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The direct-die-attached cooling solution with a diamond heat spreader and hybrid Si heat sink has been developed for hotspot cooling of a GaN-on-Si device. The hybrid heat sink combines the benefits of microchannel flow and microjet impingement. In the fabricated test chip, the small hotspot is used to represent one unit of a GaN transistor. Experimental tests have been conducted on the fabricated test vehicle to investigate the thermal and fluidic performances. Two types of simulation models have been constructed using the commercial Finite Element Method software COMSOL, using the multiphysics features and temperature-dependent material properties. A submodel in conjunction with the main model is constructed to predict the thermal performance of the GaN-on-Si structure. Various heating powers 10-150 W are loaded on eight tiny hotspots of size 450 × 300 μm (heat flux on each hotspot 0.93-13.89 kW/cm2). An overall spatially averaged heat transfer coefficient of 11.53 × 104 W/m2K has been achieved in the microjet-based hybrid heat sink. Consistent results from the experimental and simulation studies have verified the high heat dissipation capability of the designed cooling solution. Several simulations have been conducted to investigate the effects of the heat sink structure and dimensions on the performances for hotspot thermal management.