The influence of bias conditions upon the intrinsic noise parameters of short gate length GaAs MESFETs

For complete characterisation of the noise performance of FET type structures, such as MESFETs and HEMTs, accurate values for the device's intrinsic noise parameters are required. These parameters have been shown to be sensibly frequency independent below 30 GHz but are strongly bias dependent. The problem of accurate determination of the intrinsic noise parameters for GaAs devices is complicated by the carrier transport mechanisms that exist in such material. This is particularly true for sub-micron gate-length devices where channel field strengths are far in excess of the critical field required for velocity saturation of the carriers. As yet no analytic solution is available that can accurately predict the current dependency of the intrinsic noise parameters of such devices. Here results obtained from measured noise data are presented that show the evolution of these parameters with drain current for two practical devices.