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This paper presents a comparative study of the degradation of dc characteristics and drain current collapse under dc-bias stress in passivated metal-oxide-semiconductor high-electron mobility transistor (MOSHEMT), unpassivated HEMT, and passivated HEMT devices. The Al2O3 oxide thin film that is used as a gate dielectric and a passivation layer in MOSHEMTs is prepared by a simple, low-cost, and low-temperature liquid-phase deposition (LPD) technique. All devices are subjected to short-term dc-bias stress to investigate the reliability of the oxide and its passivation effect. In the case of MOSHEMTs and passivated HEMTs, the gradual reduction in drain current is found within 20-h drain-bias stress, which is apparently caused by the hot-electron injection and trapping in the buffer, and a barrier layer that is operated at a high drain voltage. However, faster degradation is found in unpassivated HEMTs, and some devices are permanently damaged due to the degradation of unpassivated surface states. Nonetheless, the current is partially recovered for all devices after gate stress, and no damage to the MOSHEMTs is observed. Therefore, it is believed that the Al2O3 thin film that is prepared through the LPD technique is effective as a gate dielectric and as a surface passivation layer in reducing device degradation during dc-bias stress and in diminishing the current collapse effect in MOSHEMTs.