A gallium nitride (GaN) based Metal-Oxide-Semiconductor (MOS) capacitor was fabricated using radio frequency (RF)-sputtered tantalum oxide (Ta2O5) as the high-k gate dielectric. Electrical characteristics of this capacitor were evaluated via capacitance-voltage (C-V), current-voltage (I-V), and interface trap density (D-it) measurements with emphasis on the substrate temperature dependence ranging from 25 degrees C to 200 degrees C. Charge trapping and conduction mechanism in Ta2O5 were investigated. The experimental results suggested that higher substrate temperature rendered higher oxide capacitance, reduced gate leakage current, and lowered mid-gap interface trap density at the expenses of high border traps and high fixed oxide charges. The gate leakage current through Ta2O5 was found to obey the Ohm's conduction at lower gate bias and the Poole-Frenkel conduction at higher gate bias. (C) 2013 Elsevier B. V. All rights reserved.