Semiconductor device

A semiconductor device, comprising:

• an InP substrate;
• a first semiconductor layer made from Alx1Gay1In1-x1-y1As where 0≦x1≦1 and 0≦y1≦1;
• a second semiconductor layer made from Alx2Gay2In1-x2-y2As where 0≦x2≦1 and 0≦y2≦1;
• a third semiconductor layer made from Inx3Ga1-x3Asy3P1-y3 where 0≦x3≦1 and 0≦y3≦1;
• and
• a fourth semiconductor layer made from Inx4Ga1-x4Asy4P1-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.