Review of the Gate Structure for Normally Off p-GaN High-Electron-Mobility Transistors Towards High Performances

As a representative wide-bandgap semiconductor material, gallium nitride (GaN) has attracted increasing attention because of its superior material properties (e.g., high electron mobility, high electron saturation velocity, and critical electric field). For power electronics applications, and to take full advantage of the superiorities of the GaN material, the normally off operation is required based on an AlGaN/GaN heterostructure. For a commercial approach, GaN HEMTs with a p-GaN gate have become a research hotspot. The characteristics of p-GaN gate HEMTs have a significant relationship with gate structure, especially the contact type on the p-GaN layer. In this review, the necessity of normally off operation and the advantages of adopting a p-GaN gate are elaborated, followed by the theory of achieving normally off operation by p-GaN and critical fabrication processes. The various gate structures are discussed, including metal gate, junction gate and hybrid gate structures on the p-GaN layer, to improve threshold voltage. Meanwhile, the methods required to optimize breakdown voltage and monolithically integrated technologies are also demonstrated. This review outlines the development and future trends of p-GaN gate HEMTs for power systems.