The objective of the proposed research is to investigate and model the circuit-relevant degradation of silicon germanium HBTs under extreme operation. Investigation will be performed of transistors at cryogenic temperatures below 25 K to investigate the acceleration of mixed-mode (MM) damage as carrier mean free path increases. Another investigation will be performed of Auger damage (high current stress and saturation mode damage) at elevated temperatures, to investigate potential increased degradation up to +300 C. RF breakdown, believed to be thermally driven, will also be investigated as a function of temperature in the same range. An in depth understanding and modeling of reliability mechanisms from temperatures ranging from 25 to 600 K is valuable for several emerging areas, including quantum computing, space exploration, industry, automotive, and defense applications, and will inform device and circuit design for these applications.