In this brief report, we discuss novel single crystal structures for electronic device and microelectromechanical system applications using processes that employ selective epitaxial growth (SEG) and silicon-on-insulator (SOI) wafers. Selective epitaxial growth of silicon is used to provide robust, reliable mechanical and electrical contacts between the SOI layer and the substrate. Subsequent removal of the buried oxide results in single crystal structures suspended in air. The films can then be thinned using wet or dry etching or thinned using sacrificial oxidation steps with the possibility of forming ultrathin SOI layers. Diodes formed at the substrate-SEG junction demonstrate high breakdowns and low leakage indicating good electrical isolation between the SOI layer and the substrate. The silicon on air regions can be used for dual-gate metal-oxide-semiconductor devices, quantum wires, cantilevers, as a substrate for lattice mismatched epitaxy, ultrathin SOIs, and lateral field emission tips.