A device includes a semiconductor substrate, a semiconductor fin, a gate structure, a first source/drain epitaxy structure, a second source/drain epitaxy structure, a first dielectric fin sidewall structure, a second dielectric fin sidewall structure. The semiconductor fin is over the semiconductor substrate. The semiconductor fin includes a channel portion and recessed portions on opposite sides of the channel portion. The gate structure is over the channel portion of the semiconductor fin. The first source/drain epitaxy structure and the second source/drain epitaxy structure are over the recessed portions of the semiconductor fin, respectively. The first source/drain epitaxy structure has a round surface. The first dielectric fin sidewall structure and the second dielectric fin sidewall structure are on opposite sides of the first source/drain epitaxy structure. The round surface of the first source/drain epitaxy structure is directly above the first dielectric fin sidewall structure.