An HEMT includes a first III-V compound layer. A second III-V compound layer is disposed on the first III-V compound layer. The composition of the first III-V compound layer is different from that of the second III-V compound layer. A gate is disposed on the second III-V compound layer. The gate includes a first P-type III-V compound layer, an undoped III-V compound layer and an N-type III-V compound layer are deposited from bottom to top. The first P-type III-V compound layer, the undoped III-V compound layer, the N-type III-V compound layer and the first III-V compound layer are chemical compounds formed by the same group III element and the same group V element. A drain electrode is disposed at one side of the gate. A drain electrode is disposed at another side of the gate. A gate electrode is disposed directly on the gate.