A nonvolatile memory device includes a semiconductor substrate of a first conductivity type, a plurality of word lines on the semiconductor substrate, each the plurality of word lines including a floating gate of a second conductivity type. A ground select line and a string select line are disposed on respective sides of word lines. An impurity region of the second conductivity type underlies a first word line adjacent the ground select line. The device may further include a second impurity region of the second conductivity type underlying a second word line adjacent the string select line. In still further embodiments, the device may further include third impurity regions of the second conductivity type underlying respective third word lines between the first word line and the second word line. Methods of forming such devices are also provided.