A semiconductor structure includes a semiconductor substrate; a first high-voltage well (HVW) region of a first conductivity type overlying the semiconductor substrate; a second well region of a second conductivity type opposite the first conductivity type overlying the semiconductor substrate and laterally adjoining the first well region; a gate dielectric extending from over the first well region to over the second well region; a drain region in the second well region; a source region on an opposite side of the gate dielectric than the drain region; and a gate electrode on the gate dielectric. The gate electrode includes a first portion directly over the second well region, and a second portion directly over the first well region. The first portion has a first impurity concentration lower than a second impurity concentration of the second portion.