A method for forming a memory device includes the steps of providing a substrate, forming an isolation structure in the substrate to define a plurality of active regions in the substrate, the active regions respectively comprising two terminal portions and a central portion between the terminal portions, forming a plurality of island features on the substrate, wherein each of the island features covers two of the terminals portions respectively belonging to two of the active regions, performing a first etching process, using the island features as an etching mask to etch the substrate to define a plurality of island structures and a first recessed region surrounding the island structures on the substrate, and removing the island features to expose the island structures.