Fin field effect transistor device structure and method for forming the same

A method for forming a fin field effect transistor device structure, comprising:

• forming a fin structure protruding from a substrate;
• forming a liner layer and an isolation structure surrounding the fin structure;
• recessing the liner layer and the isolation structure;
• growing a cap layer over the fin structure;
• forming a first gate dielectric layer covering the cap layer, the liner layer, and the isolation structure, wherein a portion of the cap layer is consumed;
• forming a gate structure over the first gate dielectric layer across the fin structure;
• growing source/drain epitaxial structures on opposite sides of the gate structure;
• and
• forming a contact structure over the gate structure.

A fin field effect transistor device structure, comprising:

• a fin structure formed over a substrate;
• a liner layer and an isolation structure surrounding the fin structure;
• a gate dielectric layer formed over the fin structure and the isolation structure;
• a gate structure formed over the gate dielectric layer;
• and
• source/drain epitaxial structures formed on opposite sides of the gate structure;
• wherein the fin structure comprises a protruding portion laterally extending over the liner layer.