An integrated circuit includes a NAND string including a string selection transistor SST and a ground selection transistor GST disposed at either end of series-connected memory storage cells MC. Each of the memory storage cells is a memory transistor having a floating gate, and at least one of the string selection transistor SST and the ground selection transistor GST is a memory transistor having a floating gate. The threshold voltage Vth of programmable string selection transistors SST and the ground selection transistor GST is variable and user controllable and need not be established by implantation during manufacture. Each of the programmable string selection transistors SST and the ground selection transistors GST in a memory block may be used to store random data, thus increasing the memory storage capacity of the flash memory device.