The authors demonstrate epitaxial growth of two-dimensional-electron-gas (2DEG)-based Al _x Ga_1−x N/GaN heterostructures on a thin silicon-on-insulator (SOI) substrate. Cross-sectional transmission electron microscopy, high-resolution x-ray diffraction, photoluminescence, atomic force microscopy and ultraviolet (UV) micro-Raman spectroscopy measurements are performed to address the structural and interface properties of the epilayers on such a Si-based composite substrate. Device processing of Schottky metal–semiconductor–metal UV photodetectors is carried out to test the applicability of such a thin Al _x Ga_1−x N layer on a GaN/SOI template for UV sensors. The high sensitivity response of such a 2DEG Al _x Ga_1−x N/GaN heterostructure in the UV spectral range shows high potential for integration with SOI-based electronics and photonics.