The main issues this work addresses, with reference to commercial GaAs PHEMTs, are the temperature dependence of the off-state breakdown voltage (BVDG), the physical mechanisms that determine it, and the relationship between BVDG and device degradation and failure, as determined by accelerated step-stress performed at temperatures ranging from 25 to 100°C. BVDG is seen to decrease with temperature between room temperature and 160°C. Temperature-dependent analysis of the gate leakage current indicates that thermionic-field emission and thermionic emission over a field-dependent barrier are the limiting mechanisms for off-state breakdown. Room-temperature, hot carrier step-stress tests with 24 h step duration show reduced IDSS after the stress and a tight correlation between the BVDG measured at IG = - 1 mA/mm and the stress bias producing substantial device degradation or catastrophic failure. Shorter (2 h) step stress experiments carried out between 25 and 100°C again show a tight correlation between the BVDG measured at IG = - 1 mA/mm and the stress bias producing dramatic degradation or failure. This correlation, coupled with the negligible temperature dependence of the breakdown voltage in this temperature range, results in temperature-independent device degradation.