A semiconductor device. The device comprises an active region isolated by an isolation structure on a substrate. The device further comprises a gate electrode extending across the active area and overlying the substrate, a pair of source region and drain region, disposed on either side of the gate electrode on the substrate in the active area, and a gate dielectric layer disposed between the substrate and the gate electrode. The gate dielectric layer comprises a relatively-thicker high voltage (HV) dielectric portion and a relatively-thinner low voltage (LV) dielectric portion, wherein the HV dielectric portion occupies a first intersection among the drain region, the isolation structure, and the gate electrode, and a second intersection among the source region, the isolation structure, and the gate electrode.