Different base layer designs for npn GaN-based heterojunction bipolar transistors (HBTs) were simulated using a drift-diffusion transport model. The use of AlGaN/GaN superlattice bases did not produce working devices for any of the conditions investigated. InGaN/GaN base layers did produce acceptable dc current gains by enhancing electron transport. Grading of the InGaN layer composition produced the highest gains. The effects of both emitter and collector doping on the dc performance of the HBTs was also investigated. (C) 2003 Published by Elsevier Science Ltd.