The purpose of NAND Flash memories as nonvolatile memories is to store the user
data for years without requiring a supply voltage. The state-of-the-art NAND memory
cell is the 1T floating gate cell. In contrast to the 1T1C DRAM cell, which
consists of an access transistor and a separate capacitor as charge storage node, the
1T floating gate cell is a MOSFET whose gate is split, having a charge storage
node (floating gate) in between. This charge storage node, usually polysilicon, is
completely surrounded by oxide and, therefore, electrically isolated. The electrical
charges stored within the floating gate represent the nonvolatile information; the
program operation adds electrons to the floating gate according to the user data.