A semiconductor device is provided. The semiconductor device includes a substrate, a field plate, a gate electrode, and a first dielectric layer. The substrate has a top surface. The substrate includes a first drift region with a first conductivity type extending from the top surface of the substrate into the substrate, and includes a second drill region with the first conductivity type extending from the top surface of the substrate into the substrate and adjacent to the first drift region. The field plate is over the substrate. The gate electrode has a first portion and a second portion, wherein the first portion of the gate electrode is located over the field plate. The first dielectric layer is between the substrate and the field plate. The first portion of the gate electrode is overlapping with a boundary of the first drift region and the second drift region in the substrate.