A discrepancy in the elementary theory of MOSFET modeling

The elementary theories of flicker noise in MOSFET’s operating at low drain bias evaluate the spectrum SVeq8f) of the equivalent gate noise emf δVeq under the assumptions of an effective mobility μeff that is independent of the gate voltage Vg and of a carrier number in the channel that varies linearly with the gate voltage. We show here that this introduces an error I2d/[g2m(Vg—VT)2] in the calculated results. Experiments show that this error can be significant, so that it makes some existing interpretations of SVeq(f) doubtful. The error comes about because at a given drain current Id the transconductance gm for large Vg−VT is smaller than the elementary theory predicts. This affects most MOSFET modeling at low drain bias.