Abstract High voltage 12 V GaAs-based pHEMT devices are a commercial work horse for higher frequency infrastructure applications, including cable television, cellular base stations, and, potentially, WiMAX™ (worldwide interoperability for microwave access). For the device described here a self-aligned field plate was integrated into Freescale’s production 12 V pHEMT process to achieve high breakdown (>30 V) and high gain for 3.55 GHz operation. The reliability of this 12 V field plate pHEMT device and a standard 12 V pHEMT device were evaluated using both a conventional three temperature DC accelerated stress test and a series of temperature step stress tests. Also, the potential effect of current acceleration in the field plate device was measured. For the targeted infrastructure applications the 12 V field plate pHEMT device exceeds the reliability target at T CHANNEL = 150 °C of 20 years of operation at a 1 ppm failure rate by a significant margin.