I. Introduction
Gallium nitride (GaN)-based high electron mobility transistors (HEMTs) have emerged as promising candidates for power device applications owing to excellent material properties such as wide bandgap, large breakdown field, and high electron mobility [1], [2]. A conventional GaN-based HEMT is usually a normally-ON (depletion mode) device. For fail-safe operation in the power switching applications, normally-OFF (enhancement mode) GaN HEMTs are required [3]. Moreover, from the viewpoint of the high efficient power switch, a high breakdown voltage (), low ON-resistance (), excellent threshold voltage () hysteresis, and good dynamic /static ratio are also desirable [3]–[5].