The effects of emitter doping and width; base doping, thickness and width; collector doping, thickness and width and finally emitter-to-base metal spacing on the rf performance of GaN-based npn bipolar junction transistors were investigated using a drift-diffusion transport model. Cut-off frequencies (f(T)) of >150 GHz at 300 K were predicted for emitter dimensions of 2 and 0.5 mum design rule between the emitter and base metal. The do current gain is >50 under these conditions. The f(T) values were a strong function of base thickness and doping. (C) 2003 Elsevier Science Ltd. All rights reserved.