Noise of field-effect transistors at very high frequencies

The minimum noise factor of a field-effect transistor has been computed at high frequencies on the basis of the thermal noise of the real parts of the equivalent circuit. A treatment of the intrinsic FET is followed by a consideration of the influence of feedback, parasitic output impedance and parasitic impedance in series with the source on the noise factor. Moreover, the difference between common-gate and common-source configuration has been considered. For frequencies smaller than the gain-bandwidth product f
gb
the factor
F_{\min} - 1
varies linearly with the frequency, whereas at higher frequencies this factor varies with f
²
. The computed results are compared with measurements on both JFETs and MOSFETs in the frequency range 100-1500 MHz at different conditions of operation. The agreement is rather good. For the JFET the value of
F_{\min}(f_{gb}) \approx 2.5
; for the MOSFET somewhat higher values are found due to the presence of substrate depletion effects.