Thickness Dependence and Anisotropy of Capped Diamond Thermal Conductivity on Cooling of Pulse-Operated GaN HEMTs

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Abstract
A capped thin-film diamond heat spreader is modeled for the reinforcement of hotspot cooling in the application of pulse-operated GaN-based high-electron mobility transistors (HEMTs). To precisely evaluate the cooling performance, thickness dependence and anisotropy of diamond film thermal conductivity are considered. A 3-D transient thermal model based on the finite element method is developed to simulate the pulse behavior. The sharp temperature oscillation occurring in the periodic pulse mode from the microsecond to millisecond time scales introduces substantial difficulty and challenge for accurate modeling. To this end, a scale-down model mimic of a recently reported GaN-based amplifier with kilowatts peak power level is built to significantly reduce the computational time and increase the accuracy while maintaining crucial thermal characteristics. The thermal characteristics under periodic pulse mode are revealed and the dynamic steady-state peak junction temperature is reliably obtained. The impacts of thermal boundary resistance (TBR), substrate material, duty cycle, and pulse period on the performance of the diamond heat spreader have been scrutinized. For the GaN-on-SiC HEMTs working at 2.5% duty cycle, 200- $\mu \text{s}$ pulse period, and 19.58 W/mm peak power density, a 2- $\mu \text{m}$ -thick diamond heat spreader can reduce the dynamic steady-state peak junction temperature by 14.7%.

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