Nonvolatile semiconductor memory device for storing multivalue information by controlling erase and plural write states of each memory cell
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United States of America Patent
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Nov 9, 1999
Issued Date -
N/A
app pub date -
Jun 11, 1998
filing date -
May 1, 1996
priority date (Note) -
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Abstract
To enable one non-volatile memory cell to store four-value information, three different kinds of threshold voltages are serially applied to a word line in a verify operation to execute a write operation, the threshold voltages of the memory cell are controlled, and two-value (one-bit) information corresponding to the four-value (two-bit) information to be written are synthesized by a write data conversion circuit for each of the write operations carried out three times. In this way, the four-value (two-bit) information are written into one memory cell, and the memory capacity of the memory cell can be increased. In the information read operation, three different kinds of voltages are applied to a word line, three kinds of two-value (one-bit) information so read out are synthesized by a read conversion circuit and the memory information of the memory cell are converted to the two-bit information.
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