A method includes forming a first semiconductor layer over a substrate, forming a second semiconductor layer over the first semiconductor layer, forming a first trench and a second trench through in the first semiconductor layer and the second semiconductor layer, wherein a width of the second trench is different from a width of the first trench, forming a dielectric region in the first trench and forming a first gate region in the first trench and over the dielectric region, and a second gate region in the second trench.