A method for forming a semiconductor device is disclosed. A substrate including a gate dielectric layer and a gate electrode layer sequentially formed thereon is provided. An offset spacer is formed on sidewalls of the gate dielectric layer and the gate electrode layer. A carbon spacer is formed on a sidewall of the offset spacer, and the carbon spacer is then removed. The substrate is implanted to form a lightly doped region using the gate electrode layer and the offset spacer as a mask. The method may also include providing a substrate having a gate dielectric layer and a gate electrode layer sequentially formed thereon. A liner layer is formed on sidewalls of the gate electrode layer and on the substrate. A carbon spacer is formed on a portion of the liner layer adjacent the sidewall of the gate electrode layer. A main spacer is formed on a sidewall of the carbon spacer. The carbon spacer is removed to form an opening between the liner layer and the main spacer. The opening is sealed by a sealing layer to form an air gap.