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

초록

The position dependence of threshold-voltage change (Delta V(th)) in floating-gate NAND Flash cell strings due to random telegraph noise was characterized. It was found that the cumulative distribution of Delta V(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 similar to 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 Delta V(th) at the same noise current fluctuation. It was also shown that the G(m) ( finally, Delta V(th)) of a read cell can be changed by controlling the pass bias since the pass bias changes the channel resistance (approximate to Rs) of the pass cells.

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