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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.

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