Parasitic conduction current in the passivation ledge of AlGaAs/GaAs heterojunction bipolar transistors

Abstract

Passivating the extrinsic base surfaces of AlGaAs/GaAs heterojunction bipolar transistors with a thin AlGaAs emitter ledge is widely utilized to reduce the extrinsic base surface recombination current. The effect on current gain of this ledge being only partially depleted is analyzed. This analysis examines the ledge parasitic conduction current as a function of device geometry. Results indicate that even with an undepleted portion of the ledge as thin as 50 Å, the elimination of base contact recombination current requires a passivation ledge length 6 times longer than from devices with a fully-depleted passivation ledge. The effect of a base quasi-electric field on the current gain as a function of passivation ledge distance is also discussed. This theoretical analysis is compared with published experimental work.