Improved InP-based double heterojunction bipolar transistors

This article develops two double heterostructure bipolar transistors (DHBTs) made by low-pressure metamorphic organic chemical vapor deposition (LP-MOCVD). One is a control DHBT with an abrupt base-collector heterojunction (abrupt DHBT) and the other is an improved DHBT with a composite-collector design (composite DHBT). Of the devices with no ledge passivation, the composite DHBT has a high common-emitter current gain, a low offset voltage and voltage-independent collector current characteristics, unlike the abrupt DHBT. The favorable performance of the composite DHBT follows mainly from the optimal collector design, which effectively lowers the potential spike at the base-collector heterojunction. Furthermore, measurements of the composite DHBT with and without n-InP emitter ledges are made. The composite DHBT with emitter ledges has a current gain of 180, which is 44% higher than that of the device with no emitter ledge. The higher current gain is attributable to the lower base surface recombination current in the composite DHBT with a ledge. The effectiveness of the composite-collector design and the emitter ledge process of the presented DHBT is proven. (c) 2007 WILEY-VCH Verlag GmbH & Co. KGaA. Weinheim.