A semiconductor device includes a substrate, a semiconductor fin, a silicon layer, a gate structure, gate spacers, and source/drain structures. The semiconductor fin is over the substrate. The silicon layer is over the semiconductor fin. The gate structure is over the silicon layer, in which the gate structure includes an interfacial layer over the silicon layer, a gate dielectric layer over the interfacial layer, and a gate electrode over the gate dielectric layer. The gate spacers are on opposite sidewalls of the gate structure and in contact with the interfacial layer of the gate structure, in which a bottom surface of the interfacial layer is higher than bottom surfaces of the gate spacers. The source/drain structures are on opposite sides of the gate structure.