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This work develops a thermally stable micromachined AlGaN/GaN high-electron mobility transistor (HEMT) with an enhanced breakdown voltage. After removal of the Si substrate beneath the HEMT, a 300-nm SiO2 and a 20-μm copper layer are deposited to form the GaN-on-insulator (G.O.I.) structure. The self-heating at high current that is exhibited by GaN HEMTs that are by previously developed full substrate removal methods is eliminated. The need for complicated substrate-transfer technology is also eliminated, increasing chip package yield. The low frequency noise measurement results also demonstrate that the trap density of the buffer/transition layer is reduced by the removal of the substrate and micromachining of the HEMTs.