A semiconductor device is provided, which includes an active region, a first structure, a second gate structure, a first gate dielectric sidewall, a second gate dielectric sidewall, a first air gap region, a second air gap region and a contact structure. The active region is formed over a substrate. The first and second gate structures are formed over the active region and between the first gate structure and the second gate structure are the first gate dielectric sidewall, the first air gap region, the contact structure, the second air gap region and a second gate dielectric sidewall.