A method for preparing a semiconductor device includes forming a ring structure over a semiconductor substrate, and etching the semiconductor substrate by using the ring structure as a mask to form an annular semiconductor fin. The method also includes epitaxially growing a first bottom source/drain structure within the annular semiconductor fin and a second bottom source/drain structure surrounding the annular semiconductor fin. The method further includes forming a first silicide layer over the first bottom source/drain structure and a second silicide layer over the second bottom source/drain structure. In addition, the method includes forming a first gate structure over the first silicide layer and a second gate structure over the second silicide layer, and epitaxially growing a top source/drain structure over the annular semiconductor fin.