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.