A semiconductor device includes a substrate, a gate dielectric layer, a gate electrode, a field plate, a source electrode and a drain electrode. The gate dielectric layer is disposed on the substrate and includes a first portion having a first thickness, a second portion having a second thickness, and a third portion having a third thickness. The first, second and third thicknesses are different from each other, and the first thickness is smaller than the second and third thicknesses. The gate electrode is disposed on the first portion of the gate dielectric layer. The field plate is separated from and electrically coupled to the gate electrode, and is disposed on the second and third portions of the gate dielectric layer. The source and drain electrodes are disposed on the sides of the gate electrode and the field plate, respectively.