A disclosed optical semiconductor device includes a first semiconductor layer having a first refractive index and a first optical absorption coefficient; and a second semiconductor layer formed on the first semiconductor layer, the second semiconductor layer having a second refractive index and a second optical absorption coefficient. The second refractive index is larger than the first refractive index, and the second optical absorption coefficient is larger than the first optical absorption coefficient. The first semiconductor layer includes a first region of p-type, a second region of n-type, a third region of p-type or n-type between the first region and the second region, a fourth region of i-type between the first region and the third region, and a fifth region of i-type between the second region and the third region. The second semiconductor layer is formed on the first region, the fourth region, and the third region.