Date of Award

2016

Document Type

Open Access Dissertation

Department

Electrical Engineering

Sub-Department

College of Engineering and Computing

First Advisor

Grigory Simin

Abstract

In modern society, the demand for power consumption is increasing rapidly and the need of energy savings is now an issue of global importance. Highly efficient power converters and power conditioning systems operating with wide range of traditional as well as novel renewable and clean energy sources, are playing crucial role in energy saving. Si converters have already reached their limitation in terms of switching frequency and breakdown voltage/on-resistance ratio. Research is going on all around the world and it is now well accepted that significant improvement in power conversion efficiency and speed can only be achieved using beyond Si devices, such as SiC and GaN based. GaN based converters have shown great promises for higher conversion efficiency and switching speed. It is now crucial to develop accurate models that can assist in design and fabrication of GaN based power electronics.

There are models for GaN HFETs. But these models are mainly focused on GaN HFET applications in RF power amplifiers. Although certain device characteristics (e.g. 2DEG density, power gain, cut-off frequencies etc.) are accurately estimated by existing models, currently there is no complete model usable in power electronics circuit/system simulators. In this work, we have developed a hybrid physics based/empirical compact model that describes the behavior of GaN HFETs in power switching high current, high voltage circuits. The complete model includes different modules from existing models that are suitable for GaN HEMTs for power switching applications and incorporate models that are non-existent but crucially important for power switching applications such as current collapse and bulk current.

The Charge-Control-model can reproduce both above-threshold and subthreshold current-voltage and transient characteristics of GaN based power HFET’s over a wide temperature range. The current–voltage (I-V) characteristics are described by a single, continuous, analytical expression for all regimes of operation, thereby improving convergence. The semi-empirical model includes effects such as velocity saturation in the channel, saturation of sheet carrier density, drain-induced barrier lowering (DIBL), self-heating, field plate effects, current collapse, substrate current and temperature dependence. Extensive TCAD simulations have been performed using a novel approach to investigate the mechanisms of bulk current and based on the results, a simple but accurate compact model for bulk current has been developed. The model is implemented using Verilog-AMS Hardware Description Language and extensively verified against a variety of experimental data for various HFET devices. This model does not require detail layer by layer device structure or technology because it uses directly measurable parameters.

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