Semiconductor memory device for storing multivalued data
Data storage circuits are connected to the bit lines in a one-to-one correspondence. A write circuit writes the data on a first page into a plurality of first memory cells selected simultaneously by a word line. Thereafter, the write circuit writes the data on a second page into the plurality of first memory cell. Then, the write circuit writes the data on the first and second pages into second memory cells adjoining the first memory cells in the bit line direction.
1. A semiconductor memory device comprising:
first and second bit lines;
first and second word lines;
first and second select transistors;
a source line;
a plurality of memory cells including first, second, third and fourth memory cells, each memory cell capable of storing n bits of data (n is a natural number equal to or larger than two), the first memory cell connected to the first word line, coupled to the first bit line and coupled to the source line via the first select transistor, the second memory cell being adjacent to the first memory cell along the first word line, connected to the first word line, coupled to the second bit line and coupled to the source line via the second select transistor, the third memory cell connected to the second word line, coupled to the first bit line and coupled to the source line via the first select transistor and the first memory cell, a source of the third memory cell connected to a drain of the first memory cell, the fourth memory cell being adjacent to the third memory cell along the second word line, connected to the second word line, coupled to the second bit line and coupled to the source line via the second select transistor and the second memory cell, a source of the fourth memory cell connected to a drain of the second memory cell; and
a write circuit configured to write n bits of data into each of the memory cells, wherein:
a first bit of data to be stored in the first memory cell is assigned to a first page address;
a second bit of data to be stored in the second memory cell is assigned to a second page address which is consecutive to the first page address;
a third bit of data to be stored in the third memory cell is assigned to a third page address which is consecutive to the second page address;
a fourth bit of data to be stored in the fourth memory cell is assigned to a fourth page address which is consecutive to the third page address;
a fifth bit of data to be stored in the first memory cell is assigned to a fifth page address which is consecutive to the fourth page address;
the write circuit is configured to write the first bit of data into the first memory cell by altering a threshold voltage of the first memory cell from a first voltage to a second voltage according to a first logical state of the first bit of data;
the write circuit is configured to write the fifth bit of data into the first memory cell by altering the threshold voltage of the first memory cell from the second voltage to a third voltage according to a first logical state of the fifth bit of data; and
the write circuit is configured to write the fifth bit of data into the first memory cell by altering the threshold voltage of the first memory cell from the second voltage to a fourth voltage higher than the third voltage according to a second logical state of the fifth bit of data.
2. The semiconductor memory device according to claim 1 , wherein the first logical state of the first bit of data is “0”.
3. The semiconductor memory device according to claim 1 , wherein:
the first logical state of the fifth bit of data is “0”; and
the second logical state of the fifth bit of data is “1”.
4. The semiconductor memory device according to claim 1 , wherein the semiconductor memory device is a NAND-type flash memory.
5. The semiconductor memory device according to claim 1 , wherein:
the write circuit includes a plurality of data storage circuits configured to store data to be written in the memory cells; and
the first and second bit lines are coupled to the same data storage circuit.
6. The semiconductor memory device according to claim 1 , wherein the write circuit is configured to write the first bit of data into the first memory cell by maintaining the threshold voltage of the first memory cell at the first voltage according to a second logical state of the first bit of data.
7. The semiconductor memory device according to claim 1 , the semiconductor memory device further comprising a plurality of flag cells, wherein
the write circuit is configured to write flag information indicating that the fifth bit of data has been written into the first memory cell, into one of the flag cells selected concurrently with the first memory cell, when the write circuit writes the fifth bit of data into the first memory cell.
8. The semiconductor memory device according to claim 1 , wherein the write circuit is configured to write the n bits of data into each of the memory cells in the order of page addresses.