Semiconductor device structure and methods of forming the same

A semiconductor device structure, comprising:

• a source/drain epitaxial feature over a substrate;
• a plurality of semiconductor layers over the substrate, and each semiconductor layer of the plurality of the semiconductor layers being in contact with the source/drain epitaxial feature;
• a gate electrode layer surrounding a portion of one of the plurality of the semiconductor layers;
• and
• a non-doped silicon layer, comprising:
• a first side in contact with a first side of the source/drain epitaxial feature;
• a second side opposing the first side and in contact with the substrate;
• a third side in contact with the substrate;
• and
• a fourth side opposing the third side and in contact with the substrate.

A semiconductor device structure, comprising:

• a source/drain epitaxial feature over a well;
• a plurality of semiconductor layers over the well, each semiconductor layer of the plurality of semiconductor layers being in contact with the source/drain epitaxial feature;
• a gate electrode layer surrounding a portion of one of the plurality of semiconductor layers;
• a first dopant region positioned between the source/drain epitaxial feature and the well, the first dopant region having a first dopant concentration that exceeds a second dopant concentration of the well;
• and
• a second dopant region positioned between the first dopant region and the well, the well having the second dopant concentration that exceeds a third dopant concentration of the second dopant region.