The observed initial gain degradation of AlGaAs/GaAs heterojunction bipolar transistors under current stress was investigated. The change in device characteristics is attributed to a dissociation of passivating hydrogen in the base layer during stress. The hydrogen passivation occurs during the implant isolation process. An activation energy of 0.75 eV was measured for the junction temperature dependence of the dissociation process. (C) 2002 Published by Elsevier Science Ltd.