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 11^th, 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.

Ragnar Ferrand-Drake Del Castillo will defend his thesis, “Trapping Effects in Gallium Nitride High Electron Mobility Transistors: Mechanisms, Modeling, and Applications” at 9 am on September 4^th, 2025, in Kollektorn, Kemivägen 9, Göteborg. The faculty opponent is Stephane Piotrowicz, III-V Lab, France.

The thesis is available here:  https://research.chalmers.se/publication/?created=true&id=682b82af-56e5-4dcd-b9e0-4a5e069d4c55

Abstract:

While GaN-based high-electron-mobility transistors (HEMTs) have become indispensable for 5G and RADAR systems, they also show potential for astronomy and space exploration. Knowledge gaps remain in how epitaxial and processing design impact device performance. Downscaling of GaN HEMTs exacerbates source-drain current dispersion due to trapping and self-heating effects. This thesis focuses on characterizing and optimizing back-barrier/buffer design and processing methods to mitigate trap-induced degradation.
Although back-barrier and buffer doping individually enhance two-dimensional electron gas (2DEG) confinement, carbon-induced trapping creates a trade-off between confinement and dispersion. This work explores variations in carbon doping levels in the GaN buffer and AlGaN back-barrier to improve 2DEG confinement. By employing extensive electrical and spectroscopic methods, trapping mechanisms and their origins are investigated. The results show that dispersion dominates over short-channel effects at the investigated carbon levels, offering guidance for RF performance optimization. Annealing during gate opening is widely used to counteract damage from fluorine-based plasma treatments. However, the influence of high-temperature pre-gate annealing (500−800◦C), particularly in relation to CF4 and CF4 chemistries, remains underexplored. This study demonstrates that fluorine implantation and surface oxidation affect device behavior via thermally activated and deactivated traps. It identifies optimal combinations of fluorine plasma and annealing treatments, showing that up to 60 % of CF4 plasma-induced F−states can be deactivated by 600◦C annealing. Buffer trapping is also studied under cryogenic conditions, where Fe-induced traps manifest slow de-trapping dynamics. Field plates are found to mitigate these effects, emphasizing epi-structure and layout design strategies critical for reliable cryogenic GaN HEMT operation.
This thesis further shows that charged states introduced during gate-defining processing can be deliberately harnessed to modulate reverse gate-bias C–V characteristics. By varying fluorine plasma chemistry and pre-gate annealing conditions, the distribution and concentration of charged states in the barrier/channel region can be tuned. This enables the development of GaN-based varactors for MMIC applications, offering low nonlinear distortion in RF systems. By addressing key challenges in reliability and performance, and exploring emerging applications such as cryogenic operation and varactor integration. This thesis is well placed to advance and diversify GaN HEMT technology.

Ding-Yuan Chen will defense his PhD Thesis

“Ohmic Contacts, Passivation, and Buffer-Free Concepts”

Date: February 7th 2025, 10:00 am

Location: Chalmers University of Technology, Kemivägen 9, Kollektorn

Online: N/A

Supervisor: Niklas Rorsman, Chalmers University of Technology

Opponent: Dr. Farid Medjdoub, CNRS senior scientist, Group leader, IEMN, France