A semiconductor device and method of forming the same is disclosed. The semiconductor device includes a semiconductor substrate, a first fin and a second fin extending from the semiconductor substrate, a first lower semiconductor feature directly over the first fin, and a second lower semiconductor feature directly over the second fin. Each of the first and second lower semiconductor features includes a top surface bending downward towards the semiconductor substrate. The semiconductor also further includes an upper semiconductor feature directly over and in physical contact with the first and second lower semiconductor features. The semiconductor device further includes a dielectric layer on sidewalls of the first and second lower semiconductor features.