Nanosheet devices with hybrid structures and methods of fabricating the same

A semiconductor structure, comprising:

• a first fin structure including a first stack of active channel layers and protruding from a substrate;
• a second fin structure including a second stack of active channel layers disposed over at least one dummy channel layer and protruding from the substrate;
• a gate structure engaged with the first stack and the second stack;
• a first source/drain (S/D) feature disposed in the first fin structure and adjacent to the first stack, wherein the first S/D feature includes a first doped epitaxial layer over an un-doped epitaxial layer;
• and
• a second S/D feature disposed in the second fin structure and adjacent to the second stack, wherein the second S/D feature includes a second doped epitaxial layer over the un-doped epitaxial layer, and wherein the un-doped epitaxial layer overlaps with the at least one dummy channel layer.

A method, comprising:

• providing a semiconductor substrate having a first region and a second region;
• forming a first fin protruding from the first region and a second fin protruding from the second region, wherein the first fin includes a first stack of active channel layers and the second fin includes a second stack of active channel layers disposed over at least one dummy channel layer, and wherein the active channel layers and the at least one dummy channel layer have different compositions;
• forming a dummy gate stack over the first fin and the second fin;
• forming first source/drain (S/D) features in the first fin and second S/D features in the second fin, wherein the second S/D features overlap with the at least one dummy channel layer;
• and
• replacing the dummy gate stack with a metal gate stack, wherein the metal gate stack is formed adjacent to the first S/D features and the second S/D features.