Abstract
The invention provides a semiconductor device and a manufacturing method thereof. The semiconductor device includes a Group III-V material layer and a gate structure. The gate structure includes a first portion and a second portion on the first portion. The first portion is located on the Group III-V material layer. The first portion has a first surface and a second surface opposite the first surface and adjacent the Group III-V material layer. A length of the second surface of the first portion of the gate structure is less than a length of the first surface of the first portion of the gate structure. A length of the second portion of the gate structure is less than the length of the first portion of the gate structure.