The present disclosure describes a semiconductor device and methods for forming the same. The semiconductor device includes a first transistor device of a first type and a second transistor device of a second type. The first transistor device includes first nanostructures, a first pair of source/drain structures, and a first gate structure on the first nanostructures. The second transistor device of a second type is formed over the first transistor device. The second transistor device includes second nanostructures over the first nanostructures, a second pair of source/drain structures over the first pair of source/drain structures, and a second gate structure on the second nanostructures and over the first nanostructures. The semiconductor device further includes a first isolation structure in contact with the first and second nanostructures and a second isolation structure in contact with a top surface of the first pair of source/drain structures.