Device structures for a metal-oxide-semiconductor field effect transistor (MOSFET) that is suitable for operation at relatively high voltages and methods of forming same. The MOSFET, which is formed using a semiconductor-on-insulator (SOI) substrate, includes a channel in a semiconductor body that is self-aligned with a gate electrode. The gate electrode and semiconductor body, which are both formed from the monocrystalline SOI layer of the SOI substrate, are separated by a gap that is filled by a gate dielectric layer. The gate dielectric layer may be composed of thermal oxide layers grown on adjacent sidewalls of the semiconductor body and gate electrode, in combination with an optional deposited dielectric material that fills the remaining gap between the thermal oxide layers.