The effects of base doping and thickness on dc current gain, collector emitter saturation voltage, saturation current and collector-emitter breakdown voltage of GaN/AlGaN heterojunction bipolar transistors were investigated using a drift-diffusion transport model. Given the low ionization efficiency of Mg acceptors in the base, it is important to design structures that avoid depletion of the base layer. The presence of a resistive base causes current to flow directly to the collector, severely reducing gain. The effect of emitter doping on current gain and of collector doping on the breakdown voltage of the C-E junction were also investigated. (C) 2003 Elsevier Science Ltd. All rights reserved.