A semiconductor includes a substrate, a semiconductor fin, an STI structure, a fin sidewall spacer, and a doped silicon layer. The semiconductor fin extends from the substrate. The STI structure laterally surrounds a lower portion of the semiconductor fin. The fin sidewall spacer extends along a middle portion of the semiconductor fin that is above the lower portion of the semiconductor fin. The doped silicon layer wraps around three sides of an upper portion of the semiconductor fin that is above the middle portion of the semiconductor fin.