A semiconductor structure is provided. The semiconductor structure includes a substrate, a first well region, a second well region, an isolation structure, and a gate structure. The first well region is disposed in the substrate. The second well region is disposed in the substrate. The second well region is adjoining the first well region. The isolation structure is disposed in the first well region. The gate structure is disposed on the substrate. The gate structure includes a first gate portion and a second gate portion. The first gate portion overlaps the first well region and the second well region. There is an opening between the first gate portion and the second gate portion exposing a portion of the isolation structure.