A device includes a semiconductor fin, a first transistor, a second transistor and a dielectric structure. The first semiconductor fin extends from a substrate. The first transistor is formed on a first region of the semiconductor fin. The second transistor is formed on a second region of the semiconductor fin laterally spaced apart from the first region of the semiconductor fin. The dielectric structure has a lower portion extending in the semiconductor fin and between the first transistor and the second transistor. The lower portion of the dielectric structure has a width increasing from a bottommost position of the dielectric structure to a first position higher than the bottommost position of the dielectric structure and decreasing from the first position to a second position higher than the first position.