Critical discussion on the flat band perturbation technique for calculating low-frequency noise

The flat band perturbation technique [Solid State Electron., vol. 30, p. 1037, 1987] has been used extensively for the last 15 years to study and model low-frequency noise in a MOS transistor. The method has been "proven" to be valid even in nonohmic regions (with a nonzero drain-to-source voltage) [Solid State Electron., vol. 32, p. 563, 1989]. This brief will demonstrate that this methodology is valid only in an ohmic region (drain-to-source voltage tending to be zero) and establish its link with the Langevin method, the most fundamental noise calculation methodology