Methods of fabricating semiconductor devices include forming a first impurity region in a substrate by implanting a first impurity of a first conductivity type in a cell region and a peripheral region of the substrate to a first target depth from a top surface of the substrate; forming a second impurity region in the cell region and the peripheral region by implanting a second impurity of the first conductivity type into the cell region and the peripheral region to a second target depth that is smaller than the first depth from the top surface of the substrate; forming a cell transistor with a channel in the cell region, wherein the first impurity region forms the channel of the cell transistor; and forming a peripheral transistor with a channel in the peripheral region, wherein the second impurity region forms the channel of the peripheral transistor.