Semiconductor device structure and method for forming the same

A semiconductor device structure includes first nanostructures formed over a substrate. The structure also includes a first gate structure wrapped around the first nanostructures. The structure also includes first source/drain epitaxial structures formed over opposite sides of the first nanostructures. The structure also includes second nanostructures formed over the first nanostructure. The structure also includes a second gate structure wrapped around the second nanostructures. The structure also includes second source/drain epitaxial structures formed over opposite sides of the second nanostructures. The first gate structure and the second gate structure have different conductivity types, and the Ge concentration of the first nanostructures and the Ge concentration of the second nanostructures are different.