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A package (base plate) level thermal management of high power density GaN High-Electron-Mobility-Transistors (HEMTs) is carried out by liquid micro-jet impingement and its subsequent phase change. Implemented on a 64-gate (9.6 mm gate periphery) device, the cooling technique demonstrates a 43% improvement in power density compared to the traditional air-cooling. Performance improvement could be significantly higher in a Monolithic Microwave Integrated Circuit (MMIC) where the internal thermal resistance (junction to case) of the device is much lower. In parallel, a high fidelity computational model is developed to explore the thermal field within the device and the peak device junction temperature. Practical methods to reduce the device temperature, such as variation of substrate thickness, are established through numerical simulation. For example, a 24% reduction in junction temperature or a 33% gain in power density is shown by reducing the SiC substrate thickness from 400 μm to 75 μm. Temperature rise due to local micro-scale hot spots (gate), gate-to-gate thermal interaction, and their combined effect towards peak junction temperature are investigated at various power levels.