The present disclosure describes a semiconductor structure and a method for forming the same. The semiconductor structure can include a substrate, a first fin structure with a first height and a first width formed over the substrate, a second fin structure with a second height and a second width formed over the substrate, and an insulating stack formed over lower portions of the first and second fin structures. The second height can be substantially equal to the first height and the second width can be greater than the first width. A top surface of the insulating stack can be below top surfaces of the first and second fin structures.