Modelling of Electrostatics and Transport in GaN-Based HEMTs under Non-Equilibrium Conditions

Abstract

High electron mobility transistors (HEMTs) consisting of GaN and its alloys, most commonly AlGaN, have been gaining popularity as the next generation of high-speed devices for radiofrequency and power applications. Although a high concentration of 2D electrons in such structures can be obtained even in equilibrium, i.e. with a zero gate bias, in recent years there has been a tendency of developing normally-off, i.e. enhancement-mode GaN HEMTs to ease integration with the associated gate-driver circuitry. Therefore, accurate simulation of key electrical properties of these devices, such as electron mobility, becomes important even in non-equilibrium conditions, i.e. with an applied gate bias. This paper describes a simulation framework designed to enable the modelling of 2DEG mobility in enhancement-mode HEMTs. Apart from the Schrödinger and Poisson equations which need to be solved for the equilibrium case, the current continuity equations for electrons and holes also need to be satisfied when a positive gate bias is applied. All these equations are solved in a self-consistent numerical procedure to obtain a correct solution of the electrostatic problem for an arbitrary gate bias, as well as the discrete states and carrier wavefunctions needed for semi-classical mobility calculations. The procedure is demonstrated by simulating an advanced enhancement-mode device with a p-GaN cap and comparing the calculated electron concentrations and mobilities with available experimental results.