High-voltage AlGaN/GaN HEMTs fabricated on a GaN-on-SOI platform were demonstrated. The GaN-on-SOI wafer features III-nitride epi-layers grown by MOCVD on a modified SOI wafer consisting of a p-type (111) Si device layer, a SiO2 buried oxide and a p-type (100) Si handle substrate. Depletion-and enhancement-mode HEMTs are monolithically integrated. The Enhancement-mode HEMTs obtained by fluorine plasma implantation technique deliver large ON/OFF current ratio (107), large breakdown voltage (1354 V with floating substrate) and low ON-resistance (3.9 m Omega.cm(2)). In addition, the impact of the buried oxide on thermal dissipation is estimated by a simple thermal resistance model.