Dynamic $V_{\mathrm{th}}$ in $p$-GaN Gate Power HEMTs and Its Impacts upon Power Switching Circuits

The p-GaN gate power HEMT exhibits a dynamic threshold voltage (V_th) during device operation, i.e. its V_th is a mathematical function of the terminal bias. The dynamic Vth phenomenon can be accurately predicted by a charge storage mechanism. The floating p-GaN layer stores negative charges during switching operating, and requires an additional gate bias to counteract these charges. The dynamic Vth has profound impacts upon power switching circuits. It affects the dynamic on-state characteristics and reverse conduction characteristics, presents restriction on the design of gate driving circuit design, and influences the switching waveforms of the circuits. Detailed analysis of these effects will be presented.

1. Introduction

The wide bandgap semiconductor GaN based power devices are attracting worldwide attention owing to their superior performances and their potential for next-generation power switching applications [1]–[3]. A critical issue that had hindered the GaN power devices from being accepted by the market was their inherent normally-on operation mode, while normally-off operation is virtually a must-have feature for a successful power device [4]–[6]. The p-GaN gate structure has been widely investigated to realize normally-off operation in GaN devices. At present, normally-off p-GaN gate HEMTs are commercially available [7], [8]. A representative p-GaN gate HEMT features a Schottky contact between the gate metal and p-GaN layer; thereby, reduced gate leakage current and enlarged gate swing are obtained [9], [10]. A large gate voltage swing allows the GaN power devices to be driven with traditional voltage-based driving circuits, and is beneficial for the realization of GaN power ICs [11]–[13].