A semiconductor device includes a semiconductor layer on an insulating layer, and a first partially depleted transistor and a first diode in the semiconductor layer. The first transistor has a first gate electrode above the semiconductor layer via an insulating film and a first source or drain of a first conductivity type in the semiconductor layer below both sides of the gate electrode. The first diode has a first impurity layer of a second conductivity type in a shallow portion of the semiconductor layer and a second impurity layer of the first conductivity type in a deep portion of the semiconductor layer. The first and second impurity layers are stacked in a depth direction of the semiconductor layer. The side surfaces of the first and second impurity layers contact the semiconductor layer just below the first gate electrode.