Structure and formation method of semiconductor device with epitaxial structures

A semiconductor device structure, comprising:

• a channel structure over a semiconductor substrate;
• a gate stack wrapped around the channel structure;
• a source/drain epitaxial structure connecting the channel structures;
• an inner spacer between the gate stack and the source/drain epitaxial structure;
• and
• an epitaxial structure between the inner spacer and the source/drain epitaxial structure.

A method for forming a semiconductor device structure, comprising:

• forming a fin structure over a semiconductor substrate, wherein the fin structure has a plurality of sacrificial layers and a plurality of semiconductor layers, and the sacrificial layers and the semiconductor layers are laid out alternately;
• forming a dummy gate stack to cover a portion of the fin structure;
• partially removing the fin structure to form a first recess;
• partially removing the sacrificial layers exposed by the first recess to form a plurality of second recesses partially exposing the semiconductor layers;
• forming a plurality of inner spacers in the second recesses to cover edges of the sacrificial layers;
• forming epitaxial structures in the second recesses to cover the inner spacers;
• forming a source/drain epitaxial structure in the first recess;
• removing the dummy gate stack and the sacrificial layers to form a plurality of semiconductor nanostructures made of remaining portions of the semiconductor layers;
• and
• forming a metal gate stack to wrap around the semiconductor nanostructures.