A high electron mobility transistor (HEMT) is disclosed. The HEMT includes a substrate, a first epitaxial layer disposed on the substrate, a second epitaxial layer disposed on the first epitaxial layer, a third epitaxial layer disposed on the second epitaxial layer, and a gate disposed on the third epitaxial layer. An upper portion of the first epitaxial layer has a plurality of first recesses. The second epitaxial layer partially fills the first recesses and surrounding a plurality of first air slits in the first recesses.