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A 1.5-μm polycrystalline diamond was deposited on the AlGaN/GaN heterojunction on SiC substrate with a 20-nm SiN dielectric layer. A (5.8 ± 0.1)% increase in 2DEG density after the diamond growth due to the increase in tensile strain of GaN layer was confirmed by micro-Raman measurements. Top-side integration of polycrystalline diamond heat spreader with AlGaN/GaN high electron mobility transistors was demonstrated. Using a dry/wet combined etching process for the removal of the diamond and SiN interlayer, the integration fabrication of diamond heat spreader and GaN HEMTs with low surface damage was achieved. After the diamond growth, Off-state drain current shows no degradation. Besides, both the drain current and peak transconductance of the GaN HEMTs were enhanced. Temperature reduction of 25 °C at power density of 20 W/mm was achieved by electro-thermal simulation using the thermal properties determined by TDTR measurements. [Display omitted] • Diamond was deposited on the AlGaN/GaN heterojunction with a 20-nm SiN layer. • A dry/wet combined etching was used for the removal of diamond and SiN interlayer. • No degradation of the off-state drain current was achieved after device fabrication. • Drain current and peak transconductance of the GaN HEMTs were enhanced. [ABSTRACT FROM AUTHOR]