Gallium nitride (GaN) high-electron-mobility-transistors (HEMTs) are designed to operate at increasing higher power density and frequency to suit specific applications in 5G communication and electric vehicles. Proper thermal management methods are required to ensure the power density output and long-term reliability of GaN HEMTs. The non-uniformly distributed high die-level heat flux (> 1 kW/cm²) coupled with microscale hotspots has imposed significant thermal management challenges,

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... ng traditional "remote" cooling methods. This dissertation aims to explore two novel "near-junction" cooling techniques. The first one is a capped diamond heat spreader used to facilitate the spreading of heat from hotspot to die. The second is embedded manifold microchannels which quickly remove the heat from the die to the ambient by convection. Firstly, the thermal impacts of capping a diamond layer on multi-finger GaN HEMTs have been investigated under steady-state operation conditions. The scale of temperature reduction highlighted the applicability of the capped diamond spreader as a promising cooling strategy. Secondly, a further study was carried out for diamond heat spreader on a commercial 380-gates GaN-based power amplifier with kW peak power-level under periodic pulse-operated conditions. A scale-down model has been built to enable practical modeling for pulse mode calculation, which can significantly reduce the computational time while maintaining dynamic peak temperature. To precisely evaluate the cooling performance, thickness dependence and anisotropy of micrometer-sized diamond thermal conductivity were considered through two proposed fitted formulas based on experimental data in the literature. The capped diamond cooling strategy is shown to be more effective under harsh thermal conditions, including smaller duty cycle, higher interfacial thermal resistance, and substrate of lower thermal conductivity. Thirdly, an embedded manifold microchannel cooling (EMMC) arrangement was presented, in which microch [...]