Semiconductor structure and high-electron mobility transistor device having the same

A semiconductor structure, comprising:

• a substrate;
• a seed layer on the substrate;
• and
• an epitaxial stack on the seed layer, the epitaxial stack comprising:
• a first superlattice part on the seed layer, wherein the first superlattice part comprises first units repetitively stacked M1 times on the seed layer, and each of the first units comprises:
• a first sub-layer including an Aly1Ga1-y1N layer with a thickness of b1 nanometers (nm);
• and
• a second sub-layer on the first sub-layer, the second sub-layer including an Alx1Ga1-x1N layer with a thickness of a1 nanometers (nm), wherein the first sub-layer is closer to the substrate than the second sub-layer, and y1 is less than x1;
• and
• a second superlattice part on the first superlattice part, wherein the second superlattice part comprises second units repetitively stacked M2 times on the first superlattice part, and each of the second units comprises:
• a third sub-layer including an Aly2Ga1-y2N layer with a thickness of b2 nanometers (nm);
• and
• a fourth sub-layer on the third sub-layer, the fourth sub-layer including an Alx2Ga1-x2N layer with a thickness of a2 nanometers (nm), wherein the third sub-layer is closer to the substrate than the fourth sub-layer, and y2 is less than x2,
• wherein M1 and M2 are positive integers, x1, y1 and y2 are greater than 0 and less than 1, x2 is greater than 0 and equal to or less than 1;
• and
• wherein x1 is less than x2, or x1 is equal to x2 and y1 is less than y2.