High electron mobility transistor and method for fabricating the same

A high electron mobility transistor (HEMT), comprising:

• a buffer layer on a substrate;
• a first barrier layer on the buffer layer;
• a p-type semiconductor layer on the first barrier layer, wherein the p-type semiconductor layer comprises a rectangular shape in a cross-section view;
• a second barrier layer adjacent to two sides of the p-type semiconductor layer and on the first barrier layer, wherein the p-type semiconductor layer contacts the second barrier layer directly, the first barrier layer and the second barrier layer comprise III-V semiconductor, bottom surfaces of the second barrier layer and the p-type semiconductor layer are coplanar, and a top surface of the second barrier layer is lower than a top surface of the p-type semiconductor layer;
• a hard mask on the second barrier layer and the p-type semiconductor layer;
• a passivation layer on and directly contacting a top surface of the hard mask and sidewalls of the second barrier layer, the first barrier layer, and the buffer layer;
• a gate electrode on the p-type semiconductor layer, wherein a sidewall of the p-type semiconductor layer is aligned to a sidewall of the gate electrode and orthogonal to a top surface of the substrate;
• and
• a source electrode and a drain electrode adjacent to two sides of the gate electrode on the buffer layer.