Patent ID: 6853015

Claim:
An optical semiconductor device having an active layer region of a quantum well structure comprising: a semiconductor substrate; and a first semiconductor layer, and a second semiconductor layer of a larger forbidden band gap than that of the first semiconductor layer, above the semiconductor substrate, the first and second semiconductor layers forming the quantum well structure, wherein the second semiconductor layer of the quantum well structure comprises a quaternary alloy of InGaAlAs and the second semiconductor layer is applied with modulation doping using Zn, and wherein the concentration of Zn is within a range surrounded by four points, a point at the concentration of Zn of 9.8×10 18 cm −3 for the Al compositional ratio of 0.04, a point at the concentration of Zn of 1.0×10 18 cm −3 for the Al compositional ratio of 0.04, a point at the concentration of Zn of 1.0×10 18 cm −3 for the Al compositional ratio of 0.44, and a point at the concentration of Zn of 1.3×10 18 cm −3 for the Al compositional ratio of 0.44 in a diagram for the relation of the Al compositional ratio and the concentration of Zn in the exponential expression to the Al composition of the second semiconductor layer.