Impact of Neutral Threshold-Voltage Spread and Electron-Emission Statistics on Data Retention of Nanoscale nand Flash

This letter presents a comparison of two different variability sources for data retention of nanoscale NAND Flash memories: the neutral threshold-voltage spread and the electron-emission statistics from the floating gate. Referring to fresh cells programmed to the same threshold-voltage level, the effect of the previous dispersion contributions on the data retention transients of a memory array is evaluated. Both effects are shown to result into a broadening of the array threshold-voltage distribution with time, but a quantitative assessment clearly shows that the neutral threshold-voltage spread dominates over the electron-emission spread, revealing that cell-to-cell parameter variations represent the major source of variability for data retention in nanoscale NAND Flash memories.