Heterojunction bipolar transistors (HBTs) are key devices for a variety of applications including L-band power amplifiers, high speed A/D converters, broadband amplifiers, laser drivers, and low phase noise oscillators. AlGaAs emitter HBTs have demonstrated sufficient reliability for L-band mobile phone applications. For applications which require extended reliability performance at high junction temperatures (>250) and large current densities (>50 kA/cm2), InGaP emitter HBTs are the preferred devices. The excellent reliability of InGaP/GaAs HBTs has been confirmed at various laboratories. At a moderate current density and junction temperature, Jc = 25 kA/cm2 and Tj = 264, no device failures were reported out to 10,000 hours in a sample of 10 devices. Reliability testing performed up to a junction temperature of 360 and at a higher current density (Jc = 60 kA/cm2) showed an extrapolated MTTF of 5 105 hours at Tj = 150. The activation energy for AlGaAs/GaAs HBTs was 0.57 eV, while the activation energy for InGaP/GaAs HBTs was 0.68 eV, which indicated a similar failure mechanism for both devices.