Thermal properties of GaN layers on foreign vs native substrates: Enabling cost-effective solutions for vertical GaN-based transistors

Gallium nitride (GaN) has gained significant traction in power electronics due to its high electron mobility and breakdown voltage. However, the high cost and small available size of native GaN substrates remain barriers to widespread commercial adoption. Using foreign substrates presents a cost-effective alternative but introduces challenges, such as potential reductions in GaN crystal quality and altered thermal and electrical properties. Insufficient thermal conductivity (λ) can lead to hotspots and self-heating, degrading the reliability of devices, especially vertical power transistors that are sensitive to temperature variations. This study examines the thermal properties of GaN layers grown on silicon (Si) and sapphire (Al2O3) substrates, in comparison with native GaN. The analysis focuses on the drain (n+-GaN) and drift (n-GaN) regions, relevant to practical device structures. Thermal conductivity was measured using scanning thermal microscopy and time domain thermoreflectance and correlated with Raman-based stress analysis. For n+-GaN on Si, λ was about 190 W/(mK), comparable to native GaN, while λ on Al2O3 was approximately 140 W/(mK). Dislocation density and point defects were identified as key factors reducing λ. The excellent thermal performance of n+-GaN on Si was attributed to stress reduction through a sophisticated buffer structure with AlN/GaN superlattices. Our results demonstrate that high-quality GaN can be fabricated on foreign substrates, reducing reliance on costly native GaN and advancing the industrialization of vertical GaN technology. Additionally, the derived parameters can improve simulations by providing realistic inputs for more accurate modeling of thermal behavior in novel device architectures.