A gate-all-around structure including a first transistor is provided. The first transistor includes a semiconductor substrate having a top surface, and a first nanostructure over the top surface of the semiconductor substrate and between a first source and a first drain. The first transistor also includes a first gate structure around the first nanostructure, and an inner spacer between the first gate structure and the first source, wherein an interface between the inner spacer and the first gate structure is non-flat. The first transistor includes an isolation layer between the top surface of the semiconductor substrate and the first source and the first drain.