The ridge waveguide type photo semiconductor device includes: a first
cladding layer of a first conductivity type formed on a semiconductor
substrate of the first conductivity type; an active layer formed on the
first cladding layer; a second cladding layer of a second conductivity
type formed on the active layer; a third cladding layer of the second
conductivity type formed on the second cladding layer and being worked so
as to have a ridge shape; and an impurity diffusion region formed in the
second cladding layer and the active layer on both sides of the
ridge-shaped third cladding layer and which has a higher resistance value
than that of the second cladding layer below the ridge shape.
