Patent ID: RE41368

Claim:
An SOI semiconductor device comprising: a first semiconductor layer; a second semiconductor layer that is formed on a first part of a first main surface of the first semiconductor layer; a third semiconductor layer with a conductivity type different from a conductivity type of the second semiconductor layer, the third semiconductor layer being formed on a second part of the first main surface of the first semiconductor layer, the second part being separated from the first part; a fourth semiconductor layer with a conductivity type different from a conductivity type of the first semiconductor layer, the fourth semiconductor layer being formed on a second main surface of the first semiconductor layer; and a first insulating layer that is formed on a main surface of the fourth semiconductor layer opposite to the first semiconductor layer, wherein the fourth semiconductor layer includes an impurity of an amount greater than 3×10 12 /cm 2 which is not completely depleted even when a reverse bias voltage is applied between the second and third semiconductor layers, the reverse bias voltage making a potential of a drain lower than a potential of a source a potential of a source lower than a potential of a drain .