Semiconductor devices and fabrication methods are provided. An exemplary fabrication method includes providing a semiconductor substrate; forming at least one gate structure having a gate dielectric layer on a surface of the semiconductor substrate; forming first sidewall spacers on a first sidewall surface region of the gate structure and covering sidewall surfaces of the gate dielectric layer; forming second sidewall spacers on a second sidewall surface region of the gate structure and top surfaces of the first sidewall spacers and made of a material different from a material of the first sidewall spacers; forming conductive plugs in the dielectric layer at both sides of the gate structure, the first sidewall spacers and the second sidewall spacers; and removing the second sidewall spacers to form air gap spacers above the first sidewall spacers and between the second sidewall surface region of the gate structure and the conductive plugs.