Semiconductor device and manufacturing method thereof

A semiconductor device, comprising a p-channel gate-all-around field effect transistor (p-GAA FET) and an n-channel gate-all-around around field effect transistor (n-GAA FET), wherein:

• each of the p-GAA FET and the n-GAA-FET comprises:
• a bottom fin structure disposed over a substrate;
• a first channel region disposed over the substrate;
• and
• a source/drain (S/D) structure formed over the bottom fin structure, and in the p-GAA FET:
• the S/D structure includes a first S/D layer and a second S/D layer having a different lattice constant from the first S/D layer,
• the first S/D layer is connected to the first channel region,
• the first S/D layer is made of a different material than the first channel region, and
• the second S/D layer is completely surrounded by the first S/D layer.

A semiconductor device including a gate-all-around field effect transistor (GAA FET), the GAA FET, comprising:

• a first channel region disposed over a substrate;
• a second channel region disposed over the substrate at a different height than the first channel region;
• a gate structure wrapping around the first channel region and the second channel region;
• a first source region disposed over the substrate and connected to the first channel region and the second channel region;
• and
• a second source region disposed over the substrate, wherein:
• a lattice constant of the first source region is different from a lattice constant of the second source region, and
• the first source region completely surrounds the second source region.