Parasitic effects of surface states on GaAs MESFET characteristics at liquid-nitrogen temperature

An abrupt increase of drain current has been observed at low drain voltages for GaAs MESFETs operated at liquid-nitrogen temperature. The drain voltage (V/sub k/) at which the anomaly occurs shifts slightly toward smaller drain voltages with increasing gate voltage. Two types of I-V behavior exist for drain voltages less than V/sub k/. For type I behavior, the devices show transistor-like characteristics with smaller drain output current and transconductance than expected. I-V characteristics similar to breakdown in short-channel devices are observed for drain voltages near V/sub k/. For type II behavior, the devices show space-charge-limited current characteristics. In both cases, the drain current returns to the normal MESFET value when the drain voltage exceeds V/sub k/. The phenomena can be explained by parasitic effects of surface states in the separation regions between the gate and the heavily doped drain/source. >