Wider and narrower trenches are formed in a substrate. A first gate material layer is deposited but not fully fills the wider trench. The first gate material layer in the wider trench and above the substrate original surface is removed by isotropic or anisotropic etching back. A first dopant layer is formed in the surface layer of the substrate at the original surface and the sidewall and bottom of the wider trench by tilt ion implantation. A second gate material layer is deposited to fully fill the trenches. The gate material layer above the original surface is removed by anisotropic etching back. A second dopant layer is formed in the surface layer of the substrate at the original surface by ion implantation. The dopants are driven-in to form a base in the substrate and a bottom-lightly-doped layer surrounding the bottom of the wider trench and adjacent to the base.