Physics-based Analytical Modelling and Optimization of the GaN HEMT with the Field-Plate Structure for Application in High-Frequency Switching Converters

摘要

The purpose of this thesis is to present a novel, physics-based, capacitive model for Gallium Nitride High Electron Mobility Transistors that contain gate field-plate structure. The proposed methodology is fully analytical, providing a set of equations between the design parameters and input, output and reverse capacitance of the device. Together with previously proposed physics-based analytical models for output characteristcs of a HEMT, proposed capacitive model gives the complete model of a GaN switching device. Thereore, it can be implemented into power loss models for the topology of interest and used for device design optimization by power losses minimization. The target application of the device modelled in this work is a high-frequency (HF) DC/DC converter used as a dynamic power supply in Envelope Tracking and Envelope Elimination and Restoration techniques. The main challenge in these HF topologies is to increase the efficiency of the signal transmission together with the bandwidth. Since AlGaN/GaN HEMTs present a technological solution that does not contain p-n junctions, their capacitacnies are significantly lower comparing to Si MOSFETs. Therefore, they present excellent candidates for switching devices in high-frequency converters. Furthermore, using proposed physics-based capacitive model, their design can be optimized for a particular application. That was the main motivation for the work presented in tis thesis. The main contributions of this thesis are the proposed capacitance model together with the optimization process of the HEMT design. The model was verified by experimental characterization of the device. Additionaly, in order to verify its applicability for the design optimization process, model was implemented into a power loss model of a high-frequency buck converter. Simulated efficiency curves showed very good agreement with the measurements, verifying the precision of the proposed methodology. Furthermore, the obtained model showed that field-plate design dominantely influences Miller’s charge in the device and determines the breakdown voltage rating. Therefore, the field-plate was optimized separately from other design parameters, by minimizing gate-to-drain charge for a target breakdown voltage. Furthermore, implementation of the obtained physics-based model into a power loss model of a buck converter and variation of different design parameters, provided directions in which parameters should be changed (increased/decreased) in order to minimize the power losses. This resulted in the optimized design of the device that reduced loss total losses in the converter for 80%, at 20MHz of switching frequency and output power of 20W. Total loss breakdown showed that switching losses that presented 81% of the total losses were decreased for 76% due to highly improved capacitance characteristics of the device.