The present disclosure describes a semiconductor structure and a method for forming the same. The semiconductor structure can include a substrate, a gate structure over the substrate, and a source/drain (S/D) region adjacent to the gate structure. The S/D region can include first and second side surfaces separated from each other. The S/D region can further include top and bottom surfaces between the first and second side surfaces. A first separation between the top and bottom surfaces can be greater than a second separation between the first and second side surfaces.