Recent progress in GaN based high electron mobility transistors (HEMTs) has revealed them to be strong candidates for future high power devices with high frequency operation. In order to extract and utilize the favorable GaN material properties, however, there is still a lot to be investigated. Reduction of the gate leakage current is one of the key issues to be solved for their further improvement. A high work function metal such as Pt, Ir, Pd or Mo was inserted to the conventional Ni/Au Schottky contact to n-GaN and AlGaN/GaN epilayers, and the Schottky diodes were studied in detail with respect to the thermal annealing in nitrogen ambient. The electrical characteristics were found to be changed by the thermal treatment in each device. A drastic improvement was attained in the Ni/Pt(Ir)/Au system whereas degradation occurred in Ni/Mo/Au by RTA at 500 degreesC for 5 min. These phenomena were confirmed to be dependent on the work function of each inserted metal. The role of Ni in the Ni/Pt/Au system was also investigated, and it was found to be essential in obtaining better electrical performance in comparison with the diodes without Ni, such as Pt/Au or Ir/Au Schottky electrodes. The AlGaN/GaN HEMTs were fabricated using Ni/Pt/Au gate contacts. Reduction of the gate leakage current by as much as four orders of magnitude was successfully recorded by thermal annealing without degrading the transconductance of the transistor, and it was concluded that this technique was promising for high power AlGaN/GaN HEMT electronics. (C) 2003 Elsevier Ltd. All rights reserved.