A high voltage (HV) device includes a gate dielectric structure over a substrate. The gate dielectric structure has a first portion and a second portion. The first portion has a first thickness and is over a first well region of a first dopant type in the substrate. The second portion has a second thickness and is over a second well region of a second dopant type. The first thickness is larger than the second thickness. A gate electrode is disposed over the gate dielectric structure. A metallic layer is over and coupled with the gate electrode. The metallic layer extends along a direction of a channel under the gate dielectric structure. At least one source/drain (S/D) region is disposed within the first well region of the first dopant type.