A semiconductor structure comprises a substrate having a first conductive type; a deep well having a second conductive type formed in the substrate; a first well having the first conductive type and a second well having the second conductive type both formed in the deep well and the second well spaced apart from the first well; a gate electrode formed on the substrate and disposed between the first and second wells; an isolation extending down from the surface of the substrate and disposed between the gate electrode and the second well; a conductive plug including a first portion and a second portion electrically connected to each other, and the first portion electrically connected to the gate electrode, and the second portion penetrating into the isolation. The bottom surface of the second portion of the conductive plug is covered by the isolation.