Inadequacy of the classical theory of the MOS transistor operating in weak inversion

The most important characteristics of the MOS transistor operating in weak inversion are discussed. When the drain voltage is greater than a few
kT/q
it is demonstrated that the drain current can be written as the product of the geometrical factor
W/L
, the minority carrier diffusion constant, and the inversion charge at the source. In the classical theory, the slope of the In I
D
versus V
G
curve is only influenced by the capture of minority carriers by surface states. It is demonstrated that the N
ss
values obtained from these current measurements are in disagreement with the values found by independent surface states measuring techniques.