Semiconductor device with tunable channel layer usage and methods of fabrication thereof

A method of fabricating a semiconductor device, comprising:

• forming an epitaxial stack of channel layers and sacrificial layers on a semiconductor substrate, the channel layers and the sacrificial layers having different material compositions and being alternatingly stacked in a vertical direction;
• patterning the epitaxial stack to form a first semiconductor fin in a first region of the semiconductor substrate and a second semiconductor fin in a second region of the semiconductor substrate;
• recessing the first semiconductor fin in a first source/drain region;
• recessing the second semiconductor fin in a second source/drain region;
• forming an epitaxial layer in the first and second source/drain regions;
• forming a dielectric layer on the epitaxial layer in the first and second source/drain regions, wherein a top surface of the dielectric layer in the first source/drain region is above the top surface of the dielectric layer in the second source/drain region;
• and
• forming a first source/drain feature in the first source/drain region and a second source/drain feature in the second source/drain region, wherein the second source/drain feature is in contact with a bottommost channel layer in the second semiconductor fin, and the first source/drain feature is free of contact with a bottommost channel layer in the first semiconductor fin.

A method, comprising:

• forming a stack of channel layers and sacrificial layers on a substrate, the channel layers and the sacrificial layers having different material compositions and being alternatingly disposed in a vertical direction;
• patterning the stack to form a fin-shaped structure;
• recessing a region the fin-shaped structure to form a trench;
• depositing inner spacers in the trench and on sidewalls of the sacrificial layers;
• depositing a first epitaxial layer in the trench, wherein a top surface of the base epitaxial layer is below a top surface of a bottommost one of the channel layers;
• depositing an insulation layer in the trench and on the first epitaxial layer, wherein a top surface of the insulation layer is above the top surface of the bottommost one of the channel layers;
• and
• forming a second epitaxial layer in the trench and on the insulating layer, wherein the second epitaxial layer abuts at least a topmost one of the channel layers.