Source/drain epi structure for device boost

A semiconductor structure, comprising:

• a substrate;
• a semiconductor fin extending from the substrate;
• a source/drain feature over the semiconductor fin;
• and
• a contact plug disposed on the source/drain feature, wherein the source/drain feature includes:
• a first semiconductor layer having silicon germanium and disposed on the semiconductor fin,
• a second semiconductor layer having silicon germanium above the first semiconductor layer and extending laterally wider than the semiconductor fin,
• a third semiconductor layer having silicon germanium over the second semiconductor layer, and
• a fourth semiconductor layer having silicon germanium and disposed at a corner portion of the source/drain feature where the source/drain feature has a largest lateral dimension,
• wherein each of the first, the second, the third, and the fourth semiconductor layers includes a p-type dopant, and the fourth semiconductor layer has a higher dopant concentration of the p-type dopant than the first, the second, and the third semiconductor layers.

A semiconductor structure, comprising:

• a substrate;
• a semiconductor fin extending from the substrate and having silicon germanium;
• a source/drain feature over the semiconductor fin;
• and
• a contact plug disposed on the source/drain feature, wherein the source/drain feature includes a first semiconductor layer, a second semiconductor layer above the first semiconductor layer, a third semiconductor layer over the second semiconductor layer, and a fourth semiconductor layer disposed at a corner portion of the source/drain feature where the source/drain feature has a largest lateral dimension,
• wherein each of the first, the second, the third, and the fourth semiconductor layers includes silicon germanium doped with boron, the fourth semiconductor layer has a higher boron doping concentration than the first, the second, and the third semiconductor layers, and a boron doping concentration in the fourth semiconductor layer gradually increases as a thickness of the fourth semiconductor layer increases.

A semiconductor structure, comprising:

• a substrate;
• a semiconductor fin extending from the substrate, wherein the semiconductor fin includes a base portion and an upper portion on the base portion, the upper portion of the semiconductor fin includes silicon germanium, the base portion of the semiconductor fin includes a different material than the upper portion of the semiconductor fin;
• a source/drain feature over the semiconductor fin;
• and
• a contact plug disposed on the source/drain feature, wherein the source/drain feature includes a first semiconductor layer, a second semiconductor layer above the first semiconductor layer, a third semiconductor layer over the second semiconductor layer, and a fourth semiconductor layer disposed at corners of the source/drain feature,
• wherein each of the first, the second, the third, and the fourth semiconductor layers includes silicon germanium doped with a p-type dopant, the fourth semiconductor layer has a higher doping concentration of the p-type dopant than the first, the second, and the third semiconductor layers, and a dopant concentration of the p-type dopant in the second semiconductor layer gradually increases and then gradually decreases as a thickness of the second semiconductor layer increases.