A Physics-Based AlGaN/GaN HFET Compact Model for Implementation in Circuit Simulators

AlGaN/GaN heterojunction field-effect transistors (HFETs) are promising RF transistors for use in high-power and high-frequency circuit applications. These HFETs possess a combination of high current density capability and high breakdown voltage due to the desirable physical properties of the materials, such as high critical electric field for breakdown, high electron mobility and saturated carrier velocity, high carrier density in the channel, lower dielectric constant compared to the conventional materials, and improved thermal conductivity when epitaxially grown on semi-insulating SiC substrates. These parameters permit the HFET to operate at high RF voltage and current, which results in high power operation at high frequency.;The technology for fabricating devices and circuits in AlGaN/GaN is developing rapidly, and this rapid development is creating a need for improved device models. To date no commercially available HFET model for use in harmonic-balance circuit simulators exists that can predict the large-signal RF operation of an HFET or a MMIC before the active device is fabricated, characterized, and fitted.;In this work, a new physics-based compact model for AlGaN/GaN HFETs has been reported. The new model is programmed in Verilog-A, an industry-standard compact modeling language, and implemented in the circuit simulator Microwave Office (MWO(TM)).;The new model is developed, based upon separating the conducting channel of the HFET into a series of zones, based upon operational physics. According to the physics that dominate in each zone and the boundary conditions at the adjacent zones, the conduction current formulation can be derived in terms of the electrical nodes in the devices. Then the charge storages and corresponding displacement current can be obtained.;The HFET model is generalized to suit different fabrication process by introducing a curvature parameter in the v-E relationship for electrons in the conducting channel of the HFET in order to control the sharpness of the knee of the I-V characteristics. The charge deficit zone has been considered in the drain access for triode operation as well as saturation operation to ensure continuity of the terminal characteristics. The pinch off voltage is modified to take into account different Al mole fractions in the AlGaN barrier layer in the AlGaN/GaN HFET. Finally, it is necessary to consider channel breakdown in order to accurately simulate RF performance (RF output power, Power Added Efficiency (PAE) and gain) at high RF power operation.;The model is written in Verilog-A language and implemented it in MWO(TM). The model has been calibrated and verified by comparison of its predictions for dc and RF performance against measured data and Silvaco(TM) simulation results for an experimental S-Band AlGaN/GaN HFET amplifier. Good agreement is obtained.;The new model permits the dc, small-signal, and large-signal RF performance for the transistor to be determined as a function of the device geometric structure and design features, material composition parameters, and dc and RF operating conditions. The new physics-based HFET model does not require extensive parameter extraction to determine model element values, as commonly employed for traditional equivalent-circuit based transistor models. Therefore, it's suitable for use in commercial harmonic-balance microwave circuit simulators, and permits the co-design and optimization of HFETs in an MMIC environment, which will enhance and speed integrated circuit development.