Multi-value level type non-volatile semiconductor memory unit and method of rewriting the same

A method of rewriting a non-volatile semiconductor memory unit, said non-volatile semiconductor memory unit comprising a memory cell which has a semiconductor substrate, a control gate formed over said semiconductor substrate, an electric charge accumulative layer formed between said semiconductor substrate and said control gate, and a source and drain both of which are formed in said semiconductor substrate, said memory cell storing tertiary or more data by accumulating electric charge in said electric charge accumulative layer;

• said method comprising said steps of:
• detecting a storage state before data rewrite of said memory cell;
• comparing the storage state before data rewrite, which is detected by said detecting step, with a storage state after data rewrite to produce a difference between the storage state before data rewrite and the storage state after data rewrite;
• and
• rewriting the storage state of said memory cell by applying predetermined voltages to said source, said drain and said control gate of said memory cell, respectively, in accordance with said produced difference.

A non-volatile semiconductor memory unit comprising:

• a memory cell having a semiconductor substrate, a control gate formed over said semiconductor substrate, an electric charge accumulative layer formed between said semiconductor substrate and said control gate, and a source and drain, both formed in said semiconductor substrate, said memory cell storing N-valued data (N being an integer more than 3) by accumulating electric charge in said electric charge accumulative layer;
• detection means for detecting a storage state before data rewrite of said memory cell;
• comparison means for comparing the storage state before data rewrite, which is detected by said detection means, with a storage state after data rewrite to produce a difference between the storage state before data rewrite and the storage state after data rewrite;
• and
• rewrite means for rewriting the storage state of said memory cell by applying N-1 levels of predetermined voltages to said source, said drain and said control gate of said memory cell, respectively, in accordance with the difference produced in said comparison means.

A method of rewriting a non-volatile semiconductor memory unit, said non-volatile semiconductor memory unit comprising a memory cell which has a semiconductor substrate, a control gate formed over said semiconductor substrate, an electric charge accumulative layer formed between said semiconductor substrate and said control gate, and a source and drain both of which are formed in said semiconductor substrate, said memory cell storing N-valued data (N being an integer more than 3) by accumulating electric charge in said electric charge accumulative layer;

• said method comprising the steps of:
• detecting a storage state before data rewrite of said memory cell;
• comparing the storage state before data rewrite, which is detected by said detecting step, with a storage state after data rewrite to produce a difference between the storage state before data rewrite and the storage state after data rewrite;
• and
• rewriting the storage state of said memory cell by applying N-1 levels of predetermined voltages to said source, said drain and said control gate of said memory cell, respectively, in accordance with said produced difference.

A non-volatile semiconductor memory unit comprising:

• a memory cell having a semiconductor substrate, a control gate formed over said semiconductor substrate, an electric charge accumulative layer formed between said semiconductor substrate and said control gate, and a source and drain, both formed in said semiconductor substrate, said memory cell storing N-valued data (N being an integer more than 3) by accumulating electric charge in said electric charge accumulative layer;
• detection means for detecting a storage state before data rewrite of said memory cell;
• comparison means for comparing the storage state before data rewrite, which is detected by said detection means, with a storage state after data rewrite to produce a difference between the storage state before data rewrite and the storage state after data rewrite;
• and
• rewrite means for rewriting the storage state of said memory cell by applying predetermined voltages to said source, said drain and said control gate of said memory cell, respectively, each of said predetermined voltages having a voltage value which is determined among N-1 of voltage values in accordance with the difference produced in said comparison means.

A method of rewriting a non-volatile semiconductor memory unit, said non-volatile semiconductor memory unit comprising a memory cell which has a semiconductor substrate, a control gate formed over said semiconductor substrate, an electric charge accumulative layer formed between said semiconductor substrate and said control gate, and a source and drain, both formed in said semiconductor substrate, said memory cell storing N-valued data (N is an integer more than 3) by accumulating electric charge in said electric charge accumulative layer;

• said method comprising said steps of:
• detecting a storage state before data rewrite of said memory cell;
• comparing the storage state before data rewrite, which is detected in said detecting step, with a storage state after data rewrite to produce a difference between the storage state before data rewrite and the storage state after data rewrite;
• and
• rewriting the storage state of said memory cell by applying predetermined voltages to said source, said drain and said control gate of said memory cell, respectively, each of said predetermined voltages having a voltage value which is determined among N-1 of voltage values in accordance with the difference produced in said comparison means.