The effects of operating bias conditions on the proton tolerance of Silicon-Germanium (SiGe) heterojunction bipolar transistors (HBTs) are investigated for the first time. While this SiGe HBT technology is relatively insensitive to different bias conditions during radiation exposure, the cases with all terminals grounded and with the emitter base junction reverse-biased show slightly enhanced degradation at proton fluences above 2 x 10(13) p/cm(2) compared to the other bias configurations, consistent with 2D simulations, and can thus be viewed as worst case conditions. (C) 2003 Elsevier Ltd. All rights reserved.