Physics Based Virtual Source Compact Model of Gallium-Nitride High Electron Mobility Transistors

Abstract

Gallium Nitride (GaN) based high electron mobility transistors (HEMTs) outperform Gallium Arsenide (GaAs) and silicon based transistors for radio frequency (RF) applications in terms of output power and efficiency due to its large bandgap (~3.4 eV@300 K) and high carrier mobility property (1500 – 2300 cm^2/(V⋅s)). These advantages have made GaN technology a promising candidate for future high-power microwave and potential millimeter-wave applications. Current GaN HEMT models used by the industry, such as Angelov Model, EEHEMT Model and DynaFET (Dynamic FET) model, are empirical or semi-empirical. Lacking the physical description of the device operations, these empirical models are not directly scalable. Circuit design that uses the models requires multiple iterations between the device and circuit levels, becoming a lengthy and expensive process. Conversely existing physics based models, such as surface potential model, are computationally intensive and thus impractical for full scale circuit simulation and optimization. To enable efficient GaN-based RF circuit design, computationally efficient physics based compact models are required. In this thesis, a physics based Virtual Source (VS) compact model is developed for GaN HEMTs targeting RF applications. While the intrinsic current and charge model are developed based on the Virtual Source model originally proposed by MIT researchers, the gate current model and parasitic element network are proposed based on our applications with a new efficient parameter extraction flow. Both direct current (DC) of drain and gate currents and RF measurements are conducted for model parameter extractions. The new flow first extracts device parasitic resistive values based on the DC measurement of gate current. Then parameters related with the intrinsic region are determined based on the transport characteristics in the subthreshold and above threshold regimes. Finally, the parasitic resistance, capacitance and inductance values are optimized based on the S-parameter measurement. This new extraction flow provides reliable and accurate extraction for parasitic element values while achieving reasonable resolutions holistically with both DC and RF characteristics. The model is validated against measurement data in terms of drain current, gate current and scattering parameter (S-parameter). This model provides simple yet clear physical description for GaN HEMTs with only a short list of model parameters compared with other empirical or physics based models. It can be easily integrated in circuit simulators for RF circuit design.