Lightly-doped channel extensions

A semiconductor structure, comprising:

• an active region comprising a channel region disposed between a first source/drain region and a second source/drain region;
• a first source/drain feature disposed over the first source/drain region;
• a second source/drain feature disposed over the second source/drain region;
• a plurality of channel members disposed over the channel region and extending between the first source/drain feature and the second source/drain feature along a direction;
• a first plurality of channel extension features disposed between the first source/drain feature and the plurality of channel members along the direction;
• and
• a bottom epitaxial feature disposed between the first source/drain region and the first source/drain feature as well as between the second source/drain region and the second source/drain feature,
• wherein the bottom epitaxial feature interfaces a surface of the first source/drain region.

A method, comprising:

• receiving a workpiece comprising:
• a substrate, and
• a fin-shaped structure over the substrate and comprising a channel region and a source/drain region adjacent the channel region, wherein the fin-shaped structure comprises a plurality of channel layers interleaved by a plurality of sacrificial layers;
• forming a dummy gate stack over the channel region;
• recessing the source/drain region to form a source/drain trench that exposes sidewalls of the plurality of channel layers and the plurality of sacrificial layers;
• selectively and partially recessing the plurality of sacrificial layers to form a plurality of inner spacer recesses;
• forming a plurality of inner spacer features in the plurality of inner spacer recesses;
• after the forming of the plurality of inner spacer features, selectively trimming the plurality of channel layers to form end surfaces;
• selectively depositing a semiconductor layer on the end surfaces to form channel extension features in contact with the end surfaces;
• after the selectively depositing, forming a source/drain feature in the source/drain trench to be in contact with the channel extension features;
• and
• after the forming of the source/drain feature, replacing the dummy gate stack with a gate structure,
• wherein the recessing also exposes a portion of the substrate,
• wherein the selectively depositing comprises depositing a bottom epitaxial layer over the exposed portion of the substrate.