A semiconductor device includes first channel layers disposed over a substrate, a first source/drain region disposed over the substrate, a gate dielectric layer disposed on and wrapping each of the first channel layers, and a gate electrode layer disposed on the gate dielectric layer and wrapping each of the first channel layers. Each of the first channel layers includes a semiconductor wire made of a first semiconductor material. The semiconductor wire extends into the first source/drain region. The semiconductor wire in the first source/drain regions is wrapped around by a second semiconductor material.