Thermal Dissipation in GaN-on-Sapphire Power HEMTs: Synergy of Substrate Thinning and High-Conductivity Thermal Interface Materials

Despite their significant potential for high-power applications, GaN-on-sapphire HEMTs face critical thermal management challenges due to the inherently low thermal conductivity of sapphire substrates. This study investigates the thermal performance of GaN-on-sapphire and GaN-on-Si HEMTs through finite element simulations of flip-chip ball grid array (FC-BGA) packaging. Simulation results reveal that substrate thinning effectively mitigates the impact of thermal conductivity on junction temperature differences between GaN-on-sapphire and GaN-on-Si devices. Reducing the substrate thickness to 0.1 mm narrows the junction temperature gap to merely 0.355°C. Furthermore, optimization of thermal interface materials (TIMs) demonstrates unique advantages for GaN-on-sapphire configurations. The electrically insulating nature of sapphire substrates permits the use of high-thermal-conductivity metal-based TIMs, whereas silicon substrates are restricted to insulating TIMs (≤ 10 W•m⁻<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup>•K⁻<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup>) to avoid electrical risks. At a 0.1 mm substrate thickness, GaN-on-sapphire HEMTs exhibit thermal performance equivalent to GaN-on-Si HEMTs when using a 15 W•m⁻<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup>•K⁻<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> TIM. Increasing the TIM's thermal conductivity to 50 W•m⁻<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup>•K⁻<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">1</sup> further reduces the junction temperature to 91.977°C. Transient thermal cycling analysis confirms the robustness of this synergistic design approach. These results highlight promising applications for high-power-density systems, such as electric vehicle fast-charging infrastructure.