This SPIE-PW paper by Handel and Morkoç examines new factors affecting HFET stability, 1/f noise and reliability. The research identifies previously unrecognized physical mechanisms that influence long-term stability and noise performance in heterostructure field-effect transistors, including piezoelectric effects, hot carrier degradation, and trap generation dynamics, providing comprehensive framework for understanding reliability physics and developing improved device designs and screening procedures for ensuring stable operation in RF power amplifiers and switches.