High electron mobility transistor and method of manufacturing the same

A high electron mobility transistor (HEMT) includes a substrate (30), an HEMT stack (60) spaced apart from the substrate, and a pseudo-insulation layer (PIL) (20) disposed between the substrate and the HEMT stack. The PIL includes at least two materials having different phases. The PIL defines an empty space (50) that is wider at an intermediate portion than at an entrance of the empty space.