Leakage reduction for multi-gate devices

A semiconductor structure, comprising:

• an active region comprising a base fin portion and a top portion disposed over the base fin portion;
• a gate structure wrapping over a channel region of the active region;
• a source/drain feature disposed over and extending into a source/drain region of the active region, the source/drain feature comprising a first epitaxial layer and a second epitaxial layer disposed over the first epitaxial layer;
• and
• a diffusion stop layer sandwiched vertically between the first epitaxial layer and the source/drain region,
• wherein the base fin portion comprises silicon,
• wherein the first epitaxial layer, the second epitaxial layer, the diffusion stop layer, and the top portion comprise silicon germanium,
• wherein a germanium concentration in the diffusion stop layer is greater than a germanium concentration of the top portion.

A semiconductor structure, comprising:

• a fin structure comprising a bottom portion and a top portion disposed on the bottom portion, the fin structure comprising a source/drain region and a channel region adjacent the source/drain region;
• a source/drain feature disposed over and extending into the source/drain region, the source/drain feature comprising a first epitaxial layer and a second epitaxial layer disposed over the first epitaxial layer;
• and
• a diffusion stop layer sandwiched vertically between the first epitaxial layer and the source/drain region,
• wherein the diffusion stop layer does not extend into the bottom portion of the fin structure,
• wherein the top portion comprises a first germanium concentration, the diffusion stop layer comprises a second germanium concentration and the first epitaxial layer comprises a third germanium concentration,
• wherein the second germanium concentration is different from the first germanium concentration or the third germanium concentration.