A method including forming an isolation trench; forming first and second liners on the isolation trench; filling the isolation trench an insulating material to form an isolation region and an active region; forming a preliminary gate trench including a first region across the isolation region to expose the first liner, the second liner, and the insulating material, and a second region across the active region to expose a portion of the substrate, the first region having a first sidewall with a planar shape, and the second region having a second sidewall with a concave central area such that an interface between the first and second regions has a pointed portion; removing a portion of the first liner exposed by the first region to form a dent having a first depth by which the pointed portion protrudes; removing the pointed portion to form a gate trench; and forming a gate electrode.