AlGaN/GaN based enhancement mode MOSHEMTs

Banerjee, Abhishek (2010) AlGaN/GaN based enhancement mode MOSHEMTs. PhD thesis, University of Glasgow.

Abstract

This thesis describes a new gallium nitride (GaN) based transistor technology for electronic switching applications. Conventional GaN based transistors are of the high electron mobility transistor (HEMT) type and are depletion mode devices. These are not suitable for switching applications since an extra DC supply
is required to bias the device in the cut-off (off-state) region and the devices are not fail-safe, i.e. incase of malfunction a short-circuit can exist between the main DC supply and ground. Enhancement mode (E-Mode) or normally-off devices
can overcome these limitations and if realized in the GaN material system would benefit from the good material properties that support large breakdown voltages and low On-resistances.
Fabrication of high performace E-mode GaN devices with low On-resistance and high breakdown voltage still remains a big challenge to date. In this thesis a new method for realizing enhancement mode aluminium gallium nitride - gallium nitride (AlGaN/GaN) devices using a localized gate-foot oxidation has been described. Thermal oxidation of the AlGaN barrier layer converts the top surface/part of this layer into aluminium oxide (Al2 O3 ) and gallium oxide (Ga2O3 ), which serve as a good gate dielectric and improve the gate leakage current by several orders of magnitude compared to a Schottky gate. The oxidation process
leaves a thinner AlGaN barrier which can result in normally o§ operation. Without special precaution, however, the oxidation of the AlGaN barrier is not uniform from the top but occurs at higher rates at the defect/dislocation sites. This makes it impossible to control the barrier thickness and so rendering the barrier useless.
To avoid the problem of non-uniform oxidation, a thin layer of aluminum is first deposited on the barrier layer and oxidized to form aluminium oxide on top. This additional oxide layer seems to ensure uniform oxidation of the AlGaN barrier layer
underneath on subsequent further oxidation. Results of the fabricated 2 um x 100 um AlGaN/GaN MOS-HEMTs with a partially oxidized barrier layer showed a threshold voltage of -0.5 V (compared to -3 V for a Schottky devive fabricated on the same epilayer structure) and a maximum drain current of 800mA/mm
at high gate bias of 5 V with very little current compression. The peak extrinsic transconductance of the device is 160 mS/mm at a drain-source voltage of 10 V with a very low specific On-resistance of 9:8 ohm.mm2 and an off-state breakdown voltage higher than 42 V. Capacitance-Voltage (C-V) measurements
of Al2O3 /AlGaN /GaN circular test metal-oxide-semiconductor structures were observed and measured. They exhibit no hysteresis, indicating the good quality of the thermally grown aluminium oxide for realizing AlGaN/GaN based E-Mode
devices for high frequency and high power applications.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: GaN, AlGaN, MOSHEMT, Enhancement Mode
• Subjects: T Technology > TK Electrical engineering. Electronics Nuclear engineering
• Colleges/Schools: College of Science and Engineering > School of Engineering
• Supervisor's Name: Wasige, Dr. Edward
• Date of Award: 2010
• Depositing User: Mr Abhishek Banerjee
• Unique ID: glathesis:2010-2104
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 19 Jan 2011
• Last Modified: 23 Aug 2018 12:16
• URI: https://theses.gla.ac.uk/id/eprint/2104