The dc and power characteristics of AlGaN/GaN MOS-HEMTs with Sc2O3 gate dielectrics were compared with that of conventional metal-gate HEMTs fabricated oil the same material. The MOS-HEMT shows higher saturated drain-source current (similar to0.75 A/mm) and significantly better power-added efficiency (PAE, 27%) relative to the HEMT (similar to0.6 A/mm and similar to5%). The Sc2O3 also provides effective surface passivation, with higher drain Current, lower leakage Currents and higher three-terminal breakdown voltage in passivated devices relative to unpassivated devices. The PAE also increases (from similar to5% to 12%) on the surface passivated HEMTs, showing that Sc2O3 is an attractive option for reducing gate and surface leakage in AlGaN/GaN heterostructure transistors. (C) 2003 Elsevier Ltd. All rights reserved.