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 1.5V,V_{gs} \approx 0), we find for the thermal drain noise currenti\min{d}\max{2} \approx 1 - 2 \times 10^{-22}A2/Hz with a noise coefficientP \approx 0.1in 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.