A new erase saturation issue in cylindrical junction-less charge-trap memory arrays

We present a new erase saturation issue compromising the performance of cylindrical charge-trap cells integrated along junction-less NAND strings. The phenomenon comes from the inability to properly induce an inversion layer in the inter-cell regions of the string during read in the cylindrical geometry, pinning the threshold voltage (V_T) resulting from cell erase. The dependence of this issue on string parameters is investigated, showing that a reduction in the inter-cell regions may relieve it. However, this originates a trade-off against the constraints raised by the lateral diffusion of electrons in the charge-trap layer during data retention. It is shown that this trade-off can be easily managed by strings with large substrate radius, while its solution is more critical for small radii.