Principles and impacts of dynamic threshold voltage in a p-GaN gate high-electron-mobility transistor

Abstract

GaN power transistors are attracting increasing attention for next-generation power switching applications. However, GaN power transistors should not be regarded as a direct replacement for silicon power metal–oxide–semiconductor field-effect transistors, since the unique properties of GaN power transistors require special treatment. In this work we review a unique and critical property of GaN power transistors, the dynamic threshold voltage (V th). GaN power transistors feature a p-GaN gate structure, while the gate metal contacts the p-GaN layer through a Schottky junction. This unique p-GaN gate structure is distinctively different from the silicon metal–oxide–semiconductor gate structure in that the p-GaN layer is actually a floating region without direct electrical contact to the external electrodes of the device. The floating p-GaN layer enables a charge storage process during the switching operation of a p-GaN gate high-electron-mobility transistor. Through carefully designed experiments, the strong correlation between charge storage and the dynamic V th shift has been revealed; this also serves as an approach to accurately predict the dynamic V th during switching operation. The influence of the dynamic V th on power switching applications is discussed. For example, with a positive dynamic V th shift, the reverse conduction voltage of a GaN power transistor is significantly increased, adding to the power loss. This positive V th shift also demands a sufficiently high gate drive voltage to ensure that the GaN power transistor can be fully turned on, which reduces the safety margin for the gate reliability of the device. To design a GaN based power switching circuit, accurate simulation of the device/circuit characteristics is necessary. Circuit simulations that do not consider the dynamic V th lead to a large deviation from the measurement circuit characteristics. We demonstrate that by incorporating the dynamic V th into the circuit model this deviation can be effectively eliminated.