High gain AlGaAs/GaAs heterojunction bipolar transistors grown on SiGe/Si substrate have been fabricated. Measured peak dc current gain of similar to 100 is obtained for a device with emitter area of similar to 1.6 x 10(3) mu m(2), with base concentration of 1 x 10(19) cm(-3). The dominant base current component is discussed and determined. The breakdown characteristic is studied and compared with that of the device grown on GaAs substrate. Our experimental results demonstrate that SiGe/Si substrate could provide a robust method for monolithic integration of high speed GaAs-based electronic devices with silicon-based circuitry. (c) 2007 American Vacuum Society.