A semiconductor device includes an isolation structure in a substrate; and a gate structure over an active region of the substrate. The isolation structure surrounds the active region. The gate structure includes a first section parallel to a second section. The semiconductor device further includes a conductive field plate extending between the first section and the second section and overlapping an edge of the active region. A portion of the conductive field plate extends beyond the edge of the active region, The conductive field plate includes a dielectric layer having a first portion and a second portion, and the first portion is thicker than the second portion. The semiconductor device includes a first well overlapping the edge of the active region. The first well extends underneath the isolation structure. The conductive field plate extends beyond an outer-most edge of the first well.