A semiconductor structure is provided. The semiconductor structure comprises a substrate, a deep well formed in the substrate, a first well and a second well formed in the deep well, a gate electrode formed on the substrate and disposed between the first well and the second well, a first isolation, and a second isolation. The second well is spaced apart from the first well. The first isolation extends down from the surface of the substrate and is disposed between the gate electrode and the second well. The second isolation extends down from the surface of the substrate and is adjacent to the first well. A ratio of a depth of the first isolation to a depth of the second isolation is smaller than 1.