A semiconductor device structure includes a layer of single crystal compound semiconductor material; and a layer of polycrystalline CVD diamond material. The layer of polycrystalline CVD diamond material is bonded to the layer of single crystal compound semiconductor material via a bonding layer having a thickness of less than 25 nm and a thickness variation of no more than 15 nm. The effective thermal boundary resistance as measured by transient thermoreflectance at an interface between the layer of single crystal compound semiconductor material and the layer of polycrystalline CVD diamond material is less than 25 m2K/GW with a variation of no more than 12 m2K/GW as measured across the semiconductor device structure. The layer of single crystal compound semiconductor material has one or both of the following characteristics: a charge mobility of at least 1200 cm2V−1s−1; and a sheet resistance of no more than 700 Ω/square.