A semiconductor structure includes a first well region of a first conductivity type overlying a substrate, a second well region of a second conductivity type opposite the first conductivity type overlying the substrate and laterally adjoining the first well region, a third well region of the second conductivity type adjacent and spaced apart from the first well region, a first deep well region of the second conductivity type underlying at least portions of the first and the second well regions, a second deep well region of the second conductivity type underlying the third well region and spaced apart from the first deep well region, an insulation region in the first well region, a gate dielectric extending from over the insulation region to over the second well region, and a gate electrode on the gate dielectric.