Semi-on DC stress experiments were conducted on AlGaN/GaN high electron mobility transistors(HEMTs) to find the degradation mechanisms during stress. A positive shift in threshold voltage(VT/ and an increase in drain series resistance(RD/ were observed after semi-on DC stress on the tested HEMTs. It was found that there exists a close correlation between the degree of drain current degradation and the variation in VT and RD.Our analysis shows that the variation in VT is the main factor leading to the degradation of saturation drain current(IDS/, while the increase in RD results in the initial degradation of IDS in linear region in the initial several hours stress time and then the degradation of VT plays more important role. Based on brief analysis, the electron trapping effect induced by gate leakage and the hot electron effect are ascribed to the degradation of drain current during semi-on DC stress. We suggest that electrons in the gate current captured by the traps in the AlGaN layer under the gate metal result in the positive shift in VT and the trapping effect in the gate–drain access region induced by the hot electron effect accounts for the increase in RD. Semi-on DC stress experiments were conducted on AlGaN / GaN high electron mobility transistors (HEMTs) to find the degradation mechanisms during stress. A positive shift in threshold voltage (VT / and an increase in drain series resistance (RD / were observed after semi It was found that there exists a close correlation between the degree of drain current degradation and the variation in VT and RD. Our analysis shows that the variation in VT is the main factor leading to the degradation of The saturation drain current (IDS /, while the increase in RD results in the initial degradation of IDS in linear region in the first few hours stress time and then the degradation of VT plays more important role. Based on brief analysis, the electron trapping effect induced by gate leakage and the hot electron effect are ascribed to the degradation of drain current during semi-on DC stress. We suggest that electrons in the gate current captured by the traps in the AlGaN lay er under the gate metal result in the positive shift in VT and the trapping effect in the gate-drain access region induced by the hot electron effect accounts for the increase in RD.