Design and fabrication of AlGaN/GaN HEMTs with high breakdown voltages

Macfarlane, Douglas James (2014) Design and fabrication of AlGaN/GaN HEMTs with high breakdown voltages. PhD thesis, University of Glasgow.

Abstract

Gallium nitride based transistors will make up a large portion of the power electronics and the microwave electronics sectors in the very near future, replacing
traditional materials such as silicon (Si) and gallium arsenide (GaAs). The work in this thesis focuses on AlGaN/GaN high electron mobility transistors (HEMTs) in
particular, with the aim of gaining the maximum potential out of them with regards to breakdown voltage. GaN based devices are able to breakdown at higher voltages compared to Si or GaAs due to its wider band gap (3.4 eV compared to 1.1 eV and 1.4 eV respectively) and although a lot of work has been invested into these devices over the last two decades or so, their full potential has yet to be realised and new
solutions are still sought to provide a complete engineering solution which will make them competitive and commercially viable. One of the main obstacles is the high
electric fields generated at the drain side of the gate which have prevented these devices from reaching their theoretical breakdown fileld of around 300 V/um. In an
attempt to overcome this, several approaches have been investigated in this thesis including metal insulator semiconductor HEMTs (MIS-HEMTs), `gate overlapping'
HEMTs, where the gate partially overlaps the source and drain contacts and finally a device employing a Schottky source and a Schottky drain contact. The results
given show that a MIS-HEMT can have a substantially larger breakdown voltage compared to a Schottky gate HEMT which is clarified through qualitative simulated electric field work and experimental work. Further, the MIS-HEMT shows a high
breakdown field of about 87 V/um when a Schottky drain contact is incorporated. The gate overlapping HEMTs attempts to mitigate completely the large electric field found at the drain edge of the gate. Simulated and experimental results are given for this device concept and reveal that the large electric field peak is indeed removed, however, low breakdown voltages are still incurred due to the closeness of the gate edge to the drain contact. Finally, results are given for a device employing Schottky source and Schottky drain contacts and reveals that present theory may
not completely describe the operation of this device.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Subjects: T Technology > TA Engineering (General). Civil engineering (General)
• Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
• Supervisor's Name: Wasige, Dr. Edward
• Date of Award: 2014
• Depositing User: Mr Douglas James Macfarlane
• Unique ID: glathesis:2014-4835
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 04 Feb 2014 10:16
• Last Modified: 04 Feb 2014 10:22
• URI: https://theses.gla.ac.uk/id/eprint/4835