A semiconductor device and a fabrication method are provided. The method includes forming a first fin structure and a second fin structure on a substrate. The first fin structure includes a first sidewall surface, facing to the second fin structure, and a second sidewall surface opposite to the first sidewall surface. The method also includes forming an isolation layer to cover a portion of sidewall surfaces of the first fin structure and the second fin structure. The top surface of the isolation layer is lower than the top surfaces of the first fin structure and the second fin structure. The method further includes forming a first sidewall on the first sidewall surface; forming a first doped layer in the first fin structure; and forming a second doped layer in the second fin structure. The first sidewall covers a portion of a sidewall surface of the first doped layer.