:

$f_T = cutoff\ frequency=\frac{\mu_n\ (V_{GS}-V_T)}{2 \pi L^2}\\=\frac{400 * (3 - 1)}{2 \pi (4*10^{-6})^2}\\\\. \ \ =25/\pi *10^{12} Hz$

where    μ_n = electron mobility  : speed / electric field
          V_GS = voltage difference from Gate to Source
           V_T = Threshold voltage  for V_GS
             L =  channel length between source and drain.