Nonerratic behavior of overerased bits in flash EEPROM

We report the effects of flash “channel” programming, or severe gate disturb, on the threshold voltage of fast or overerased bits. Experiments have been performed to establish that this class of fast bits are nonerratic and remain fast after 250 °C bake. These fast bits exhibit identical subthreshold characteristics similar to that of a normal bit after ultraviolet erase, thus establishing that the initial charge stored on the floating gate is the same for both normal and fast bits. Polysilicon grain boundary enhanced electric fields which result in impact ionization by tunneling electrons, thus generating trapped positive charges in the grain boundary oxide ridges are believed to play an important role in the generation of fast bits.