A semiconductor device includes a substrate, a dielectric layer, a source region, a drain region, and a metal structure. The substrate has a trench therein, and the dielectric layer is conformally formed over the substrate and the trench. The source region and the least one drain region are in the substrate. The metal structure is filled in the trench and surrounded by the dielectric layer, and the metal structure is disposed between the source region and the drain region. Moreover, the metal structure has a first metal portion and a second metal portion which has a height greater than a height of the first metal portion, and the first metal portion is disposed between the drain region and the second metal portion.