A semiconductor device and tunnel field-effect transistor, and methods of fabrication thereof are provided. The device includes first and second semiconductor regions, an intermediate region, and an epitaxial layer. The intermediate region separates the first and second semiconductor regions, and the epitaxial layer extends at least partially between the first and second regions over or alongside of the intermediate region. A gate electrode is provided for gating the circuit structure. The epitaxial layer is disposed to reside between the gate electrode and at least one of the first semiconductor region, the second semiconductor region, or the intermediate region. The epitaxial layer includes an epitaxially-grown, ultra-thin body layer of semiconductor material with a thickness less than or equal to 15 nanometers. Where the semiconductor device is a tunneling field-effect transistor, the intermediate region may be a large band-gap semiconductor region, with a band-gap greater than that of the epitaxial layer.