A method includes epitaxially growing a first III-V compound layer over a semiconductive substrate. A second III-V compound layer is epitaxially grown over the first III-V compound layer. A source/drain contact is formed over the second III-V compound layer. A gate structure is formed over the second III-V compound layer. A pattern is formed shielding the gate structure and the source/drain contact, in which a portion of the second III-V compound layer is free from coverage by the pattern.