Nonvolatile memory devices include a string of nonvolatile memory cells on a substrate. This string of nonvolatile memory cells includes a first vertical stack of nonvolatile memory cells on the substrate and a string selection transistor on the first vertical stack of nonvolatile memory cells. A second vertical stack of nonvolatile memory cells is also provided on the substrate and a ground selection transistor is provided on the second vertical stack of nonvolatile memory cells. This second vertical stack of nonvolatile memory cells is provided adjacent the first vertical stack of nonvolatile memory cells. A conjunction doped semiconductor region is provided in the substrate. This conjunction doped region electrically connects the first vertical stack of nonvolatile memory cells in series with the second vertical stack of nonvolatile memory cells so that these stacks can operate as a single NAND-type string of memory cells.