Electron irradiation effects in polyimide passivated InP/InGaAs single heterojunction dipolar transistors

In this paper, we report the effects of high-energy electron irradiation on the DC characteristics of polyimide passivated InP/InGaAs single heterojunction bipolar transistors. In contrast with the results of electron irradiation of unpassivated devices, the polyimide-passivated devices show much less degradation of current gain and no change in the collector output conductance. The decrease of collector current in the active regime is found to be typically similar to 9 percent for a cumulative equivalent 1-MeV dose of 2.7x10(16) e/cm(2) (similar to 620 Mrad (InGaAs)) For low base currents, the devices show an increase in the current gain for smaller doses (2.5x10(15) e/cm2) followed by a decrease at the higher doses. The increase in the current gain at low doses is attributed to the trapped charge in the polyimide layer near the periphery of the B-E junction. The most significant effect of electron irradiation on the passivated devices is a decrease in the slope of the I-C-V-CE characteristics of some devices in the saturation regime. We believe this decrease in slope is caused by an increase in the collector series resistance after irradiation. Finally, devices with smaller emitter size are shown to have less radiation degradation than the larger emitter devices. This is explained by the smaller radiation damage at the junction peripheries of the passivated devices.