The effect of thermal fluctuation of two-dimensional electron gases (2DEGs) on the leakage in AlGaN/GaN heterostructure-based high-electron-mobility transistors was investigated. Results indicate that the gate leakage current in AlGaN/GaN heterojunction was enhanced by the increase of the 2DEG density at low voltages. The gate leakage current was also associated with bias voltages, in which it increases with bias voltage to a saturation value because of the limit in the increase of 2DEG density. The results for the gate leakage current at room temperature are in order of 10(-2) to 10(-1) A/cm(2) at low voltages. It is shown that the thermal fluctuation mechanism is one of the gate leakage currents in AlGaN/GaN heterostructure. (C) 2016 Elsevier B.V. All rights reserved.