MASc seminar - Hao Zhang

Tuesday, April 11, 2017 10:30 am - 10:30 am EDT (GMT -04:00)


Hao Zhang


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


Slim Boumaiza and Lan Wei


Gallium Nitride (GaN) based high electron mobility transistors (HEMTs) outperforms Gallium Arsenide (GaAs) and silicon based devices for radio frequency (RF) applications in terms of output power and efficiency due to its high band gap voltage (~3.4 eV@300K) and high carrier mobility property (1500~2300cm^2/(V·s)). These advantages have made GaN technology a promising candidate for future high-power and high data rate RF and potentially millimeter-wave applications.

Current mainstream GaN HEMT models used by the industry, such as Angelov Model, EEHEMT Model and DynaFET model, are empirical or semi-empirical. Lacking the physical description of the device operations, these empirical models are non-scalable. Circuit design uses the models requires multiple iterations between the device and circuit levels, becoming a lengthy and expensive process. Currently 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 as well as a holistic device and circuit interactive design and optimization framework, computationally efficient physics based compact models are required.

In this thesis, a complete physics based compact modeling framework is developed for GaN HEMTs targeting RF applications. First, the direct current (DC) and RF measurements required for modeling the device are introduced. Then a new gate current based resistive elements estimation technique is demonstrated for modeling both parasitic and channel resistance. Moreover, the intrinsic current and charge model are developed based on the virtual source (VS) model originally proposed by MIT researchers. Finally, the model is validated against measurement data in terms of drain current, gate current and S-parameter.

The 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. In particular, a new gate current based resistive elements estimation technique is proposed in this thesis. By using DC current for resistive element extraction, this technique provides more reliable and accurate device resistive values, compared to the conventional Cold-FET technique, where S-parameter are used and uncertainties are introduced into resistance extraction due to the effect of parasitic inductance and capacitance.