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Schottky diodes with Ni-Ti-Pt-Au Schottky electrodes on AlGaN-GaN heterostructures were fabricated and subjected to rapid thermal annealing. The electrical influence on them was investigated in terms of the existence of a thin Ni or Ti layer. The diodes of the Ni-Pt-Au system showed a drastic improvement in their electrical properties, such as an increase in the Schottky barrier height and a decrease in the leakage current, after the 600°C treatment whereas the thermal annealing effect was found to be small in the Ti-Pt-Au and the Pt-Au systems. The Ni was considered to play a significant role in realizing a clean Pt contact to AlGaN and reducing surface traps, which were revealed from Auger electron spectroscopy measurement and frequency-dependent capacitance-voltage measurement, respectively. The thermally-treated Ni-Pt-Au gate electrode was concluded to be practicable for realizing high performance HEMTs.