In this paper, we investigated ON-state electroluminescence (EL) characteristics in fresh GaN-based HEMTs under different ON-state bias conditions. It demonstrated that: 1) the intensity of the ON-state EL emitted by the GaN-based HEMTs with relatively high VDS (V-DS > 10 V) monotonically increased as the gate voltage increased; and 2) the distribution of the intensity of the ON-state EL was uniform along the gate and across the device before stress tests. The degradation process and mechanisms of the devices in the high temperature operation (HTO) stress were also analyzed based on the ON-state EL characterization, the electrical measurement, and failure analysis. It demonstrated the following. First, the ON-state EL distribution of the device after step HTO (S-HTO) stress tests was no longer uniform along one of the fingers, which indicated that the degradation was related to this finger. This hypothesis was confirmed by the SEM observation after the deprocess of the gate metal and the SiN passivation layer. Second, a new degradation mechanism of AlGaN/GaN HEMTs that caused the abrupt degradation in the S-HTO stress was identified and related to the emergent, localized, and jagged crack under the gate metal. Third, there was no abrupt degradation in the long-term HTO (LT-HTO) stress test. However, the ON-state EL intensity of the inner fingers in the device after the LT-HTO stress test decreased more than that of the outer fingers, which meant that the degradation of the inner fingers was faster than that of the outer fingers as a result of the higher temperature caused by self-heating in the inner region of the device. Finally, the gradual degradation in the LT-HTO stress test was ascribed to the gradual formation of structural damage along the drain side of the whole gate edge, which was confirmed by the SEM observation.
