US Patent 7105910 Semiconductor device having SOI construction

A semiconductor device includes: a semiconductor substrate including a first semiconductor layer, an insulation layer and a second semiconductor layer, which are laminated in this order; a trench penetrating both of the second semiconductor layer and the insulation layer and reaching the first semiconductor layer; and a third semiconductor layer. The trench has a ring shape on a principal surface of the substrate so that a part of the second semiconductor layer and a part of the insulation layer are surrounded with the trench. The third semiconductor layer is disposed in the trench through a first insulation film disposed on a sidewall of the trench so that the third semiconductor layer contacts the first semiconductor layer at a bottom of the trench.