Method for fabricating MOS device

A method for fabricating a metal oxide semiconductor (MOS) device is described, including following steps. A gate stack structure is formed on a substrate. A first spacer is formed at a sidewall of the gate stack structure, and then source and drain extension regions are formed in the substrate beside each sidewall of the gate stack structure. Recesses are formed in the substrate beside the gate stack structure. Semiconductor compound layers are formed in each recess, and source and drain regions are formed in semiconductor compound layers. A second spacer is formed at the sidewall of the gate stack structure. The source and drain regions are formed by in-situ doping during the formation of the semiconductor compound layer instead of performing an ion implantation process after the semiconductor compound layer is formed.