Structure & manufacturing method for a fin field effect transistor

There are provided a structure and a manufacturing method of a semiconductor device. The semiconductor device includes a semiconductor substrate and a rib structure on the semiconductor substrate. The semiconductor device also includes a gate stack of a portion of the rib structure is covered, and an epitaxially grown source / drain structure above the fin structure and adjacent to the gate stack. The semiconductor device further includes a semiconductor protective layer over the epitaxially grown source / drain structure. The semiconductor protective layer has a higher silicon atom concentration than the epitaxially grown source / drain structure.