Position-Dependent Threshold-Voltage Variation by Random Telegraph Noise in nand Flash Memory Strings
The position dependence of threshold-voltage change (ΔVth) in floating-gate NAND Flash cell strings due to random telegraph noise was characterized. It was found that the cumulative distribution of ΔVth's measured from 100 cell devices at word line 31 (WL31) is broader than that at WL0 due to smaller transconductance (Gm). As the position of a read cell in the string is changed from WL0 to WL31, maximum Gm decreases by ~50% since the equivalent source resistance (Rs) of the read cell increases. Decreasing Gm makes the slope of the Iread - Vread curve low, which increases ΔVth at the same noise current fluctuation. It was also shown that the Gm (finally, ΔVth) of a read cell can be changed by controlling the pass bias since the pass bias changes the channel resistance (≈ Rs) of the pass cells.