A method of forming an active region structure includes preparing a semiconductor substrate including a cell array region and a peripheral circuit region, forming preliminary cell active regions in the cell array region of the semiconductor substrate, and forming cell active regions in the preliminary cell active regions and at least one peripheral active region in the peripheral circuit region of the semiconductor substrate, such that the preliminary cell active regions, the cell active regions, and the at least one peripheral active region are integrally formed with the semiconductor substrate and protrude from the semiconductor substrate.