High electron mobility transistor and fabrication method thereof

A method for fabricating a high-electron mobility transistor (HEMT), comprising:

• providing a substrate;
• forming a channel layer on the substrate;
• forming an AlGaN layer on the channel layer;
• forming a P—GaN gate on the AlGaN layer;
• and
• after forming the P—GaN gate on the AlGaN layer, subjecting the AlGaN layer to a thermal treatment, thereby forming a first region comprising an entire thickness of the AlGaN layer located directly under the P—GaN gate and a second region in the AlGaN layer, wherein the second region does not overlap with the first region in a thickness direction of the AlGaN layer, wherein the first region has a composition that is different from that of the second region, and wherein a thickness of the AlGaN layer in the first region is the same as a thickness of the AlGaN layer in the second region.