High temperature reverse bias (HTRB) stress effects on static and dynamic characteristics of 0.15 µm AlGaN/GaN high-electron mobility transistors (HEMTs) are reported. The HEMTs were stressed to off-state bias (VGS = -7 V, VDS = 30 V) at a high temperature of 175 °C for 1000 hours of duration. The HTRB induced changes in drain current (IDS) are analyzed. The IG-VG characteristics are evaluated at VDS = 0 V (emulating gate Schottky diode leakage) before and after the stress to examine the Schottky gate diode properties upon aging. The gate leakage current variations are further inspected with VG and VD sweeps. After aging test, a considerable drift in the output power (Pout) is observed during the interim RF measurement of the stressed HEMTs. The pulsed IDS-VDS characteristics reveal the reduction in IDS, particularly in the transition between linear and saturation regions; which is referred as knee-voltage smoothing. The output-admittance (Y22) and drain current transient (DCT) measurements are conducted to track any new defects created in the stressed HEMT and also assess the evolution of the preexisting trap parameters. Especially, physical location of the traps responsible for the knee-voltage smoothing and output power drift is identified by TCAD simulations.