FET noise sources and their effects on amplifier performance at low frequencies

In this brief review paper analytical results concerning the low-frequency (LF) amplifier noise performance of FET's are presented. The effects of interaction between the device basic noise sources, the small-signal model parameters, and the signal source admittance parameters are clearly indicated. The noise performance is found to be essentially determined by the effective surface-state density and the gate insulator thickness product (
N_{ss}t_{ox}
) in the case of MOSFET's, whereas in the case of JFET's, this is determined by the bulk density of impurity and/or defect generation-recombination (g-r) centers within the depletion region and the half-channel height squared product (
N_{TT}a^{2}
). Although an increase in the gate electrode area can reduce the equivalent gate noise resistance, this does not improve the noise performance of the device. Quantitative results based on typical device parameters are graphically presented with proper indications as to the upper limit of the LF range, the excess minimum noise figure, and the frequency range within which the noise figure remains below 3 dB level for specified source resistance values. The effects of gate leakage current on the noise performance of JFET's are included in these results.