Fabricating method of semiconductor device

A method of forming a semiconductor device, comprising:

• providing a substrate;
• forming a nucleation layer on the substrate;
• forming a buffer layer on the nucleation layer, the buffer layer comprising:
• a first superlattice layer, having at least two materials alternately arranged in a first periodically ordered relation in a horizontal direction;
• and
• a second superlattice layer, having a plurality of first layers and a plurality of second layers alternately stacked in a second periodically ordered relation along a vertical direction, wherein the plurality of the first layers and the plurality of the second layers each is consisted of a single material, and each simultaneously overlays the at least two materials of the first superlattice layer;
• forming an active layer on the buffer layer;
• and
• forming a gate electrode on the active layer.