A semiconductor device includes at least one n-channel, at least one p-channel, at least one first high-k dielectric sheath, at least one second high-k dielectric sheath, a first metal gate electrode and a second metal gate electrode. The first high-k dielectric sheath surrounds the n-channel. The second high-k dielectric sheath surrounds the p-channel. The first high-k dielectric sheath and the second high-k dielectric sheath comprise different high-k dielectric materials. The first metal gate electrode surrounds the first high-k dielectric sheath. The second metal gate electrode surrounds the second high-k dielectric sheath.