Semiconductor source/drain regions comprising multi-layers with different dopant concentrations and methods of forming the same

A device comprising:

• a first nanostructure over a substrate;
• a second nanostructure over the first nanostructure and the substrate, wherein the first nanostructure vertically overlaps the second nanostructure;
• and
• a first source/drain region, the first source/drain region comprising:
• a first epitaxial layer having a first portion and a second portion, the first portion of the first epitaxial layer covering a first sidewall of the first nanostructure, the second portion of the first epitaxial layer covering a second sidewall of the second nanostructure, the first portion of the first epitaxial layer having a first thickness measured at a midpoint of the first nanostructure, the first epitaxial layer having a second thickness measured at a point level with a top surface of the first nanostructure, a ratio of the second thickness to the first thickness being in a range of 0.7 to 1.0;
• and
• a second epitaxial layer between the first portion of the first epitaxial layer and the second portion of the first epitaxial layer.

A device comprising:

• a first nanostructure over a substrate
• a first inner spacer in contact with the first nanostructure;
• and
• a first source/drain region in contact with the first nanostructure, the first source/drain region comprising:
• a first epitaxial layer covering a first sidewall of the first nanostructure and a first portion of a sidewall of the first inner spacer, wherein the first epitaxial layer has a round convex profile opposite the first sidewall of the first nanostructure in a cross-sectional view;
• a second epitaxial layer covering the round convex profile of the first epitaxial layer and being in physical contact with a second portion of the sidewall of the first inner spacer, wherein the first epitaxial layer comprises a first dopant with a first concentration and the second epitaxial layer comprises the first dopant with a second concentration different from the first concentration;
• and
• a third epitaxial layer on the second epitaxial layer, the third epitaxial layer having a convex top surface in the cross-sectional view, the third epitaxial layer having a third concentration of the first dopant, the third concentration being greater than the first concentration and less than the second concentration.