to the cells of the sector for a short time period for example 40 u03bcs then the method returns to the block 409 for repeating the same operations the above described loop ends when the verify phase has a positive result block 412 or after such maximum number of attempts block 415 in this latest case the sector is set as failed at the block 425 in both cases the method thus ends at the block 430 in such a way there is obtained a controlled erasing of the sector that strongly reduces the cells having a very low threshold voltage with reference to fig4 b a preferred implementation of the compacting phase block 330 realizes a method that starts at the block 440 the block 443 a follows the block 440 wherein there is set a first limit value vla for example u2212 3 5v which is slightly lower then the desired value i e u2212 3v in the example at issue such limit value vla is used for performing a coarse compacting of the distribution of the threshold voltages of the erased cells thus a first loop is executed starting at the block 446 a wherein a selected block of the cells of the sector starting from the first one is read with respect to the limit value vla the block is determined by the bit lines of a desired page for each bit line the block includes a group of cells connected to one or more selected word lines in particular each group is formed by a single cell performing a page level reading or by an entire string performing a string level reading in the first case page level reading the select lines of the strings corresponding to the selected page are brought to the.