A high electron mobility transistor (HEMT) includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer and is different from the first III-V compound layer in composition. A carrier channel is located between the first III-V compound layer and the second III-V compound layer. A source feature and a drain feature are disposed on the second III-V compound layer. Each of the source feature and the drain feature comprises a corresponding intermetallic compound at least partially embedded in the second III-V compound layer. Each intermetallic compound is free of Au and comprises Al, Ti or Cu. A p-type layer is disposed on a portion of the second III-V compound layer between the source feature and the drain feature. A gate electrode is disposed on the p-type layer. A depletion region is disposed in the carrier channel and under the gate electrode.