Impact of AlN buffer thickness on electrical and thermal characteristics of AlGaN/GaN/AlN HEMTs

TL;DR

This study examines how varying AlN buffer thickness affects the structural, electrical, and thermal properties of AlGaN/GaN HEMTs, revealing trade-offs between strain, surface quality, electron mobility, and thermal conductivity.

Contribution

It provides insights into optimizing AlN buffer layers to enhance HEMT performance by balancing strain relaxation, surface quality, and thermal management.

Findings

Thinner AlN buffers exhibit compressive strain and smoother surfaces.

Thicker buffers develop tensile strain and increased roughness.

Thermal conductivity increases with buffer thickness, reaching 188 W/m.K at 2 μm.

Abstract

We investigate the influence of AlN buffer thickness on the structural, electrical, and thermal properties of AlGaN/GaN high-electron mobility transistors (HEMTs) grown on semi-insulating SiC substrates by metal-organic chemical vapor deposition. X-ray diffraction and atomic force microscopy reveal that while thin AlN layers (120 nm) exhibit compressive strain and smooth step-flow surfaces, thicker single-layer buffers (550 nm) develop tensile strain and increased surface roughness. Multi-layer buffer structures up to 2 {\mu}m alleviate strain and maintain surface integrity. Low-temperature Hall measurements confirm that electron mobility decreases with increasing interface roughness, with the highest mobility observed in the structure with a thin AlN buffer. Transient thermoreflectance measurements show that thermal conductivity (ThC) of the AlN buffer increases with the thickness, reaching 188 W/m.K at 300 K for the 2 {\mu}m buffer layer, which is approximately 60% of the bulk AlN ThC value. These results highlight the importance of optimizing AlN buffer design to balance strain relaxation, thermal management, and carrier transport for high-performance GaN-based HEMTs.