A semiconductor device includes a fin-shaped structure on the substrate, a shallow trench isolation (STI) around the fin-shaped structure, a single diffusion break (SDB) structure in the fin-shaped structure for dividing the fin-shaped structure into a first portion and a second portion; a first gate structure on the fin-shaped structure, a second gate structure on the STI, and a third gate structure on the SDB structure. Preferably, a width of the third gate structure is greater than a width of the second gate structure and each of the first gate structure, the second gate structure, and the third gate structure includes a U-shaped high-k dielectric layer, a U-shaped work function metal layer, and a low-resistance metal layer.