I. Introduction
GaN high-electron mobility transistors (HEMTs) are known as the promising devices for high efficiency and high power amplifiers [1]–[3]. To take full advantage of a device, a compact device model that can be used in commercial microwave circuit simulators is required. Typically, compact models for GaN HEMTs can be classified as empirical and physical models. Empirical models can accurately predict dc and large-signal RF characteristics of a device and can be easily implemented in circuit simulators [4]–[10]. However, empirical models for GaN HEMTs usually contain dozens of fitting parameters to account for self-heating, ambient temperature, and trapping effects [11]. So the extraction of empirical models usually needs lots of measurements and complicated extraction methods. This makes it inefficient to update models with process variations.