Abstract: An intermediate layer made of an Si.sub.1-X Ge.sub.X mixed crystal layer is formed on an Si crystal substrate, where the character X denotes a value in the range of 0 to 1. The intermediate layer is connected to the substrate via a heterojunction generating a first heterojunction barrier. An Si crystal layer is formed on the intermediate layer and is connected to the intermediate layer via a heterojunction generating a second heterojunction barrier. The substrate, the intermediate layer, and the Si layer form a laminated structure. The first and second heterojunction barriers form a charge storage well in an energy band of the intermediate layer. Charges in the well are excited by absorbing infrared light. The intermediate layer contains unevenly-distributed impurities whose concentration montonically varies in a direction along a thickness of the laminated structure. The variation in the concentration generates an internal electric field in the energy band of the intermediate layer.

Abstract: An infrared sensor includes a metal and a semiconductor contacted to each other via a rectifying potential barrier. The semiconductor includes a p-type strained Si.sub.1-X Ge.sub.X epitaxial layer grown on a p-type substrate, wherein the character X denotes a Ge atomic fraction and wherein X increases from zero to a predetermined upper limit in a direction away from the Si substrate toward the metal. The metal may be selected from the group of Pt, an alloy of Pt and Si, and an alloy of Pt, Si, and Ge. In addition, the metal may also be selected from the group of Ir, an alloy of Ir and Si, and an alloy of Ir, Si, and Ge.