This paper presents a review of a Gallium Nitride (GaN) High Electron Mobility Transistor (HEMT) technology appropriate in high-power and high-frequency applications. GaN, as wide bandgap materials (WBG) has demonstrated superior material properties such as improved thermal conductivity and excellent electrical properties, has become an attractive candidate for the next generation of high-power and high-frequency applications. The occurrence of the two- dimensional electron gas (2DEG) channel at AlGaN/GaN heterostructures interface makes GaN HEMTs intrinsically Normally-on devices. However, the Normally-off operation is often desired in many power electronics applications. Several methods had been explained to obtain Normally-off devices. Therefore, Normally-off GaN-based HEMTs with a p-GaN gate method is among the most promising and the only commercially available today. First, this paper would summarize the physical properties and device performance of GaN material over Silicon (Si) and Gallium Arsenide (GaAs) and related to other WBG material. Then, a normally-on and normally-off configuration are explored. After that, the most relevant technological issues for normally-off HEMTs focusing on the p-GaN gate are discussed. Finally, the p-GaN interface's role and the impact of the thermal processes on the electrical characteristics are widely discussed.
