A method is provided for fabricating a semiconductor device. The method includes providing a semiconductor substrate having a gate structure; and forming offset sidewall spacers around the gate structure. The method also includes forming trenches in the semiconductor substrate at outside of the gate structure; and forming isolation layers on side surfaces of the trenches to prevent diffusions between subsequently formed doping regions. Further, the method includes removing at least portions of the offset sidewall spacers to expose portions of the surface of the semiconductor substrate between the gate structure and the trenches; and forming filling layers with a top surface higher than the surface of the semiconductor substrate by filling the trenches and covering portions of the surface of the semiconductor substrate between the trenches and the gate structure. Further, the method also includes forming doping regions configured as raised source/drain regions in the filling layers.