SiGe alloys have been successfully applied to a number of semiconductor devices, including bipolar heterojunction transistors, field effect transistors (FET's), tunneling and optoelectronic devices and structures. This review paper will first summarize the results obtained to-date in bipolar transistors, highlighting the design flexibility and the trade-offs offered by SiGe heterojunction technology and bandgap engineering, like junction field/capacitance control, liquid nitrogen operation and complementary processes. The leverage of this technology in high speed circuits will be discussed, including the record 75 GHz fT and 53 GHz fmax heterojunction bipolar transistors, and the achievement of sub-25 ps ECL ring oscillator delay. The applications of this technology to field effect transistors, to increase the channel mobility, to resonant tunneling structures, and to detectors and waveguides, to extend the use of silicon technology in optoelectronics are also reviewed. © 1992.