To fabricate a semiconductor device, a fin is formed to protrude from a substrate. The fin is extended in a first direction. A gate line is formed on the fin and the substrate. The gate line is extended in a second direction crossing the first direction. An amorphous material layer is conformally formed to cover the substrate, the fin, and the gate line. The amorphous material layer is partially removed, thereby forming a first remaining amorphous layer on side walls of the fin and a second remaining amorphous layer on side walls of the gate line. The first remaining amorphous layer and the second remaining amorphous layer are annealed and the first remaining amorphous material layer and the second remaining amorphous material layer are crystallized into a monocrystalline material layer and a polycrystalline material layer, respectively. The polycrystalline material layer is removed.