The invention provides a method for forming a semiconductor structure. A substrate is provided. A conductive layer is formed on the substrate. A first patterned mask layer is formed on the conductive layer. The conductive layer exposed by the first patterned mask layer is removed to expose a first sidewall of the conductive layer. A doped region is formed in the substrate by a doping step using the first patterned mask layer as a mask. The first patterned mask layer is removed. A second patterned mask layer is formed on the conductive layer. The conductive layer exposed by the second patterned mask layer is removed to expose a second sidewall opposite to the first sidewall of the conductive layer. The second patterned mask layer is removed.