Position-Dependent Threshold-Voltage Variation by Random Telegraph Noise in nand Flash Memory Strings

The position dependence of threshold-voltage change (ΔV
_th
) in floating-gate NAND Flash cell strings due to random telegraph noise was characterized. It was found that the cumulative distribution of ΔV
_th
's measured from 100 cell devices at word line 31 (WL31) is broader than that at WL0 due to smaller transconductance (G
_m
). As the position of a read cell in the string is changed from WL0 to WL31, maximum G
_m
decreases by ~50% since the equivalent source resistance (R
_s
) of the read cell increases. Decreasing G
_m
makes the slope of the I
_read
- V
_read
curve low, which increases ΔV
_th
at the same noise current fluctuation. It was also shown that the G
_m
(finally, ΔV
_th
) of a read cell can be changed by controlling the pass bias since the pass bias changes the channel resistance (≈ R
_s
) of the pass cells.