Nitride semiconductor device

A nitride semiconductor device comprising:

• (i) a first semiconductor layer, which satisfies Alx1Ga(1-x1)N,
• wherein x1 is greater than 0 and less than or equal to 1;
• (ii) a second semiconductor layer, which satisfies Iny2Alx2Ga(1-x2-y2)N,
• wherein x2 is greater than 0 and less than 1,
• wherein y2 is greater than 0 and less than 1, and
• wherein x2+y2 is greater than 0 and less than or equal to 1;
• (iii) a third semiconductor layer, which satisfies Alx3Ga(1-x3)N,
• wherein x3 is greater than or equal to 0 and less than 1;
• and
• (iv) a fourth semiconductor layer, which satisfies Iny4Alx4Ga(1-x4-y4)N,
• wherein x4 is greater than 0 and less than 1,
• wherein y4 is greater than or equal to 0 and less than 1, and
• wherein x4+y4 is greater than 0 and less than or equal to 1,
• wherein the second semiconductor layer is disposed on the first semiconductor layer,
• wherein the third semiconductor layer is disposed on the second semiconductor layer, and
• wherein the fourth semiconductor layer is disposed on the third semiconductor layer.