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In this work, assembly and packaging technologies for high-temperature high-power GaN high electron mobility transistors (HEMTs) are presented. GaN HEMTs with epitaxial growth on Silicon substrates were used during these experiments. Both die-attachment and interconnection techniques were investigated and a performance comparison is given before and after the assembly process. State-of-the-art silver sintering and transient liquid phase bonding were used as die-attachment methods [2], [3]. For the die-attach material, various characterizations such as shear strength, Energy Dispersive X-ray (EDX) spectroscopy and Differential Scanning Calorimetery (DSC) were performed to characterize the operation up to 500 °C. An estimation of the thermal behavior of the sintered and TLP-bonded GaN HEMTs is performed. For interconnection, gold- and palladium-based materials were investigated for wire-bonding. The complete bonding process was characterized. Estimations about the current carrying capabilities are made for both materials. Passive temperature cycling from -40 to +150 °C was performed as an indication of initial reliability for both dieattachments and interconnections. A systematic electrical characterization of HEMTs is performed starting from the on wafer measurements up to the final assembly process. The influence of thermal effects on the electrical properties, such as on-state resistance at higher power levels, i.e., 350 W were studied before and after the assembly process. A combination of sintered device with the gold wire bonds is considered as the optimum packaging of GaN HEMTs