Semiconductor device and manufacturing method thereof

A method for manufacturing a semiconductor device, comprising:

• forming a plurality of first semiconductor layers vertically stacked over a substrate, wherein the first semiconductor layers are vertically spaced apart from each other;
• forming a plurality of second semiconductor layers vertically stacked over a substrate, wherein the second semiconductor layers are vertically spaced apart from each other;
• forming a p-type source/drain epitaxial structure on sides of the first semiconductor layers;
• performing an ion implantation process and a first annealing process to diffuse a p-type doping species in the p-type source/drain epitaxial structure into a topmost one of the first semiconductor layers;
• after the ion implantation process and the annealing process, forming a n-type source/drain epitaxial structure on sides of the second semiconductor layers;
• performing a second annealing process to activate the p-type source/drain epitaxial structure and the n-type source/drain epitaxial structure;
• forming a first source/drain contact over the p-type source/drain epitaxial structure;
• and
• forming a second source/drain contact over the n-type source/drain epitaxial structure.

A semiconductor device, comprising:

• a plurality of semiconductor layers vertically spaced apart from each other;
• a gate structure wrapping around the semiconductor layers;
• a gate spacer alongside the gate structure;
• and
• a p-type source/drain epitaxial structure on sides of the semiconductor layers, wherein the p-type source/drain epitaxial structure comprises a p-type dopant species, and a side portion of a topmost one of the semiconductor layers directly below the gate spacer has a concentration of the p-type dopant species greater than that of a side portion of a second topmost one of the semiconductor layers.