A MOSFET device (100) in a mono-crystalline semiconductor material (101) of a first conductivity type, which comprises a source and a drain of the opposite conductivity type, each having regions of polycrystalline semiconductor (110, 120) and respective junctions (112a, 122a) in monocrystalline semiconductor. Localized buried insulator regions (113, 123) are below the polycrystalline source and drain regions, and a gate (130) between the source and drain regions is located so that the gate channel (134) is formed in bulk mono-crystalline semiconductor material. As an example, the semiconductor is silicon, the first conductivity type is p-type, and the localized buried insulator is silicon dioxide. The semiconductor material may also include silicon germanium.