This paper describes the high temperature reverse bias (HTRB) stress effects on 0.15 μm AlGaN/GaN high-electron mobility transistors (HEMTs) with Fe-doped buffer. The HEMTs were stressed to OFF-state bias (VGS = -7 V, VDS = 25 V) at 175°C for 1000 hours of duration. The subsequent degradation in the drain current is analyzed. The changes in IG-VG at VDS= 0 V (emulating gate Schottky diode leakage) are investigated before and after the stress to inspect the Schottky gate interface quality. The gate leakage current evolutions are further examined with the VG and VD sweeps. After ageing test, the deviation in the emission rate of Fe-related trap at Ec - 0.5 eV (located in buffer) is quantified by means of low-frequency (LF) output-admittance (Y22) and drain current transient (DCT) spectroscopy. Moreover, the drift in the output power (Pout) during first interim measurement of stressed HEMT is evaluated, with the supporting TCAD simulations. These preliminary HTRB test results are useful to understand the fundamental failure mechanisms of the HEMT due to the ageing in actual operating conditions.