Nonvolatile memory devices and methods of making the same are described. A nonvolatile memory device includes a string selection transistor, a plurality of memory cell transistors, and a ground selection transistor electrically connected in series to the string selection transistor and to the pluralities of memory cell transistors. Each of the transistors includes a channel region and source/drain regions. First impurity layers are formed at boundaries of the channels and the source/drain regions of the memory cell transistors. The first impurity layers are doped with opposite conductivity type impurities relative to the source/drain regions of the memory cell transistors. Second impurity layers are formed at boundaries between a channel and a drain region of the string selection transistor and between a channel and a source region of the ground selection transistor. The second impurity layers are doped with the same conductivity type impurities as the first impurity layers and have a higher impurity concentration than the first impurity layers.