Patent ID: 7573060

Claim:
An optical semiconductor device comprising: a lower cladding layer made of semiconductor material of a first conductivity type; an active layer disposed over the lower cladding layer and having a p-type quantum dot structure; an upper cladding layer disposed over the active layer, made of semiconductor material, and comprising a ridge portion and a cover portion, the ridge portion extending in one direction on a surface of the active layer, and the cover portion covering the surface of the active layer on both sides of the ridge portion; and a capacitance reducing region disposed on both sides of the ridge portion and reaching at least a lower surface of the cover portion from an upper surface of the cover portion, wherein: the capacitance reducing region has the first conductivity type or a higher resistivity than a resistivity of the ridge portion, and the ridge portion has a second conductivity type opposite to the first conductivity type; and if the lower cladding layer is an n-type, the capacitance reducing region reaches at least an upper surface of the lower cladding layer.