A novel AlGaN/GaN based enhancement-mode high electron mobility transistor with sub-critical barrier thickness

Brown, Raphael (2015) A novel AlGaN/GaN based enhancement-mode high electron mobility transistor with sub-critical barrier thickness. PhD thesis, University of Glasgow.

Abstract

Power-switching devices require low on-state conduction losses, high-switching speed, high thermal stability, and high input impedance. Using gallium nitride (GaN) based field-effect transistors, these properties for switching devices can be satisfied. GaN-based High Electron Mobility Transistors (HEMTs) are emerging as promising candidates for high-temperature, high-power (power electronics) and radio-frequency (RF) electronics due to their unique capabilities of achieving higher current density, higher breakdown voltage, higher operating temperatures and higher cut-off frequencies compared to silicon (Si). Conventional GaN HEMTs with an aluminium gallium nitride (AlGaN) barrier are of depletion-mode (d-mode) or normally-on which require a negative polarity power supply to turn off. On the other hand, enhancement-mode (e-mode) or normally-off AlGaN/GaN HEMTs are attracting increasing interest in recent years because no negative gate voltage is necessary to turn off the devices. This leads to the advantage of simple circuit design and low stand-by power dissipation. For power electronics applications, power switches which incorporate e-mode devices provide the highly desirable essential fail-safe operation.

In this research, a new high performance normally-off GaN-based metal-oxide-semiconductor (MOS) high electron mobility transistor (HEMT) that employs an ultrathin sub-critical 3nm Al_0.25Ga_0.75N barrier layer and relies on an induced two dimensional electron gas (2DEG) for operation was designed, fabricated and characterized. The device consists of source and drain Ohmic contacts nominally overlapped by the gate contact and employs a gate dielectric. With no or low gate-to-source voltage (V_GS), there is no two dimensional electron gas (2DEG) channel at the AlGaN/GaN interface to allow conduction of current between the drain and source contacts as the AlGaN barrier thickness is below the critical thickness required for the formation of such channel. However, if a large enough positive bias voltage V_GS is applied, it causes the formation of a quantum well at the AlGaN/GaN interface into which electrons from the source and drain Ohmic regions are attracted (by the positive gate voltage), effectively creating a 2DEG channel, and so the structure is a normally-off field effect transistor.

Normally-off GaN MOS-HEMT devices were fabricated using plasma enhanced chemical vapour-deposited (PECVD) silicon dioxide (SiO_2) as the gate dielectric. They demonstrated positive threshold voltages (V_th) in the range of +1V to +3 V, and very high maximum drain currents (I_DSmax) in the range of 450mA/mm to 650mA/mm, at high gate voltage (V_GS) of around 6 V. The devices also exhibited breakdown voltages in the range of 9V and 17V depending on the gate dielectric thickness, making them suitable for realising high current low voltage power devices required, for instance, for buck converters for mobile phones, tablets, laptop chargers, etc.

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: GaN, AlGaN, HEMT, normally-off, enhancement-mode, E-mode, transistor, thin-barrier, MOS-HEMT, PECVD
• Subjects: Q Science > QC Physics; T Technology > TK Electrical engineering. Electronics Nuclear engineering
• Colleges/Schools: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering
• Supervisor's Name: Wasige, Dr. Edward
• Date of Award: 2015
• Depositing User: Dr Raphael Brown
• Unique ID: glathesis:2015-6590
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 31 Jul 2015 07:28
• Last Modified: 07 Aug 2015 08:40
• URI: https://theses.gla.ac.uk/id/eprint/6590