Optical semiconductor device and optical transceiver

1. An optical semiconductor device comprising:

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,

wherein 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,

wherein 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; and

wherein the second semiconductor layer is formed on the first region, the fourth region, and the third region,

wherein the optical semiconductor device further includes:

a first metal film in ohmic contact with the first region; and

a second metal film in ohmic contact with the second region,

wherein the first region includes:

a sixth region in contact with the first metal film; and

a seventh region in contact with the fourth region, and

wherein the sixth region includes a p-type dopant at a higher concentration than the seventh region.

2. The optical semiconductor device as claimed in claim 1 , wherein

the third region is n-type, and

the second region includes an n-type dopant at a higher concentration than the third region.

3. The optical semiconductor device as claimed in claim 1 , wherein

a recess is formed in a region where the first semiconductor layer is in contact with the second semiconductor layer.

4. The optical semiconductor device as claimed in claim 1 , further comprising:

an opto-electric converter including the first semiconductor layer and the second semiconductor layer that overlap each other in a plan view,

a mode converter connected to the opto-electric converter, wherein

in the mode converter, the first semiconductor layer has a planar shape having a width that increases toward the opto-electric converter.

5. The optical semiconductor device as claimed in claim 1 , wherein

the first semiconductor layer is a Si layer, and

the second semiconductor layer is a Si x Ge 1-x layer (0≤x<1).

6. The optical semiconductor device as claimed in claim 1 , wherein

the first semiconductor layer is a Si layer, and

the second semiconductor layer is a Ge 1-x Sn x layer (0≤x<1).

7. An optical transceiver comprising:

the optical semiconductor device as claimed in claim 1 .