TECHNOLOGY FOCUS - ELECTRICAL POWER AND ENERGY - Preferred Components: The semiconductor layer comprises a pair of i-type semiconductor layers made of the first compound semiconductor material sandwiching the quantum layer. It comprises at least three stacked i-type semiconductor layers made of the first compound semiconductor material and the quantum well layers inserted between the i-type semiconductor layers. The projections on the quantum well layers between the three stacked i-type semiconductor layers are arranged in increasing order of size from the quantum well layer closer to the p-type or n-type semiconductor layer which serves as a light receiving surface to the quantum well layer farther from the light receiving surface.Preferred Process: On the quantum well layer formed on the semiconductor base layer, the formation of a semiconductor intermediate layer and another quantum well layer is carried out at least once, and then forming the semiconductor cap layer is carried out. The quantum well layer is formed by providing a base portion of the second compound semiconductor material by lithography and providing projections of different sizes on the base portion by selective etching or spontaneously via crystal growth on the base portion by making use of a self-growing mechanism of the second compound semiconductor material. It is formed by molecular beam epitaxy in which a source of the second compound semiconductor material is evaporated under vacuum and the source is supplied on an underlying layer to deposit the source into a thin film crystal, while controlling the source supply by observing a reflection high energy electron diffraction pattern.    TECHNOLOGY FOCUS - INORGANIC CHEMISTRY - Preferred Materials: A lattice constant of the first compound semiconductor material is 101.5-107.5% of a lattice constant of the second compound semiconductor material. The first compound semiconductor material is gallium arsenide (GaAs) and the second compound semiconductor material is indium gallium arsenide (InGaAs). The InGaAs is InxGa1-xAs, where x is greater than 0.2 but less than 1. The second compound semiconductor is made of a Group III element and a Group V element in the Periodic table and the lattice constant or the forbidden band width of the source of the material to be supplied is varied by changing the compound crystal ratio of the elements.