A device includes a well region over a substrate, and a heavily doped well region over the well region, wherein the well region and the heavily doped well region are of a same conductivity type. A gate dielectric is formed on a top surface of the heavily doped well region. A gate electrode is formed over the gate dielectric. A source region and a drain region are formed on opposite sides of the heavily doped well region. The source region and the drain region have bottom surfaces contacting the well region, and wherein the source region and the drain region are of opposite conductivity types.