A semiconductor structure includes a first semiconductor substrate, a second semiconductor substrate, a depletion layer, an isolation structure, a first gate structure, and a second gate structure. The first and second semiconductor substrates respectively have a first active region and a second active region overlapping the first active region. The depletion layer is disposed between the first active region and the second active region. The isolation structure surrounds the first and second active regions. The first gate structure is disposed in the second active region. The second gate structure is disposed in the second active region. The second active region has a portion between the first gate structure and the second gate structure.