A compound semiconductor substrate, such as a GaAs substrate, is eutectically bonded to a metallic substrate (400). The semiconductor substrate is optionally thinned, and vias or trenches (601) are formed from the front side for making easily aligned interconnects to the metal substrate (400). The mechanical support given by the metallic substrate (400) permits the vias or trenches (601) to be any shape. Trenches (601) can surround a particular circuit element, to provide thermal isolation or heat spreading, or (in combination with metallic air bridges) to provide electromagnetic screening. Trench structures (601) also provide lower impedance ground connections at very high frequencies, in comparison to standard via holes. The metallic substrate (600) can be used as a ground plane or as a heat sink.