High power bipolar transistors often have multiple emitters, to achieve high currents, and efficient use of the whole emitter area. The emitters experience high current densities and are self-heated above the ambient temperature, leading to concerns about thermal run-away and damage to the device. Here we use a multi-emitter SiGe HBT, with multiple emitter contacts, to examine the temperature distribution in the emitters in such devices. We have measured the temperature increase in different emitters by biasing one emitter at a time and using the other base-emitter junctions as thermometers. We show that use of a selectively implanted collector does not alter the temperature increase or thermal coupling between the emitters. Crown Copyright (C) 2004 Published by Elsevier Ltd. All right's reserved.