The Reduction of Backgating in GaAs MESFET's by Impact Ionization

The reduction of drain current due to reverse substrate bias in GaAs MESFET's fabricated on EL2-compensated substrates is recovered on the application of sufficient drain bias. This recovery is shown to be due to the compensation of the negative space charge at the channel-substrate interface by holes generated by impact ionization in the MESFET channel. Illumination raises the value of drain bias needed for current recovery due to the requirement of additional hole flux to offset the effects of optically generated electrons on EL2 occupancy. Simulation results show that the channel current becomes independent of substrate bias when the bias value is sufficient to completely delete the p-type surface layer.