Nonvolatile memory devices include a first NAND-type string of EEPROM cells having a first plurality of string selection transistors therein electrically connected in series within the string. This first plurality of string selection transistors includes a first plurality of depletion-mode transistors and a first enhancement-mode transistor. A second NAND-type string of EEPROM cells is provided with a second plurality of string selection transistors therein that are electrically connected in series. The second plurality of string selection transistors includes a second plurality of depletion-mode transistors and a second enhancement-mode transistor. The first enhancement-mode transistor is stacked vertically relative to one of the second plurality of depletion-mode transistors and the second enhancement-mode transistor is stacked vertically relative to one of the first plurality of depletion-mode transistors. A first string selection plug is configured to electrically connect gate electrodes of the first enhancement-mode transistor and one of the second plurality of depletion-mode transistors.