The reliability of AlGaN/GaN high electron mobility transistors (HEMTs) is traditionally determined via thermal lifetime acceleration stress tests. More recently it has been proposed that electric field has a prominent role in limiting lifetimes. Multiple failure mechanisms have been proposed as a result of device degradation observed when stressed under high applied electric fields, as typical when the device is biased into the OFF-state. One potential reason for multiple mechanisms could be due to varying levels of quality and maturity of the GaN processes in the reported literature.The work presented in this dissertation seeks to provide clarity and understanding into the failure mechanism of AlGaN/GaN HEMT devices under high electric fields. The devices in this study were fabricated in a commercial GaN process, notable for exceptional ruggedness and industry leading $65V$ qualified operational bias for RF power amplifiers. A series of OFF-state, high electric field step-stress experiments, as described in literature, were performed to assess if any were applicable to this process.It was discovered that device degradation could only be induced when stressed close to the breakdown limits. This lead to the development of a unique stress method that enables the device to be held close to catastrophic breakdown, while avoiding an over stress event that would prevent the device from being studied at the conclusion of the experiment. It was discovered via careful electrical and optical analysis that failure was due to a localized degradation of the Schottky gate diode properties. The physical analysis found the failure inconsistent with the widely reported inverse piezoelectric effect. Instead the failures resemble recently proposed time dependent dielectric breakdown of the AlGaN barrier layer.