Charge collection in GaAs MESFETs fabricated in semi-insulating substrates

Charge-collection in GaAs MESFETs fabricated in semi-insulating substrates is investigated. Current transients are measured at short times (~few picoseconds) after either an alpha-particle strike or a laser pulse. In addition, the total charge is obtained by integrating the collected current. Measurements show the existence of three mechanisms for charge collection: (1) the drift of holes and electrons to the gate and drain electrodes, respectively, (2) bipolar-gain, and (3) channel-modulation. The charge collected by drift of holes or electrons gives rise to an instrument limited response (within 20 ps) after a laser pulse. The bipolar-gain mechanism peaks in approximately ~200 ps and is responsible for most of the collected charge. The channel-modulation mechanism is responsible for charge collection at longer times. These results are different than previous results for MESFETs fabricated on top of a buried p-layer, where most of the charge was found to be collected by the channel-modulation mechanism. Our results indicate that in order to harden GaAs transistors to single event upset, one must use techniques that reduce the effects of the bipolar-gain and channel-modulation mechanisms