Embodiments of the invention provide an asymmetrical transistor device comprising a semiconductor substrate, a source region, a drain region and a channel region. The channel region is provided between the source and drain regions, the source, drain and channel regions being provided in the substrate. The device has a layer of a buried insulating medium provided below the source region and not below the drain region thereby forming an asymmetrical structure. The layer of buried insulating medium is provided in abutment with a lower surface of the source region.