Step-stress experiments on high-voltage Npn InGaP/GaAs HBTs are shown to reveal a number of degradation mechanisms, singly or in combinations: defect buildup in the emitter depletion region, defect buildup in the neutral base region, possible degradation of Ohmic contacts or increase in epitaxial layer resistances. Defect buildup in the emitter depletion region often precedes other types of degradation. Two less commonly reported degradation mechanisms are also suggested: base Ohmic metal punch-through to the collector and deterioration of the die attach material. It was found that the vast majority of devices failed within 5% of a maximum attainable power level, but when stressed just shy of this catastrophic level, electrical characteristics typically degraded gradually.