Thermal noise measurements in GaAs MESFET's

The absolute magnitude of the thermal drain current fluctuations and the associated effective thermal noise coefficient of 1-µm gate-length MESFET's have been measured under various bias conditions. At low drain-source voltages the magnitude of current fluctuations are in good agreement with the thermal noise theory which is based on the gradual channel approximation. However, under normal operating conditions (
V_{ds} \geq 1.5
V,
V_{gs} \approx 0
), we find for the thermal drain noise current
i\min{d}\max{2} \approx 1 - 2 \times 10^{-22}
A
²
/Hz with a noise coefficient
P \approx 0.1
in disagreement with the commonly used, theoretically predicted value P = 1.1. Our results are qualitatively consistent with a more comprehensive FET noise theory which properly takes into account high-field effects.