Björn Hult will defend his thesis, “Downscaled III-Nitride Power HEMTs with Thin GaN Channel Layers: Fabrication, Characterization, and Physics-Based Modeling”, at 9 am on September 11th, 2025, in Kollektorn, Kemivägen 9, Göteborg. The faculty opponent is Dr. Oliver Hilt, Head of Department, Ferdinand-Braun-Institut, Germany.
The thesis is available here: https://research.chalmers.se/publication/547840
Abstract:
The unique polarization properties of the III-nitride materials have motivated research into gallium nitride (GaN)-based high-electron-mobility transistors
(HEMTs) for both power electronics and microwave applications. In these devices, compensation-doped buffer layers and strain-relief layers are typically incorporated into the III-nitride layer stack to reduce off-state currents and to achieve high-crystal-quality GaN and aluminum GaN (AlGaN) layers. However, thin-channel AlGaN/GaN/AlN heterostructures have been presented as a viable alternative to the conventional technology. Among these types of heterostructures, the buffer-free´QuanFINE® concept has been suggested. This material uses the AlN nucleation layer and the silicon carbide substrate to improve the electron confinement in the GaN channel layer. In this thesis, high-voltage buffer-free GaN power HEMTs are evaluated.
The devices are characterized in terms of their on-state, off-state, and dynamic performance. The impact of critical processing modules – including isolation techniques, dielectrics, and field plate configurations—is investigated. Due to the high electron confinement in the GaN channel layer, a power figure of merit of 729 MW/cm2 at sub-100 nA/mm drain-source current could be achieved, which is comparable to most state-of-the-art technologies reported in the literature. In contrast to heterostructures with buffer designs, no compensation dopants that can adversely affect the dynamic performance are intentionally incorporated into GaN or AlN layers. However, it is not fully understood how, or to what extent, unintentional defects and impurities will affect the dynamic performance in buffer-free HEMTs. A physics-based technology computer-aided design model is presented to explain the capture and emission processes involved during and after high-voltage conditions. It is hypothesized that a highly ionized donor concentration exists in the GaN layer near the GaN/AlN interface. The trap is thought to be related to defects and impurities that naturally coalesce near the GaN/AlN interface. These states are needed to prevent a semi-permanent current reduction after high-voltage conditions. However, it is also shown that the spatial distribution has to be controlled to prevent excessive off-state drain-source leakage currents.
An alternative measurement technique for estimating drain-induced barrier lowering in GaN HEMTs is also suggested. The new method is based on the drain
current injection technique (DCIT), which facilitates the measurement of threshold voltage variations at different drain-source voltages. GaN HEMT with short gate lengths (LG) and different epitaxial designs were used to demonstrate the viability of the method. For high-voltage buffer-free HEMTs, the DCIT can be used in the optimization of channel layer thickness and LG to improve dynamic performance while minimizing the adverse effects of LG reduction. Overall, the thesis contributes to the advancement of III-nitride technologies tailored toward power applications through the development of thin-channel buffer-free materials.
