The wide bandgap GaN-based transistors are attractive for power electronics applications owing to the superior intrinsic properties of the materials. In addition to the breakdown electric-field that is one order of magnitude higher than the mainstream semiconductor silicon, GaN-based heterostructrues (e.g. AlGaN/GaN) enhanced by spontaneous and piezoelectric polarization effects can yield a two-dimensional electron gas (2DEG) channel with a high sheet charge concentration and high electron mobility, both of which lead to low ON-state resistance. On the other hand, the high density 2DEG also presents the conventional AlGaN/GaN high electron mobility transistors (HEMTs) as depletion-mode transistors with a negative threshold voltage (V_th). For simpler circuit configuration and inherent fa...[ Read more ]

The wide bandgap GaN-based transistors are attractive for power electronics applications owing to the superior intrinsic properties of the materials. In addition to the breakdown electric-field that is one order of magnitude higher than the mainstream semiconductor silicon, GaN-based heterostructrues (e.g. AlGaN/GaN) enhanced by spontaneous and piezoelectric polarization effects can yield a two-dimensional electron gas (2DEG) channel with a high sheet charge concentration and high electron mobility, both of which lead to low ON-state resistance. On the other hand, the high density 2DEG also presents the conventional AlGaN/GaN high electron mobility transistors (HEMTs) as depletion-mode transistors with a negative threshold voltage (V_th). For simpler circuit configuration and inherent fail-safe operation, normally-off GaN based HEMTs with positive threshold voltage are highly desired in modern power electronics systems. This thesis is focused on the process modeling and device technology of normally-off GaN power transistors.

A process based on fluorine plasma ion implantation has been recently developed for achieving a robust control of the threshold voltage of AlGaN/GaN HEMTs, and is emerging as one of the most promising technologies for normally-off GaN power devices. To understand the underlying physical mechanisms of this process and also provide tools for process optimization, an atomistic hybrid molecular dynamics (MD)/kinetic Monte Carlo (KMC) model is developed for modeling the implantation and the subsequent annealing/diffusion behavior of the fluorine ions in AlGaN/GaN heterostructures. The MD simulation reveals the F distribution profiles and the corresponding defect profiles. Most importantly, the potential energy profiles of fluorine ions in the III-nitride material system are calculated for the first time, and the results fundamentally explained the stability of fluorine ions in AlGaN/GaN heterostructrues. Based on the results from the MD simulation, the diffusion process is modeled with KMC method, and the modeling results are validated by the secondary-ion-mass-spectrum (SIMS) measurement.

The buffer leakage current is a common challenge being faced by the existing technologies of normally-off GaN power transistors due to the lack of high-quality blocking junctions in the leakage current path. To address this issue, a novel AlGaN/GaN metal-2DEG tunnel junction field-effect transistor (TJ-FET) is proposed and experimentally demonstrated in this work. The TJ-FET, when realized in AlGaN/GaN HEMT structure, delivers desirable features including normally-off operation (V_th = + 1.35 V), low leakage current (~10 pA/mm) and high on/off current ratio (10¹⁰). A maximum ON-state drain current density as high as 320 mA/mm has been obtained. The leakage current is effectively suppressed by the source Schottky junction that is naturally reverse biased in the OFF-state. The threshold voltage controlling scheme of TJ-FET is also fundamentally different from that of the conventional HEMTs, adding another degree of freedom in realizing normally-off GaN power transistors. The gate induced Schottky barrier lowering effect (SBL) in TJ-FET has been investigated to further characterize the effective Schottky barrier height (SBH) of source tunnel junction. The larger SBH at lower gate voltage enables off-state blocking, while the lower SBH at higher gate voltage leads to high on-state current driving capability in the AlGaN/GaN TJ-FETs.