GaN HEMT based technology development for millimetre wave applications

Alathbah, Moath 2022. GaN HEMT based technology development for millimetre wave applications. PhD Thesis, Cardiff University. Item availability restricted.

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Abstract

High Electron Mobility Transistors (HEMTs) based on Gallium Nitride (GaN) and grown on Silicon (Si) substrates are emerging as one of the most promising candidates for high-power, switching-speed, low-losses, cost-effectiveness, and high-frequency Integrated Circuits (ICs) applications operating at microwave and millimetre frequencies (3-300 GHz). As silicon approaches its theoretically expected limitations, enhancements in energy efficiency of silicon power electronics are becoming less significant. Due to a unique combination of features that offer higher power density per unit area, compared to Si, GaN transistors are alternatives for the next generation of power electronics. Because GaN can be processed on current Si process lines employing GaN-on-Si heteroepitaxy, the industrial shift to GaN is also cost effective. In GaN HEMTs devices, the mesa structure is a three-dimensional device structure that is conventionally created by traditional dry etching methods applied to establish device-isolation used for GaN-based transistor technologies. As a result, this leads to significant leakage currents which have an immense influence on the device's noise performance and breakdown voltages. In this thesis, a novel technique designed to establish a device-isolation in GaN HEMTs is proposed to overcome mesa etch concerns such as sidewall profile, common gate discontinuity and gate leakage originated from a gate direct contact with two-dimensional electron gas (2DEG). The proposed method requires a miniature extension of the mesa to deposit the gate-feed to ensure a fully planar gate formation. As a result, this reduced the gate leakage by an order of magnitude. However, since the gate-feed is situated on an active layer with a very low resistance due to a relatively larger length in comparison with the actual gate, the gate became conductive above pinch-off point and gate leakage increased. Consequently, a dielectric is deposited below the gate-feed to counter the arising problem which led to an increase in the drain leakage at the off-state due to a substantial dielectric thickness. This can be prevented using methods such as atomic layer deposition for a denser yet a shallower (a few nm) dielectric deposition below the gate feed. Finally, the proposed structure will allow a deeper etch of the active layer for heat management purposes while keeping the integrity of the gate metal intact. Leakage currents reduction of unity A/mm were attained, which are equivalent to those obtained using the more sophisticated and costly ion implantation process. Additionally, a small-signal-model was developed to evaluate the behavior of the GaN HEMT devices with respect to the high frequency operation and the parasitic associated with it. The process began with the extraction of the extrinsic elements using various methods followed by the extraction of the intrinsic parameters. Finally, novel GaN Schottky barrier diodes (SBDs) with multi-channel trenches positioned below the anodes to reduce the turn-on voltage caused by the reduction of the diode barrier height due to the direct contact to the 2DEG. To compete with existing III-V technologies, GaN-based SBDs with low reverse�current leakage (IS), low onset voltage (VON), high switching speed (RON), high reverse-breakdown (VB) voltage, and high cutoff frequency (fc) are mainly essential to be considered. Conventional GaN-based SBD DC and RF performance, when fabricated using LR-Si substrates, is still constrained by their significant VON, RF leakage, and switching losses. Methods such as recessed anode, dual-channel field-effect rectifier, regrowth cathodes, and dual-filed plates have been employed to suppress these problems. In this work, a cost-effective RF AlGaN/GaN SBDs on LR-Si structure is developed utilizing the technique of multi-channel and trenches below anodes, which is completely compatible with III-V THz monolithic integrated circuit (THz-MIC) technology. The newly designed devices outperformed traditional SBDs in terms of switching loss, turn-on characteristics, ideality factor (�!), and cutoff frequency, with RON = 0.97 Ω.mm, VON = 0.84 V, VB > 30V, �! = 1.69, and fc = 600 GHz. These outcomes are attributed to the direct contact acquired between the Schottky anode and the 2DEG channel occurring at the sidewalls of the trenches due to multi-mesa, as well as the precise design geometries utilized to limit substrate coupling effects.

Item Type:	Thesis (PhD)
Date Type:	Completion
Status:	Unpublished
Schools:	Engineering
Uncontrolled Keywords:	GaN HEMTs , GaN Schottky Diodes , GaN HEMTs for X-band power amplifiers , GaN HEMTs on Silicon substrate , GaN Diodes for Frequency Mixers and Multipliers , Gallium Nitride Devices
Date of First Compliant Deposit:	11 October 2022
Last Modified:	10 Oct 2023 01:30
URI:	https://orca.cardiff.ac.uk/id/eprint/153255

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