A semiconductor structure includes a substrate; an isolation structure over the substrate; a first fin extending from the substrate and through the isolation structure; a first source/drain structure over the first fin; a contact etch stop layer over the isolation structure and contacting a first side face of the first source/drain structure; and a first dielectric structure contacting a second side face of the first source/drain structure. The first side face and the second side face are on opposite sides of the first fin in a cross-sectional view cut along a widthwise direction of the first fin. The first dielectric structure extends higher than the first source/drain structure.