A discrepancy in the elementary theory of MOSFET modeling

Abstract

The elementary theories of flicker noise in MOSFET’s operating at low drain bias evaluate the spectrum S_Veq8f) of the equivalent gate noise emf δV_eq under the assumptions of an effective mobility μ_eff that is independent of the gate voltage V_g and of a carrier number in the channel that varies linearly with the gate voltage. We show here that this introduces an error I²_d/[g²_m(V_g—V_T)²] in the calculated results. Experiments show that this error can be significant, so that it makes some existing interpretations of S_Veq(f) doubtful. The error comes about because at a given drain current I_d the transconductance g_m for large V_g−V_T is smaller than the elementary theory predicts. This affects most MOSFET modeling at low drain bias.