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Degradation of AlGaAs/GaAs and InGaP/GaAs heterojunction bipolar transistors under high current stress

The effect of current stressing on the degradation of the current - voltage characteristics of carbon-doped base AlGaAs/GaAs and InGaP/GaAs heterojunction bipolar transistors (HBTs) is reported here. The devices tested comprised the following: compositionally graded and abrupt emitter/base junction AlGaAs/GaAs HBTs and emitter edge thinned (EET) compositionally graded junction AlGaAs/GaAs and InGaP/GaAs HBTs. Results for the EET and InGaP/GaAs devices are presented here for the first time. It is shown experimentally that after current stressing the base current of graded junction AlGaAs/GaAs HBTs increases appreciably whilst its collector current shows little change. The lower potential faced by the carriers of the graded junctions increases the recombination at the surface and stressing increases the heterointerface trap density, which accounts for the device deterioration. Eventually these devices exhibit a current gain degradation, as well as an increased turn-on voltage. The superior stability of the abrupt junction AlGaAs/GaAs and InGaP/GaAs HBT to current stressing is demonstrated over periods of more than 50 h; no notable changes in the base and collector currents were observed. Nevertheless, the InGaP/GaAs system is preferable to an abrupt AlGaAs/GaAs system due to its lower value of turn-on voltage and reduced surface trap density.