FinFET and fabrication method thereof

A fin field effect transistor (FinFET), comprising:

• a substrate including an NMOS (N-type metal-oxide-semiconductor) region and a PMOS (P-type metal-oxide-semiconductor) region;
• a plurality of fins formed on the substrate;
• an isolation layer formed between adjacent fins of the plurality of fins and on the substrate;
• a gate structure across a length portion of the plurality of fins and covering a portion of each of a top surface and sidewalls of the plurality of fins;
• an in-situ doped epitaxial layer formed on one of the plurality of fins on both sides of the gate structure in the NMOS region, wherein doping ions in the in-situ doped epitaxial layer are N-type ions;
• and
• a P-type source-drain doped region formed another one of the plurality of fins on both sides of the gate structure in the PMOS (P-type metal-oxide-semiconductor) region;
• wherein a bottom full width of the P-type source-drain doped region is greater than a maximum width of the another one of the plurality of fins in a direction perpendicular to an extension direction of the plurality of fins, and a bottom full width of the in-situ doped epitaxial layer is greater than a maximum width of the one of the plurality of fins in the direction perpendicular to the extension direction of the plurality of fins.