We investigated transfer characteristics of AlGaN/GaN high electron mobility transistors (HEMTs) and the effect of the barrier height of Schottky gate metals. It is found that the threshold voltage of the HEMT structures with the Ni Schottky contact shows a positive shift compared to that of the Ti Schottky contacts (Delta V-th = 2.9V). The maximum saturation current of the HEMT structures with the Ti Schottky contact (similar to 1.4 x 10(7) A/cm(2)) is found to be similar to 2.5 times higher than that of the Ni Schottky contact (2.9 x 10(7) A/cm(2)). The conduction mechanisms have been examined by comparing the experimental results with numerical simulations, which confirm that the increased barrier height is mainly attributed to the reduction of 2-DEG concentration. (C) 2014 Elsevier Ltd. All rights reserved.