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Impact of Thermal Boundary Resistance on the Thermal Design of GaN-on-Diamond HEMTs

The thermal boundary resistance is the key feature of innovative approach of high-thermal-conductivity diamond substrate for high-power GaN HEMTs, and has significant impact on the thermal design of GaN chip. A three-dimensional thermal simulation for analysis of the heat dissipation capability of different thermal designs is presented by finite element method. The model accounts for the nonlinear thermal conductivity of GaN and diamond materials by employing Kirchhoff's transformation with the aims to improve calculation accuracy. We investigate impact rules of thermal boundary resistance on the thermal design of the GaN chip, including the GaN buffer, diamond substrate, gate-gate pitch spacing, and chip size. Those results indicate that the impacts of diamond thickness and chip size on junction temperature are have a rule, the thermal boundary resistance has no influence upon the rules, but it influences the variation of the junction temperature. In addition, the rule for impact of gate-gate pitch spacing on junction temperature is limited by the thermal boundary resistance, and the impact is proportional to the value of thermal boundary resistance. What is worth our special attention is the impact of GaN thickness on junction temperature, the optimal GaN thickness should exist for the junction temperature, and is also affected by the thermal boundary resistance. Overall, we provide thermal structure design guidelines for GaN-based diamond substrate.