Semiconductor light-receiving device with graded layer

A semiconductor photodetector comprising:

• a substrate consisting of a compound semiconductor crystal having a first conductivity type;
• a first electrode formed on a first surface of said substrate;
• a graded layer having stacked mixed crystal sublayers sequentially epitaxially grown on a second surface of said substrate and consisting of at least two types of compound semiconductors having the first conductivity type and a different composition ratio;
• a light-absorbing layer epitaxially grown on a surface of said graded layer and having a lattice constant smaller than that of an uppermost mixed crystal sublayer of said graded layer;
• a first conductive layer having a second conductivity type and formed by diffusing an impurity near a first region of a surface of said light-absorbing layer from said first region toward said graded layer;
• a second conductive layer formed on a surface of said first conductive layer, having a lattice constant substantially similar to but not equal to that of said uppermost layer of said graded layer, and having the second conductivity type;
• a capping layer, formed on said surface of said light-absorbing layer next to said second conductive layer, having a lattice constant substantially similar to but not equal to that of said uppermost layer of said graded layer, and consisting of a compound semiconductor mixed crystal having the first conductivity type;
• and
• a second electrode formed on a surface of said second conductive layer.