Trapping Effects in Gallium Nitride High Electron Mobility Transistors: Mechanisms, Modeling, and Applications

Doktorsavhandling, 2025

While GaN-based high electron mobility transistors (HEMTs) have become indispensable for 5G and RADAR systems and shown 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 fluorinebased 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.