SOI substrate and semiconductor integrated circuit device

A semiconductor integrated circuit device comprising:

• a substrate comprising a first semiconductor layer of a first conductivity type disposed in said substrate, and a second semiconductor layer of a second conductivity type which is disposed in said substrate below said first semiconductor layer, an upper surface of said second semiconductor layer contacting said first semiconductor layer substantially in an entire upper surface of said second semiconductor layer;
• a first potential being applied to the first semiconductor layer;
• an insulating film disposed on said substrate;
• an upper surface of said first semiconductor layer contacting said insulating film substantially in an entire upper surface of said first semiconductor layer;
• a semiconductor region disposed on the insulating film;
• a third semiconductor layer of a second conductivity type which is disposed in said substrate below said first and second semiconductor layers, and an integrated circuit disposed in the semiconductor region, wherein an impurity concentration of said second semiconductor layer is higher than that of said third semiconductor layer, wherein a second potential is being applied to said second semiconductor layer, wherein a decoupling capacitor is formed between said first and second semiconductor layers, wherein the second semiconductor layer is disposed on the entire upper surface of the third semiconductor layer, and the first semiconductor layer is disposed on the entire upper surface of the second semiconductor layer.