Patent ID: 7889773

Claim:
A semiconductor device, comprising: an InP substrate; a first semiconductor layer made from Al x1 Ga y1 In 1-x1-y1 As where 0≦x1≦1 and 0≦y1≦1; a second semiconductor layer made from Al x2 Ga y2 In 1-x2-y2 As where 0≦x2≦1 and 0≦y2≦1; a third semiconductor layer made from In x3 Ga 1-x3 As y3 P 1-y3 where 0≦x3≦1 and 0≦y3≦1; and a fourth semiconductor layer made from In x4 Ga 1-x4 As y4 P 1-y4 where 0≦x4≦1 and 0≦y4≦1; and wherein said first, second, third and fourth semiconductor layers are provided in order in a contacting relationship with each other over said InP substrate; said first and second semiconductor layers have compositions which are same as or substantially same as each other on an interface therebetween; the composition of said second semiconductor layer continuously varies such that a band gap continuously increases from the first semiconductor layer side toward the third semiconductor layer side; the compositions of said second and third semiconductor layers are set such that energy levels of a valence band maximum are equal to or substantially equal to each other on an interface between said second and third semiconductor layers; the composition of said third semiconductor layer continuously varies such that a band gap continuously increases from the second semiconductor layer side toward the fourth semiconductor layer side; and the compositions of said third and fourth semiconductor layers are same as or substantially same as each other on an interface between said third and fourth layers.