AlGaN/GaN metal-insulator-semiconductor heterostructure field-effect transistors (MIS-HFETs) with a HfO2/Al2O3 bilayer gate dielectric have been fabricated, characterized, and compared with HfO2 gate dielectric MIS-HFETs. Physical and electrical characterizations have revealed the enhanced properties of the HfO2/Al2O3 bilayer dielectric over that of the single HfO2 layer. As a result, the fabricated HfO2/Al2O3 MIS-HFETs exhibit better electrical performance and thermal stability than the HfO2 transistors. The maximum drain current of the HfO2/Al2O3 MIS-HFETs has increased by similar to 8.5%, while the off-state drain current has reduced by nearly 1 order of magnitude than that of the HfO2 MIS-HFETs. After the thermal stress at elevated temperatures (400 and 500 degrees C) for a prolonged duration up to 500 min, the irreversible device performance degradation, including drain current, peak transconductance, threshold voltage, and gate leakage current for the HfO2/Al2O3 MIS-HFETs, is substantially less than that for the HfO2 MIS-HFETs. A longer lifetime of similar to 4x10(6) h at 150 degrees C has also been estimated for the former, compared to that of the latter of similar to 2x10(5) h.