Optical semiconductor device

An optical semiconductor device comprising:

• an optical waveguide formed of a first semiconductor layer having a bandgap larger than a photon energy of light propagating through the optical waveguide;
• a light absorbing region coupled to the optical waveguide, formed of a second semiconductor layer disposed over only the first semiconductor layer, and having a bandgap smaller than the photon energy of the light to be used;
• a first conductive region and a second conductive region disposed at both sides of the light absorbing region so as to sandwich the light absorbing region;
• and
• a conductor coupled to the first conductive region and the second conductive region to let the first conductive region and the second conductive region short-circuit,
• wherein the light is propagated from the first semiconductor layer to the second semiconductor layer by evanescent coupling.