In a split gate type nonvolatile memory device, a supplementary layer pattern is disposed on a source region of a semiconductor substrate. Since the source region is vertically extended by virtue of the presence of the supplementary layer pattern, it is therefore possible to increase an area of a region where a floating gate overlaps the source region and the supplementary layer pattern. Accordingly, the capacitance of a capacitor formed between the source and the floating gate increases so that it is possible for the nonvolatile memory device to perform program/erase operations at a low voltage level.