Epitaxial source/drain structure and method of forming same

A device comprising:

• a gate stack over an active region;
• an epitaxial source/drain region in the active region adjacent the gate stack, the epitaxial source/drain region comprising silicon germanium, the epitaxial source/drain region comprising:
• a first semiconductor layer having a first boron concentration;
• a second semiconductor layer over the first semiconductor layer, the second semiconductor layer having a second boron concentration greater than the first boron concentration;
• a third semiconductor layer over the second semiconductor layer, the third semiconductor layer having a third boron concentration greater than the second boron concentration;
• and
• a fourth semiconductor layer over the third semiconductor layer, the fourth semiconductor layer having a fourth boron concentration less than the third boron concentration;
• and
• a contact plug over the epitaxial source/drain region, the contact plug extending through the fourth semiconductor layer and into the third semiconductor layer.

A method comprising:

• forming a gate stack over an active region;
• forming an opening in the active region adjacent the gate stack;
• forming a source/drain region in the opening, forming the source/drain region comprising:
• epitaxially growing a first semiconductor layer along a bottom and sidewalls of the opening, the first semiconductor layer having a first germanium concentration;
• epitaxially growing a second semiconductor layer over the first semiconductor layer, the second semiconductor layer having a second germanium concentration greater than the first germanium concentration;
• epitaxially growing a third semiconductor layer over the second semiconductor layer, the third semiconductor layer having a third germanium concentration greater than the second germanium concentration;
• and
• epitaxially growing a fourth semiconductor layer over the third semiconductor layer, the fourth semiconductor layer having a fourth germanium concentration less than the third germanium concentration;
• and
• forming a contact plug over the source/drain region, the contact plug extending through the fourth semiconductor layer and into the third semiconductor layer.