Fin field effect transistor (finfet) device and method for forming the same

A fin field effect transistor (FinFET) device structure, comprising:

• a substrate;
• an isolation structure formed on the substrate;
• a fin structure extending above the substrate, wherein the fin structure is embedded in the isolation structure;
• and
• an epitaxial structure formed on a first portion of the fin structure, wherein a cross section of the epitaxial structure has a pentagon-like shape, an interface between the epitaxial structure and the first portion of fin structure is lower than a top surface of the isolation structure, a top of the first portion of the fin structure is lower than the top surface of the isolation structure, and a width of a bottom side of the pentagon-like shape is smaller than a width of the pentagon-like shape at a location level with the top surface of the isolation structure.