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In this letter, we present a novel approach to enhance the breakdown voltage (VBD) for AlGaN/GaN/AlGaN double-heterostructure FETs (DHFETs), grown by metal-organic chemical vapor deposition on Si (111) substrates through a silicon-substrate-removal and a layer-transfer process. Before removing the Si substrate, both buffer isolation test structures and DHFET devices showed a saturation of VBD due to the electrical breakdown through the Si substrate. We observed a VBD saturation of 500 V for isolation gaps larger than 6 μm . After Si removal, we measured a VBD enhancement of the AlGaN buffer to 1100 V for buffer isolation structures with an isolation gap of 12 μm. The DHFET devices with a gate-drain (LGD) distance of 15 μm have a VBD > 1100 V compared with ~300 V for devices with Si substrate. Moreover, from Hall measurements, we conclude that the substrate-removal and layer-transfer processes have no impact on the 2-D electron gas channel properties.