A nonvolatile memory device includes a string selection gate and a ground selection gate on a semiconductor substrate, and a plurality of memory cell gates on the substrate between the string selection gate and the ground selection gate. First impurity regions extend into the substrate to a first depth between ones of the plurality of memory cell gates. Second impurity regions extend into the substrate to a second depth that is greater than the first depth between the string selection gate and a first one of the plurality of memory cell gates immediately adjacent thereto, and between the ground selection gate and a last one of the plurality of memory cell gates immediately adjacent thereto. Related fabrication methods are also discussed.