A structure includes first nanostructures vertically spaced one from another over a substrate in a core region of the semiconductor structure, a first interfacial layer wrapping around each of the first nanostructures, a first high-k dielectric layer over the first interfacial layer and wrapping around each of the first nanostructures, second nanostructures vertically spaced one from another over the substrate in an I/O region of the semiconductor structure, a second interfacial layer wrapping around each of the second nanostructures, a second high-k dielectric layer over the second interfacial layer and wrapping around each of the second nanostructures. The first nanostructures have a first vertical pitch, the second nanostructures have a second vertical pitch substantially equal to the first vertical pitch, the first nanostructures have a first vertical spacing, the second nanostructures have a second vertical spacing greater than the first vertical spacing by about 4 Å to about 20 Å.