Non-Quasi-Static (NQS) Thermal Noise Modelling of the MOS Transistor
A non-quasi-static (NQS) thermal noise model of the MOS transistor is presented that is valid in all modes of operation, from weak to strong inversion, and up to frequencies which are near or above the NQS cut-off frequency. It is shown that in addition to the well known induced gate noise (IGN) there is also an induced substrate noise that is generated and that the source and drain noises are also affected. All prior publications on the subject only deal with IGN in the strong inversion regime. It is shown that significant differences are obtained for moderate and weak inversion operation. A brief review is given of the NQS model valid in all modes of operation. The general thermal noise model using four noisy current sources is described. The power spectral and cross-power spectral densities of these noise sources are computed. A first-order approximation is then derived and compared to the complete model. It is shown that the correlation coefficient between the drain and the gate noise is always null in triode (VD = VS), and varies in saturation between j 0.6 in weak inversion to j 0.4 in strong inversion. To the author's knowledge, it is the first time that a complete HF thermal noise model of the MOST is presented, that is valid in all modes of inversion and up to and above the NQS cut-off frequency. The impact of this complete NQS noise model on RF-CMOS circuit design is illustrated by two examples.