The effects of partial ionization of Mg acceptors in the emitter and collector of pup GaN/InGaN heterojunction bipolar transistors were investigated by simulations with a drift-diffusion model. The contact resistances are all significantly decreased at high temperatures where ionization efficiency is maximized. The cut-off frequency f(T), is predicted to be >45 GHz at 700 K at high V-CE. (C) 2003 Published by Elsevier Ltd.