| 摘要 |
PURPOSE: To provide a non-volatile semiconductor storage in which a processing time of whole erasing processing can be shortened without performing needless rewriting even if a memory cell in which depletion is serious exists, also as distribution of threshold voltage of memory cells can be narrowed, margin to be read out can be secured sufficiently, and which is suitable for low power source voltage operation. CONSTITUTION: When distribution D1 of a memory cell in which depletion is serious exists, threshold voltage distribution D2 of a normally erased memory looks like that the distribution is reduced to distribution D3. Then, after erasion is performed by a higher erasion discrimination level 1 than a target erasion discrimination level 2, only a memory cell of distribution D1 is rewritten by a lower rewriting discrimination level 1 than the target rewriting discrimination level 2. Consequently, as influence by a memory cell in which depletion is serious is dissolved and threshold voltage distribution is made distribution D7, after erasion is performed by the erasion discrimination level 2, only memory cells in which depletion are slight are rewritten by a rewriting discrimination level 2.
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