A multivalued memory has data of state “0”, state “1”, state “2”, and state “3” whose threshold voltages increase in that order. In a first-page write operation, a memory cell whose data is in state “0” is brought into state “1”. In a second-page write operation, a memory cell whose data is in state “0” is brought into state “3” and a memory cell whose data is in state “1” is brought into state “2”. As a result, in reading the data, the data on the first page can be read in two read operations. Furthermore, the operation of writing the data onto the second page can be made faster, because a high initial write voltage can be used.
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0. 23. A semiconductor memory device comprising:
at least one memory array with word lines and bit lines, said memory array storing multivalued data in memory cells, said multivalued data including at least a first page and a second page of data;
at least one control circuit connected to said at least one memory array,
wherein said at least one control circuit reads one of said first page of data and said second page of data for a first set of said memory cells and then reads the same one of said first page of data and said second page of data for a second set of memory cells.
0. 28. A semiconductor memory device comprising:
at least one memory array with word lines and bit lines, said memory array storing multivalued data in memory cells, said multivalued data including at least a first page, a second page of data, and a third page of data;
at least one control circuit connected to said at least one memory array,
wherein said at least one control circuit reads one of said first page, said second page, and said third page of data and then reads the same one of said first page, said second page, and said third page of data for a second set of memory cells.
0. 19. A semiconductor memory device comprising:
a nonvolatile memory cell array having bit lines and word lines;
a plurality of memory cell transistors, commonly connected to one of the word lines, forming a physical row to store data for first and second addressable pages; and
a control voltage generator configured to generate reading voltages to be applied to the word lines, wherein
in a first read operation, the control voltage generator generates a single reference voltage to sense data of the second addressable page, and in a second read operation, the control voltage generator generates only two reference voltages to sense data of the first addressable page.
0. 29. A semiconductor memory device comprising:
a memory cell array having bit lines and word lines, said memory cell array storing multilevel data in at least a first page and a second page of data wherein said first page and said second page of data are distinguished by threshold voltages;
a control circuit connected to said memory cell array for controlling potentials applied at least to one of said word lines,
wherein a potential applied to said one of said word lines has a first value for reading one of said first page and said second page of data from memory cells and has no more than two values for reading the other of said first page and said second page of data.
10. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and stores one of an n number of data items made up of state “0”, state “1”, . . . , state “n” (3≦n where n is a natural number);
a data storage circuit which stores data of a first or a second logical level externally inputted; and
a control circuit which controls not only the potential on said bit line and that on said word line but also the operation of said data storage circuit, wherein
said control circuit, in a final write operation, charges state “0” of the smallest data stored in said memory element into state “n” of the largest data when the data in said first storage circuit is data of the first logical level externally supplied, and keeps the data in said memory element when the data in said first storage circuit is data of the second logical level.
0. 17. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and stores one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage;
a control circuit which controls a potential on said bit line and that on said word line, wherein
said control circuit operates in such a manner that
in a first read operation, the bit line receives data of a first logical level when the data in said memory element is in either state “0” or state “1”, and receives data a second logical level when the data in said memory element is in either state “2” or state “3”, and
that in a second read operation, the bit line receives data of the first logical level when the data in said memory element is in either state “0” or state “3”, and receives data of the second logical level when the data in said memory element is in either state “1” or state “2”.
0. 45. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and stores one of page data state “11”, page data state “10”, page data state “00”, and page data state “01” of data that differ in threshold voltage;
a control circuit which controls a potential on said bit line and that on said word line, wherein
said control circuit operates in such a manner that
in a first read operation, the bit line receives data of a first logical level when the data in said memory element is in either page data state “11” or page data state “10”, and receives data a second logical level when the data in said memory element is in either page data state “00” or page data state “01”, and
that in a second read operation, the bit line receives data of the first logical level when the data in said memory element is in either page data state “11” or page data state “01”, and receives data of the second logical level when the data in said memory element is in either page data state “10” or page data state “00”.
0. 47. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and stores one of page data state “11”, page data state “10”, page data state “00”, and page data state “01” of data that differ in threshold voltage;
a control circuit which controls a potential on said bit line and that one said word line, wherein
said control circuit operates in such a manner that
in a read operation of one of a first page and a second page of data, the bit line receives data of a first logical level when the data in said memory element is in either page data state “11” or page data state “10”, and receives data a second logical level when the data in said memory element is in either page data state “00” or page data state “01”, and
that in a read operation of the other of said first page and said second page of data, the bit line receives data of the first logical level when the data in said memory element is in either page data state “11” or page data state “01”, and receives data of the second logical level when the data in said memory element is in either page data state “10” or page data state “00”.
15. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and stores one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage;
a data storage circuit which is connected to said bit line and stores the data read from said memory element; and
a control circuit which controls not only the potential on said bit line and that on said word line but also the operation of said data storage circuit, wherein
said control circuit operates in such a manner that
in a first read operation, the control circuit sets data of a first logical level in said data storage circuit when the data in said memory element is in either state “0” or state “1”, and sets data of a second logical level in said data storage circuit when the data in said memory element is in either state “2” or state “3”, and
that in a second read operation, the control circuit sets data of the first logical level in said data storage circuit when the data in said memory elements is in either state “0” or state “3”, and sets data of the second logical level in said data storage circuit when the data in said memory element is in either state “1” or state “2”.
1. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and stores one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage;
a data storage circuit which is connected to said bit line and stores not only data of a first or a second logical level externally supplied but also the data of the first or second level read from said memory element; and
a control circuit which controls not only the potential on said bit line and that on said word line but also the operation of said data storage circuit, wherein
said control circuit operates in such a manner that
in a first operation, the control circuit changes the data in said memory element from said state “0” to state “1” when the data in said data storage circuit is data of the first logical level and keeps the data in said memory element in said state “0” when the data in said data storage circuit is data of the second logical level,
that in a first verify operation of verifying whether said data has reached state “1”, the control circuit brings the data in said data storage circuit to the second logical level when the data in said data storage circuit is at the first logical level and said data has reached state “1”, keeps the data in said data storage circuit at the first logical level when said data has not reached state “1”, keeps the data in said data storage circuit at the second logical level when the data in said data storage circuit is at the second logical level, and carries out said first operation until the data in said data storage circuit has reached the second logical level, and
that in a second operation, the control circuit changes the data in said memory element from state “1” to state “2” when the data in said data storage circuit is data of the first logical level externally supplied and the data in said memory element is in state “1”, and changes the data in said memory element from state “0” to state “3”, when the data in said memory element is in state “0”.
6. A nonvolatile semiconductor memory device comprising:
a memory element which is connected to a bit line and a word line and store s one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage;
a first storage circuit which is connected to said bit line and stores data of a first or a second logical level externally supplied;
a second storage circuit which is connected to said bit line and stores the data of the first or second level read from said memory element; and
a control circuit which controls not only the potential on said bit line and that on said word line but also the operation of said first and second storage circuits, wherein
said control circuit operates i n such a manner that
in a first operation, the control circuit changes the data in said memory element from state “0” to state “1” when the data in said first data storage circuit is data of the first logical level and keeps the data in said memory element at said state “0” when the data in said first storage circuit is data of the second logical level,
that in a first verify operation of verifying whether said data has reached state “1”, the control circuit brings the data in said first storage circuit to the second logical level when the data in said first storage circuit is at the first logical level and said data has reached state “1”, keeps the data in said first storage circuit at the first logical level when said data has not reached state “1”, keeps the data in said first storage circuit at the second logical level when the data in said first storage circuit is at the second logic level, and carries out said first operation until the data in said first storage circuit has reached the second logical level,
that in a second operation, the control circuit stores the data read from said memory element into said second storage circuit, changes the data in said memory element from state “1” to state “2” when the data in said first storage circuit is data of the first logical level externally supplied, changes the data in said memory element from state “0” to state “3” when the data in said memory element is in state “0”, and keeps the data in said memory element when the data in said first storage circuit is data of the second logical level,
that in a second verify operation of verifying whether the data in said memory element has reached state “2”, the control circuit brings the data in said first storage circuit to the second logical level when the data has reached state “2” in a case where the data in said first storage circuit is at the first logical level and the data in said memory element is in state “1” before the second operation is carried out, keeps the data in said first storage circuit at the first logical level when said data has not reached state “2”, and brings the potential on the bit line to which the memory element is connected to the first logical level and the data in said first storage circuit to the first logical level when the data in said second storage circuit is at the second logical level in a case where the data in said memory element is in state “0” before said second operation is carried out, and
that in a third verify operation of verifying said data has reached state “3”, the control circuit brings the data in said first storage circuit to the second logical level when the data in said first storage circuit is at the first logical level and said data has reached state “3”, keeps the data in said first storage circuit at the first logical level when said data has not reached state “3”, keeps the data in said first storage circuit at the second memory logical level when the data in said first storage circuit is at the second logical level, and carries out said second operation and second and third verify operations unit the data in said first storage circuit has reached the second logical level.
2. The nonvolatile semiconductor memory device according to
said control circuit, in a second verify operation of verifying whether the data in said memory element has reached state “2”, brings the data in said data storage circuit to the second logical level when the data has reached state “2” in a case where the data in said data storage circuit is at the first logical level and the data in said memory element is in state “1” before said second operation is carried out, keeps the data in said data storage circuit at the first logical level when the data has not reached state “2”, prevents the logical level in said data storage circuit from changing when the data in said memory element is in state “0” before said second operation is carried out, and keeps the data in said data storage circuit at the second logical level when the data in said data storage circuit is at the second logical level, and furthermore
said control circuit, in a third verify operation of verifying whether the data in said memory element has reached state “3”, brings the data in said data storage circuit to the second logical level when the data in said data storage circuit is at the first logical level and the data has reached state “3”, keeps the data in said data storage circuit at the first logical level when the data has not reached state “3”, keeps the data in said data storage circuit at the second memory logical level when the data in said data storage circuit is at the second logical level, and carries out said second operation, second and third verify operations until the data in said data storage circuit has reached the second logical level.
3. The nonvolatile semiconductor memory device according to
said control circuit, in said second operation, omits the verify operation of verifying whether said data has reached state “3” in the first half of the verify operation of verifying whether said data has reached state “2” and omits the verify operation of verifying whether said data has reached state “2” in the latter half of the verify operation of verifying whether said data h as reached state “3”.
4. The nonvolatile semiconductor memory device according to
said control circuit sets an initial write voltage in changing the data in said memory element from state “0” to state “3” and in changing the data from state “1” to state “2” higher than an initial write voltage in changing the data in said memory element from state “0” to state “1”.
5. The nonvolatile semiconductor memory device according to
said control circuit, judges whether the data in said memory element is in either state “2” or below or state “3” when the data in said memory element is read, stores the result of the judgment in said data storage circuit, thereafter judges whether the data is in either state “0” or state “1” or above, and, if the data stored in said data storage circuit is in state “3”, brings the potential on th e bit line connected to the memory element in which the data has been stored to a low level and keeps the potential of the bit lines connected to the memory elements whose data is in either state “1” or state “2” at a high level.
7. The nonvolatile semiconductor memory device according to
said control circuit, in said second operation, omits the verify operation of verifying whether said data has reached state “3” in the first half of the verify operation of verifying whether said data has reached state “2” and omits the verify operation of verifying whether said data has reached state “2” in the latter half of the verify operation of verifying whether said data has reached state “3”.
8. The nonvolatile semiconductor memory device according to
said control circuit sets an initial write voltage in changing the data in said memory element from state “0” to state “3” and in changing the data from state “1” to state “2” higher than an initial write voltage in changing the data in said memory element from state “0” to state “1”.
9. The nonvolatile semiconductor memory device according to
said control circuit, judges whether the data in said memory element is in either state “2” or below or state “3” when the data in said memory element is read, stores the result of the judgment in said data storage circuit, thereafter judges whether the data is in either state “0” or state “1” or above, and, if the data stored in said data storage circuit is in state “3”, brings the potential on the bit line connected to the memory element in which the data has been stored to a low level and keeps the potential of the bit lines connected to the memory elements whose data is in either state “1” or state “2” at a high level.
11. The nonvolatile semiconductor memory device according to
said control circuit, in a second verify operation of verifying whether the data in said memory element has reached state “2”, charges the data in said data storage circuit to the second logical level when the data has reached state “2”, in a case where the data in said data storage circuit is at the first logical level and the data in said memory element is in state “1” before said second operation is carried out, keeps the data in said data storage circuit at the first logical level when the data has not reached state “2”, prevents the logical level in said data storage circuit from changing when the data in said memory element is in state “0” before said second operation is carried out, and keeps the data in said data storage circuit at the second logical level when the data in said data storage circuit is at the second logical level, and furthermore
said control circuit, in a third verify operation of verifying whether the data in said memory element has reached state “3”, charges the data in said data storage circuit to the second logical level when the data in said data storage circuit is at the first logical level and the data has reached state “3”, keeps the data in said data storage circuit at the first logical level when the data has not reached state “3”, keeps the data in said data storage circuit at the second memory logical level when the data in said data storage circuit is at the second logical level, and carries out said second operation, second and third verify operations until the data in said data storage circuit has reached the second logical level.
12. The nonvolatile semiconductor memory device according to
said control circuit, judges whether the data in said memory element is in either state “2” or below or state “3” when the data in said memory element is read, stores the result of the judgment in said data storage circuit, thereafter judges whether the data is in either state “0” or state “1” or above, and, if the data stored in said data storage circuit is in state “3”, charges the potential on the bit line connected to the memory element in which the data has been stored to a low level and keeps the potential of the bit lines connected to the memory elements whose data is in either state “1” or state “2” at a high level.
13. The nonvolatile semiconductor memory device according to
said control circuit, in said second operation, omits the verify operation of verifying whether said data has reached state “3” in the first half of the verify operation of verifying whether said data has reached state “2” and omits the verify operation of verifying whether said data has reached state “2” in the latter half of the verify operation of verifying whether said data has reached state “3”.
14. The nonvolatile semiconductor memory device according to
said control circuit sets an initial write voltage in changing the data in said memory element from state “0” to state “3” and in changing the data from state “1” to state “2” higher than an initial write voltage in changing the data in said memory element from state “0” to state “3”.
16. The nonvolatile semiconductor memory device according to
said control circuit judges whether the data in said memory element is in either state “1” or below or state “2” or above in said first read operation and judges not only whether the data in said memory element is in either state “0” or state “1” or above but also whether the data is in either state “2” or below or state “3” in said second read operation.
0. 18. The nonvolatile semiconductor memory device according to
said control circuit judges whether the data in said memory element is in either state “1” or below or state “2” or above in said first read operation and judges not only whether the data in said memory element is in either state “0” or state “1” or above but also whether the data is in either state “2” or below or state “3” in said second read operation.
0. 20. The semiconductor memory device according to
the nonvolatile memory cell array includes a plurality of memory cell transistor strings, each of the strings including serially connected memory cell transistors and select gate transistors.
0. 21. The semiconductor memory device according to
a data storage circuit connected to the bit lines;
an I/O buffer connected to the data storage circuit; and
an input/output terminal connected to the I/O buffer circuit.
0. 22. The semiconductor memory device according to
0. 24. The semiconductor memory device according to
0. 25. The semiconductor memory device according to
0. 26. The semiconductor memory device according to
data storage circuits for receiving one of said first page of data and said second page of data.
0. 27. The semiconductor memory device according to
at least one input/output buffer connected to the data storage circuit; and
an input/output terminal connected to the input/output buffer circuit.
0. 30. The semiconductor memory device of
a data storage circuit receiving said one of said first page and said second page of data and then receiving said other of said first page and said second page of data.
0. 31. The semiconductor memory device of
0. 32. The semiconductor memory device of
0. 33. The semiconductor memory device of
0. 34. The semiconductor memory device of
0. 35. The semiconductor memory device of
0. 36. The semiconductor memory device of
0 V or below,
0.3 V to 0.5 V,
0.8 V to 1.0 V,
1.3 V to 1.5 V.
0. 37. The semiconductor memory device of
0. 38. The semiconductor memory device of
0. 39. The semiconductor memory device of
0. 40. The semiconductor memory device of
0. 41. The semiconductor memory device of
0. 42. The semiconductor memory device of
0. 43. The semiconductor memory device of
0. 44. The semiconductor memory device of
0. 46. The nonvolatile semiconductor memory device according to
said control circuit judges whether the data in said memory element is in either page data state “10” or below or page data state “00” or above in said first read operation and judges not only whether the data in said memory element is in either page data state “11” or page data state “10” or above but also whether the data is in either page data state “00” or below or page data state “01” in said second read operation.
0. 48. The nonvolatile semiconductor memory device according to
said control circuit judges whether the data in said memory element is in either page data state “10” or below or page data state “00” or above in said read operation of said one of said first page and said second page of data and judges not only whether the data in said memory element is in either page data state “11” or page data state “10” or above but also whether the data is in either page data state “00” or below or page data state “01” in said read operation of the other of said first page and said second page of data.
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This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-266085, filed Sep. 20, 1999, the entire contents of which are incorporated herein by reference.
This invention relates to a nonvolatile semiconductor memory device capable of storing, for example, multivalued data.
A NAND flash memory using an EEPROM has been proposed as an electrically rewritable nonvolatile semiconductor memory. In the NAND flash memory, the sources and drains of memory cells arranged side by side are connected in series and the series connection of the memory cells is connected as one unit to a bit line. In the NAND flash memory, all or half of the cells arranged in the direction of row are written into or read from all at once. Recently, a multivalued memory that enables data items to be stored in one cell in a NAND flash memory has been developed.
It is assumed that the data in the memory cell in the erased state is in state “0”. First, the first-page data is written into the memory cell. When the write data is “1”, the data in the memory cell remains in state “0”. When the write data is “0”, the data in the memory cell goes to state “1”.
Next, the second-page data is written. At this time, when write data “0” is externally supplied to the memory cell whose data has become state “1” as a result of the first-page write operation, the data in the memory cell is brought into state “3”. Moreover, when data “0” is externally supplied to the memory cell whose data has remained in state “0” as a result of the first-page write operation, the data in the memory cell is brought into state “2”.
Furthermore, when data “1” is externally supplied to the memory cell whose data has become state “1” as a result of the first-page write operation, the data in the memory cell is allowed to remain in state “1”. In addition, when data “1” is externally supplied to the memory cell whose data has remained in state “0” as a result of the first-page write operation, the data in the memory cell is allowed to remain in state “0”.
On the other hand, when the data stored in the memory cell is read, the second-page data is read first and the first-page data is read. With the definition of
In contrast, when the first page data is read, if the data in the memory ell is in state “0” or state “2”, the data to be read will be “1”. If the data in the memory cell is in state “1” or state “3”, the data to be rad will be “0”. Consequently, the first page requires a total of three read operations for the following judgments: a judgment whether the data in the memory cell is in either state “0” or state “1” or above, a judgment whether the data in the memory cell is in either state “1” or below or state “2”, or above, and a judgment whether the data in the memory cell is in either state “2” or below or state “3”.
Therefore, an ordinary nonvolatile semiconductor memory device requires many operations in reading the data from the memory cells, taking a long time to read the data.
It is, accordingly, an object of the present invention to overcome the above disadvantage by providing a nonvolatile semiconductor memory device capable of reducing the number of operations in reading data and shortening the data read time.
The foregoing object is accomplished by providing a nonvolatile semiconductor memory device comprising: a memory element which is connected to a bit line and a word line and stores one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage; a date storage circuit which is connected to the bit line and stores not only data of a first or a second logical level externally supplied but also the data of the first or second level read from the memory element; and a control circuit which controls not only the potential on the bit line and that on the word line but also the operation of the data storage circuit, wherein the control circuit operates in such a manner that in a first operation, the control circuit changes the data in the memory element from the state “0” to state “1” when the data in the data storage circuit is data of the first logical level and keeps the data in the memory element in the state “0” when the data in the data storage circuit is data of the second logical level, that in a first verify operation of verifying whether the data has reached state “1”, the control circuit brings the data in the data storage circuit to the second logical level when the data in the data storage circuit is at the first logical level and the data has reached state “1”, keeps the data in the data storage circuit at the first logical level when the data has not reached state “1”, keeps the data in the data storage circuit at the second logical level when the data in the data storage circuit is at the second logical level, and carries out the first operation until the data in the data storage circuit has reached the second logical level, and that in a second operation, the control circuit changes the data in the memory element from state “1” to state “2” when the data in the data storage circuit is data of the first logical level externally supplied and the data in the memory element is in state “1”, and changes the data in the memory element from state “0” to state “3” when the data in the memory element is in state “0”.
The foregoing object is further accomplished by providing a nonvolatile semiconductor memory device comprising: a memory element which is connected to a bit line and a word line and stores one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage; a first storage circuit which is connected to the bit line and stores data of a first or a second logical level externally supplied; a second storage circuit which is connected to the bit line and stores the data of the first or second level read from the memory element; and a control circuit which controls not only the potential on the bit line and that on the word line but also the operation of the first and second storage circuits, wherein the control circuit operates in such a manner that in a first operation, the control circuit changes the data in the memory element from state “0” to state “1” when the data in the first data storage circuit is data of the first logical level and keeps the data in the memory element at the state “0” when the data in the first storage circuit is data of the second logical level, that in a first verify operation of verifying whether the data has reached state “1”, the control circuit brings the data in the first storage circuit to the second logical level when the data in the first storage circuit is at the first logical level and the data has reached state “1”, keeps the data in the first storage circuit at the first logical level when the data has not reached state “1”, keeps the data in the first storage circuit at the second logical level when the data in the first storage circuit is at the second logical level, and carries out the first operation until the data in the first storage circuit has reached the second logical level, that in a second operation, the control circuit stores the data read from the memory element into the second storage circuit, changes the data in the memory element from state “1” to state “2” when the data in the first storage circuit is data of the first logical level externally supplied, changes the data in the memory element from state “0” to state “3” when the data in the memory element is in state “0”, and keeps the data in the memory element when the data in the memory element is data of the second logical level, that in a second verify operation of verifying whether the data in the memory element has reached state “2”, the control circuit brings the data in the first storage circuit to the second logical level when the data has reached state “2” in a case where the data in the first storage circuit is at the first logical level and the data in the memory element is in state “1” before the second operation is carried out, keeps the data in the first storage circuit at the first logical level when the data has not reached state “2”, and brings the potential on the bit line to which the memory element is connected to the first logical level and the data in the first storage circuit to the first logical level when the data in the second storage circuit is at the second logical level in a case where the data in the memory element is in state “0” before the second operation is carried out, and that in a third verify operation of verifying the data has reached state “3”, the control circuit brings the data in the first storage circuit to the second logical level when the data in the first storage circuit is at the first logical level and the data has reached state “3”, keeps the data in the first storage circuit at the first logical level when the data has not reached state “3”, keeps the data in the first storage circuit at the second memory logical level when the data in the first storage circuit is at the second logical level, and carries out the second operation and second and third verify operations until the data in the first storage circuit has reached the second logical level.
The foregoing object is further accomplished by providing a nonvolatile semiconductor memory device comprising: a memory element which is connected to a bit line and a word line and stores one of an n number of data items made up of state “0”, state “1”, . . . , state “n” (3≦n where n is a natural number); a data storage circuit which stores data of a first or a second memory logical level externally inputted; and a control circuit which controls not only the potential on the bit line and that on the word line but also the operation of the data storage circuit, wherein the control circuit, in a final write operation, brings state “0” of the smallest data stored in the memory element into state “n” of the largest data.
The foregoing object is further accomplished by providing a nonvolatile semiconductor memory device comprising: a memory element which is connected to a bit line and a word line and stores one of state “0”, state “1”, state “2”, and state “3” of data that differ in threshold voltage; a data storage circuit which is connected to the bit line and stores the data read from the memory element; and a control circuit which controls not only the potential on the bit line and that on the word line but also the operation of the data storage circuit, wherein the control circuit operates in such a manner that in a first read operation, the control circuit sets data of a first logical level in the data storage circuit when the data in the memory element is in either state “0” or state “1”, and sets data of a second logical level in the data storage circuit when the data in the memory element is in either state “2” or state “3”, and that in a second read operation, the control circuit sets data of the first logical level in the data storage circuit when the data in the memory element is in either state “0” or state “3”, and sets data of the second logical level in the data storage circuit when the data in the memory element is in either state “1” or state “2”.
With the present invention, the number of operations in reading the data can be reduced, which makes it possible to provide a nonvolatile semiconductor memory device capable of shortening the time required to read the data.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.
The accompanying drawings, which are incorporated in and constitute a part of the specification, illustrate presently preferred embodiments of the invention, and together with the general description given above and the detailed description of the preferred embodiments given below, serve to explain the principles of the invention.
Hereinafter, referring to the accompanying drawings, embodiments of the present invention will be explained.
(First Embodiment)
The principle of a first embodiment of the present invention will be explained.
As shown in
When the data on the second page is read, if the data in the memory cell is in state “0” or state “1”, the data to be read will be “1”. If the data in the memory cell is in state “2” or state “3”, the data to be read will be “0”. As a result, the reading of the data on the second page is determined by only one judgment whether the data in the memory cell is in either state “1” of below or state “2” or above as shown in
On the other hand, when the data on the first page is read, if the data in the memory cell is in state “0” or state “3”, the data to be read will be “1”. If the data in the memory cell is in state “1” or state “2”, the data to be read will be “0”. As a result, the data on the first page is read on the basis of a judgment whether the data in the memory cell is in either state “0” or state “1” or above and a judgment whether the data in the memory cell is in either state “2”, or below or state “3”. Namely, the data on the first page can be read by a total of two operations.
As described above, when the data on the first page is read, the reading of data requires only two read operations in the present invention, whereas three read operations are needed to read the data in
In the erased state, the data in the memory cell is in state “0”. First, the data on the first page is written into the memory cell. If the write data is “1”, the data in the memory cell into which no data is written will remain “0”. If the write data is “0”, the data in the memory cell into which the data is written will go to state “1”.
Next, the data on the second page is written. At this time, if the write operation of the first page causes write data “0” to be supplied to the memory cell whose data is in state “1”, the data in the memory cell will be brought into state “2”. In addition, if the write operation of the first page causes write data “0” to be supplied to the memory cell whose data is in state “0”, the data in the memory cell will be brought into state “3”.
Furthermore, if the write operation of the first page causes write data “1” to be externally supplied to the memory cell whose data is in state “1”, the data in the memory cell will be allowed to remain in state “1”. Moreover, if the write operation of the first page causes write data “1” to be externally supplied to the memory cell whose data is in state “0”, the data in the memory cell will be allowed to remain in state “0”.
In the case of a multivalued memory, the threshold voltage of the memory cell has to be controlled accurately according to the write data. For this reason, when the data is written into the memory cell, the voltage applied to the control gate of the memory cell is increased gradually, thereby writing the data. This writing method is called a step-up writing method.
The threshold voltage of the cell after the data has been erased (the data in the memory cell is in state “0”) is assumed to be, for example, −3.5V. As described above, in the present invention, when the data in the memory cell is changed from state “0” to state “3”, 16V is applied as an initial program voltage to the control gate of the cell. Thereafter, writing is done by increasing the write voltage in 0.2-V steps, with the result that the threshold voltage increases as represented by “0”→“3”, in the figure. On the other hand, when the data in the memory cell is changed from state “0” to “1”, the initial write voltage is set to 14V and then writing is started. The reason is that the threshold voltage when the data is in state “1” is 0.2V. Thus, if the initial write voltage were set to 16V and writing started, the threshold voltage would reach the threshold voltage for the data in state “1” between step 3 and step 4, which might result in an overprogam. To avoid this, the initial write voltage is set to 14V.
Since the data in the memory cell is moved from state “0” to state “1” in writing data on the first page by both the method of FIG. 3 and the method of the present invention, the data in the memory cell can be set to the threshold voltage of state “1” through 13 writes.
In writing the data on the second page, the method of
In contrast, with the present invention, the data in the memory cell is moved from state “0” to state “3” or from state “1” to state “2”. Furthermore, as shown in
(Embodiment)
A memory cell array 1 includes bit lines, word lines, and common source lines. In the memory cell array 1, electrically rewritable memory cells composed of, for example, EEPROM cells are arranged in a matrix. A bit line control circuit 2 for controlling the bit lines and a word line control circuit 6 are connected to the memory cell array 1.
The bit line control circuit 2 includes data storage circuits as described later and read the data in a memory cell in the memory cell array 1 via a bit line or senses the state of a memory cell in the memory cell array 1 via a bit line. In addition, the bit line control circuit 2 applies a write control voltage to a memory cell in the memory cell array 1 via a bit line, thereby writing the data into the memory cell. A column decoder 3 and a data input/output buffer 4 are connected to the bit line control circuit 2. A data storage circuit in the bit line control circuit 2 is selected by the column decoder 3. The data in the memory cell read into the data storage circuit is outputted via the data input/output buffer 4 at a data input/output terminal to the outside.
The write data externally inputted to the data input/output terminal 5 is inputted via the data input/output buffer 4 to the data storage circuit selected by the column decoder 3.
The word line control circuit 6 is connected to the memory cell array 1. The word line control circuit 6 selects a word line in the memory cell array 1 and applies to the word line the voltage necessary for reading, writing, or erasing.
The memory cell array 1, bit line control circuit 2, column decoder 3, data input/output buffer 4, and word line control circuit 6 are connected to a control signal generator circuit 7a and a control voltage generator circuit 7b and are controlled by the control signal generator circuit 7a and control voltage generator circuit 7b. The control signal generator circuit 7a and control voltage generator circuit 7b are connected to a control signal input terminal 8 and are controlled by a control signal inputted through the control input terminal 8 from the outside. Namely, the control voltage generator circuit 7b generates the voltage necessary to program, verify, read, or erase the data and supplies the voltage to each section of the memory cell array 1.
A pair of bit lines is connected to each of the data storage circuits 310, 311 to 312111. Specifically, bit lines BL0, BL1 are connected to the data storage circuit 310, bit lines BL2, BL3 are connected to the data storage circuit 311, and bit lines BL4222, BL4223 are connected to the data storage circuit 312111.
In the memory cell array 1, NAND cells are arranged. A NAND cell is composed of a series connection of, for example, memory cells M1, M2, M3 to M6, or 16 EEPROMs, a select gate S1 connected to memory cell M1, and a select gate S2 connected to memory cell M16. The first select gate S1 is connected to bit line BL0 and the second select gate S2 is connected to source line SRC. The control gates of the memory cells M1, M2, M3 to M16 arranged in each row are connected to word line WL1, WL2, WL3 to WL16, respectively. The first select gate S1 is connected to select line SG1 and the second select gate S2 is connected to select line SG2.
A block is composed of 4223 NAND cells. Data is erased in blocks. The memory cells connected to a word line constitute a sector. Data is written and read in sectors. In a sector, for example, two pages of data are stored.
One end of the current path of an n-channel transistor 61a is connected to bit line BLi. Signal BLTR is supplied to the gate of the transistor 61a. The other end of the current path of the transistor 61a is connected to one end of the current path of the transistor 61b and one end of the current path of a transistor 61c. The other end of the current pat of the transistor 61b is connected to a terminal 62a. Voltage VBLA is supplied to the terminal 62a. Signal PREA is supplied to the gate of the transistor 61b. Signal BLSA is supplied to the gate of the transistor 61c.
One end of the current path of an n-channel transistor 61d is connected to bit line BLi+1. The signal BLTR is supplied to the gate of the transistor 61d. The other end of the current path of the transistor 61d is connected to one end of the current path of a transistor 61e and one end of the current path of a transistor 61f. The other end of the current path of the transistor 61e is connected to a terminal 62b. Voltage VBLB is supplied to the terminal 62b. Signal PREB is supplied to the gate of the transistor 61e. Signal BLSB is supplied to the gate of the transistor 61f. The transistors 61b, 61e precharge the unselected bit lines to the potentials VBLA, VBLB according to the signals PREA, PREB. The transistors 61c, 61f select a bit line according to the signals BLSA, BLSB.
The other ends of the current paths of the transistors 61c, 61f are connected to not only a terminal 62c via a transistor 61g but also a node NE. Signal BIAS is supplied to the gate of the transistor 61g and voltage VCC is supplied to the terminal 62c. The transistor 61g precharges a bit line according to signal BIAS in reading the data.
One end of the current path of a transistor 61h is connected to the node NE. Signal BLC1 is supplied to the gate of the transistor 61h. A first latch circuit LAT(A) is connected to the other end of the current path of the transistor 61h. The first latch circuit LAT(A) is composed of two clocked inverter circuits 61i, 61j. The clocked inverter circuit 61i is controlled by signals SEN1, SEN1B (B indicates the inverted signal). The clocked inverter circuit 61j is controlled by signals LAT1, LAT1B. The first latch circuit LAT(A) latches write data.
Transistors 61k, 61l are connected in series with the node NE. The gate of the transistor 61k is connected to a node NC in the first latch circuit LAT(A). Signal VRFY1 is supplied to the gate of the transistor 61l. Signal VREG is supplied to the current path of the transistor 61l. These transistors 61k, 61l set a potential on the bit line according to the data latched in the first latch circuit LAT(A).
A node NA in the first latch circuit LAT(A) is connected via a p-channel transistor 61m to a terminal 62d. Signal PRSTB1 is supplied to the gate of the transistor 61m and voltage VCC is supplied to the terminal 62d. The transistor 61m sets the node NA of the first latch circuit LAT(A) at the high level in writing or reading the data. The node NA is connected via a capacitor 61n to the ground. The capacitor 61n holds the charge at node NA in reading the data.
The node NA is connected to the column gate (not shown) via a transistor 61o and a clocked inverter circuit 61p connected in parallel. Signal SPB is supplied to the gage of the transistor 61o. The clocked inverter 61p is controlled by signals Osac, Osacb (b indicates the inverted signal). The transistor 61o transfers to the first latch circuit LAT(A) the data supplied via the column select gate in writing the data. The clocked inverter circuit 61p functions as a buffer in reading the data. on the other hand, one end of the current path of a transistor 61q is connect ed to the node NE. Signal BLC2 is supplied to the gate of the transistor 61q. A second latch circuit LAT(B) is connected to the other end of the current path of the transistor 61q. The second latch circuit LAT(B) is composed of two clocked inverter circuits 61r, 61s. The clocked inverter circuit 61r is controlled by signals SEN2, SEN2B. The clocked inverter circuit 61s is controlled by signal s LAT2, LAT2B. The second latch circuit LAT(B) latches the data read from a cell.
Transistors 61r, 61u are connected in series with the node NE. The gate of the transistor 61r is connected to a node ND in the second latch circuit LAT(B). Signal VRFY2 is supplied to the gate of the transistor 61u. Signal VREG is supplied to the current path of the transistor 61u. These transistors 61t, 61u set a potential on the bit line according to the data latched in the second latch circuit LAT(B).
A node NB in the second latch circuit LAT(B) is connected via a p-channel transistor 61v to a terminal 62e. Signal PRSTB2 is supplied to the gate of the transistor 61v and voltage VCC is supplied to the terminal 62e. The transistor 61v sets the node NB of the second latch circuit LAT(B) at the high level in verify read. The node NB is connected via a capacitor 61w to the ground. The capacitor 61w holds the charge at node NB in verify read.
An explanation of LAT(B1) and LAT(B2) will be given in a second embodiment of the present invention.
The operation of the above configuration will be explained.
As described earlier, the data in the memory cell and the threshold voltage of the memory cell have been defined as shown in
With the states defined as described above, when the first-page address is specified in reading the data, if the data stored in the memory cell is in state “0” or state “3”, the data read out will be “1”.
If the data stored in the memory cell is in state “1” or state “2”, the data read out will be “0”. Therefore, the reading of the first page requires a total of two operations or judgments: a judgment whether the data in the memory cell is in state “0” or state “1” or above and a judgment whether the data in the memory cell is in state “2” or below or state “3”.
When the second-page address is specified, if the data stored in the memory cell is in state “0” or state “1”, the data read out will be “1”. If the data stored in the memory cell is in state “2” or state “3”, the data read out will be “0”. Therefore, the reading of the second page requires only one operation or a judgment whether the data in the memory cell is in either state “1” or below or state “2” or above.
After an erase operation has been carried out, the data in the memory cell goes to state “0”.
(Cell Selecting Method)
In a data read operation, a program verify operation and a program operation, one of the two bit lines BLi, BLi+1 (i=0, 1, 2 . . . ) connected to the data storage circuits 310 to 312111 is selected according to an external address supplied from the outside. Furthermore, according to the external address, one word line is selected and two pages (one sector) represented by a solid line in
An erase operation is carried out in blocks represented by the dotted lines in FIG. 9. Erasing is done simultaneously on the two bit lines (BLi, BLi+1) connected to each data storage circuit.
An erase verify operation includes two operations. In a first operation, verify reading is done on one (BLi) of the two bit lines (BLi, BLi+1) connected to the data storage circuit. The data read out is stored in the latch circuit LAT(A) of FIG. 12. Then, a verify operation is carried out on the other bit line (BLi+1). The logical add of the read-out data and the result of the preceding verify reading is stored in the first latch circuit LAT(A). The erase verify operation is repeated until the nodes NA of all the first latch circuits LAT(A) go low.
After the erase operation and erase verify operation have been carried out, the data in the memory cell go into state “0”. Even when either of the first-page address and second-page address is specified, the data read out is “0”. Specifically, because the node NA of the first latch circuit LAT(A) is at the low level, the data read via the clocked inverter circuit 61p is “1”.
{Program or Program Verify}
(First-page Program)
In a program operation, two pages (one sector) of memory cells shown in
In the first-page program operation, the data on the first page is externally inputted (
Then, the program (
Here, VCC is applied to the select line SG1 of the selected block, VPGM (20V) is applied to the selected word line, and Vpass (10V) is applied to the unselected word lines. Then, when the bit line is at VSS, the channel of the cell is at VSS and the word line is at VPGM, with the result that electrons are injected into the floating gate of the cell, thereby writing the data. On the other hand, when the bit line is at VCC, the first selected gage S1 is off. This raises VPGM, not VSS, with the result that the channel of the cell is brought to VPGM/2 through coupling. Consequently, the cell is not programmed.
In this way, the data in the memory cell into which data “0” is to be written goes to state “1”. The data in the memory cell into which data “1” is written remains in state “0”.
(First-page Program Verify Read)
Next, program verify read is executed (
As shown in
Next, as shown in
Thereafter, the select line SG2 on the source side of the memory cell is made high (Vread). When the threshold voltage of the memory cell is higher than the voltage “b′”, the memory cell is off, with the result that the bit line remains high. When the threshold voltage of the memory cell is lower than the voltage “b′”, the memory cell is on, with the result that the potential on the bit line goes low (VSS).
When writing is done, the low level (data “0”) is latched at the node NA of the latch circuit LAT(A) of FIG. 12. When no writing is done, the high level (data “1”) is latched at the node NA. Thus, when signal VREG supplied to the current path of the transistors 61l is brought to VCC and signal VRFY1 supplied to the gate is made high, fixing the bit line from the floating state to the high level only when no writing is done. After this operation, the potential on the bit line is read into the first latch circuit LAT(A). It is when the potential at the memory cell has reached to the threshold voltage or when no writing is done that the high level is latched in the first latch circuit LAT(A). Furthermore, it is only when the potential at the memory cell has not reached the threshold voltage that the low level is latched in the first latch circuit LAT(A).
Therefore, when the first latch circuit LAT(A) is at the low level, the write operation is carried out again and the program operation and verify read operation are repeated until the data in all the data storage circuits have become high (
(Second-page Program)
As in the first-page program, in the second-page program, the data on the second page is inputted from the outside (ST11). These data items are stored in the first latch circuits LAT(A) of the data storage circuits 310 to 312111.
The operation of the second-page program differs greatly from that of the first-page program in internal data load (ST12). The operation of the second-page program varies according to the result of the operation of the first-page program.
Specifically, as shown in
As described above, the operation of the second-page program varies according to the result of the operation of the first-page program. For this reason, it is necessary to check whether the data in the memory cell is in state “0” or state “1” before writting the data on the second page and store the result of the check. Then, the data in the memory cell is read and loaded into the second latch circuit LAT(B) in the data storage circuit of
The internal data load operation is almost the same as the first-page verify operation of FIG. 16. While in the first-page verify operation, the data read from the memory cell has been stored in the first latch circuit LAT(A), the data is stored in the second latch circuit LAT(B) in the internal data load. Specifically, the bit line is precharged in the same manner as in the first-page verify operation of FIG. 16 and the data in the memory cell is read onto the bit line. Thereafter, as shown in
When the data in the memory cell is in state “0”, the memory cell turns on according to the read operation, with the result that the potential on the bit line goes low. This causes the low level to be latched at node NB and the high level to be latched at node ND in the second latch circuit LAT(B). When the data in the memory cell is in state “1”, the memory cell turns off, with the result that the potential on the bit line goes high. This causes the high level to be latched at node NB and the low level to be latched at node ND in the second latch circuit LAT(B). The data in the second latch circuit LAT(B) of
Next, as in the first-page program, specific voltages are applied to various sections. In this state, all the cells selected are written into according to the data on the second page stored in the first latch circuits LAT(A) (
(Second-page Verify)
A second page verify includes a first verify read (ST14) and a second verify read (ST15). The first verify read (ST14) verifies whether or not the data in the memory cell is in state “2”. The second verify read (ST15) verifies whether or not the data in the memory cell is in state “3”.
{Second-page First Verify Read}
In the verify operation, a read operation is carried out, with the potential “b′” applied to the word line, as shown in FIG. 1. As a result, when the threshold voltage of the memory cell has reached “b′”, the bit line is at the high level. When threshold voltage has not reached “b′”, the bit line is at the low level. At this time, the cells that bring the data in the memory cells into state “3” are also turned off. This results in verify OK. Thus, no writing is done in the first-page write operation, making low the potential on the bit lines connected to the memory cells whose data is in state “0”.
Specifically, when no writing is done in the first-page write operation and the data in the memory cell is in state “0”, the internal data load makes high node ND of the second latch circuit LAT(B). In this state, signal VREG supplied to the current path of the transistor 61u of
Next, as in the first-page verify operation, signal VREG is made the power-supply voltage VCC and signal VRFY1 supplied to the gate of the transistor 61l is made high. Then, when the high level is latched at node NA of the first latch circuit LAT(A) (or when no writing is done), transistor 61k turns on. As a result, the bit line goes high. After this operation, the potential on the bit line is read into the first latch circuit LAT(A).
As shown in
{Second-page Second Verify Read}
This verify is completely the same as the first-page verify operation. It is because there is no cell whose potential is higher than the potential “c′” of
As a result, as shown in
Therefore, in the second-page verify , two operations are carried out: a first verify read whereby the data in the memory cell is written to obtain state “2” and a second verify read whereby the data in the memory cell is written to obtain state “3”. Moreover, when the first latch circuit LAT(A) is at the low level, writing is done again. In this way, the program operation and verify operation are repeated until the data in all the data storage circuits have become high (
When the data in the memory cell is in state “3”, the threshold voltage is high. This makes it hard to write the data into the memory cell. For this reason, of the repeated program verify operations, the first several verify operations of verifying whether the data in the memory cell is in state “3” can be omitted. For the memory cells into which data in state “2” with the low threshold voltage is written after several repeated operations, writting must have been completed. Consequently, after the program verify operation has been repeated several times, verifying the data in the memory cell which is in state “2” can be omitted.
{Read Operation}
In reading the memory cell, the data on the second page is first read.
(Second-page Reading)
In the second page read, the potential “c”, in reading is applied to the selected word line.
Next, as shown in
Next, the potentials on these bit lines are read into the first latch circuits LAT(A). As shown in
(First-page Read)
Next, the data on the first page is read. When the data outputted in the first-page read is “1”, the data in the memory cell is in state “0” or state “3” as shown in FIG. 2.
Therefore, it is first judged whether the data in the memory cell is in either state “2” or below or state “3”. Next, it must be judged whether the data in the memory cell is in either state “0” or state “1” or above.
{First Read Operation}
In the first read operation, it is judged whether the data in the memory cell is in either state “2” or below or state “3”.
First, to check whether the data in the memory cell is in either state “2” or below or state “3”, the potential “c” is applied to the word line, thereby reading the data in the memory cell. As a result, it is only when the data in the memory cell is in state “3” that the high level is latched in the first latch circuit LAT(A) as shown in FIG. 21A. Moreover, it is when the data in the memory cell is in any one of state “0”, state “1”, and state “2” that the low level is latched in the first latch circuit LAT(A).
{Second Read Operation}
Next, in the second read operation, it is judged whether the data in the memory cell is in either state “0” or state “1” or above.
First, to check whether the data in the memory cell is in either state “0” or state “1” or above, the potential “a” is applied to the word line, thereby reading the data in the memory cell. As a result, it is only when the data in the memory cell is in state “0” that the low level is latched in the first latch circuit LAT(A). Moreover, it is when the data in the memory cell is in any one of state “1”, state “2”, and state “3” that the high level is latched in the first latch circuit LAT(A).
Here, signal VREG of
Next, when the potentials on these bit lines are read into the first latch circuits LAT(A), if the data in the memory cell is in either state “0” of state “3” as shown in
With the first embodiment, a memory cell whose data is in state “0” is brought into state “1” in the first-page write operation, and a memory cell whose data is in state “0” is brought into state “3” and a memory cell whose data is in state “1” is brought into state “2” in the second-page write operation. As a result, in reading the data, the data on the first page is read in only two operations: a first read operation for judging whether the data in the memory cell is in either state “2” or below or state “3”, and a second read operation for judging whether the data in the memory cell is in either state “0” or state “1” or above. Consequently, the number of times the first-page is read is decreased.
In the second-page write operation, the data written in the first-page write operation is first latched in the second latch circuit LAT(B). Thereafter, the second-page write operation is carried out. In a verify operation with the data in state “2”, the potential on the bit line of the memory cell whose data is in state “0” and is to be written to obtain state “3” is made low on the basis of the data latched in the second latch circuit LAT(B). This prevents the memory cell to be brought into state “3” from having been written into. As a result, the operation of writing data in state “3” into the memory cell into which data in state “0” has been written and the operation of writing data in state “2” into the memory cell into which data in state “1” has been written can be executed simultaneously, enabling a high-speed operation.
Furthermore, in both cases where data in state “3” is written into the memory cell with data in state “0” and where data in state “2” is written into the memory cell with data in state “1”, a high initial write voltage can be used, which realizes a high-speed write operation.
In the first embodiment, 4-valued (2-bit) data has been stored. The present invention is not limited to this and may be applied to a case where n-valued (n≧8, 3 bits or more) data is stored.
(Second embodiment)
The third-page write operation is carried out as shown in FIG. 25. First, a case where the write data is “1” will be explained. When the data in the memory cell before writing is done is in state “0”, the data in the memory cell is allowed to remain in state “0”. In addition, when the data in the memory cell before writing is done is in state “1”, the data in the memory cell is allowed to remain in state “1”. Moreover, when the data in the memory cell before writing is done is in state “2”, the data in the memory cell is allowed to remain in state “2”. Furthermore, when the data in the memory cell before writing is done is in state “3”, the data in the memory cell is allowed to remain in state “3”.
In a case where the write data is “0”, when the data in the memory cell before writing is done is in state “0”, the data in the memory cell is brought into state “7”. In addition, when the data in the memory cell before writing is done is in state “1”, the data in the memory cell is brought into state “6”. Moreover, when the data in the memory cell before writing is done is in state “2”, the data in the memory cell is brought into state “5”. Furthermore, when the data in the memory cell before writing is done is in state “3”, the data in the memory cell is brought into state “4”.
Next, the verify operation will be explained. The verify operation is carried out as in the case of four-valued data. Specifically, in an internal data load, the data in the memory cell has been stored in the second latch circuit LAT(B) beforehand. Next, when the result of verifying a memory cell into which data larger than the data in the memory cell to be verified is written has shown verify OK, the data stored in the first latch circuit LAT(A) is kept at write data “0”. As a result, the process of writing large data into the memory cell is continued.
With the second embodiment, the third-page program brings the data in the memory cell into state “4”, state “5”, state “6”, and state “7”. To store three of these states, that is, state “5”, state “6”, and state “7”, for example, three second latch circuits LAT(B) are provided in the data storage circuits shown in FIG. 12. LAT(B1) and LAT(B2) represented by broken lines indicate second latch circuits. In the internal data load, the state of data in each memory cell is stored in each of the three second latch circuits LAT(B), LAT(B1), and LAT(B2).
In the verify operation of verifying whether or not the data in the memory cell has become state “4”, when the data in the memory cell has reached state “4”, the result of verifying the cell whose data takes state “5”, state “6”, or state “7” has also shown OK. As a result, the data in the first latch circuit LAT(A) becomes “1”. To avoid this, the data stored in the first latch circuit LAT(A) connected to the memory cell whose data takes state “5”, state “6”, or state “7” is kept at write data “0” according of the data stored in the second latch circuit LAT(B). Then, the data is changed to data “1” which allows only the first latch circuit LAT(A) connected to the memory cell whose data has been written to obtain state “4” to show verify OK.
Similarly, in the verify operation of verifying whether or not the data in the memory cell has become state “5”, when the data in the memory cell has reached state “5”, the result of verifying the cell which is being written into to bring the data in the memory cell into state “6” or state “7” has also shown OK. As a result, the data stored in the first latch circuit LAT(A) connected to the memory cell which is being written into to bring the data in the memory cell into state “6” or state “7” is kept at write data “0” according to the data stored in the second latch circuit. Then, the data is changed to data “1” which allows only the first latch circuit LAT(A) connected to the memory cell whose data has been written to obtain state “5” to show verify OK.
Similarly, in the verify operation of verifying whether or not the data in the memory cell has become state “6”, when the data in the memory cell has reached state “6”, the result of verifying the cell which is being written into to bring the data in the memory cell into state “7” has also shown OK. As a result, the data stored in the first latch circuit LAT(A) connected to the memory cell which is being written into to bring the data in the memory cell into state “7” is kept at write data “0” according to the data stored in the second latch circuit. Then, the data is changed to data “1” which allows only the first latch circuit LAT(A) connected to the memory cell whose data has been written to obtain state “6” to show verify OK.
In this way, the third-page write operation and verify operation are repeated until the data in all the first latch circuits LAT(A) have become “1”.
Next, the read operation will be explained. The data in the memory cell is read from the third page, second page, and first page in that order. The data in the memory cell and the data to be written onto and read from the first, second, and third pages have been defined as shown in FIG. 24. Therefore, in the third-page read operation, if the data in the memory cell is “3” or less, the data read out will be “1”. If the data in the memory cell is “4” or more, the data read out will be “0”. This enables the data to be read in one read operation.
In the second-page read operation, if the data in the memory cell is either “1” or less or “6” or more, the data read out will be “1”. If the data in the memory cell is “2” or more and “5” or less, the data read out will be “0”. This enables the data to be read in two read operations.
In the first-page read operation, if the data in the memory cell is “0” or less, or “3” or more and “4” or less, or “7” or more, the data read out will be “1”. If the data in the memory cell is “1” or more and “2” or less, or “5” or more and “6” or less, the data read out will be “0”. This enables the data to be read in four read operations.
With the second embodiment, in the third-page write operation, as a result of the first-page and second-page write operations, the memory cell whose data is in state “0” is brought into state “7”, the memory cell whose data is in state “1” is brought into state “6”, the memory cell whose data is in state “2” is brought into state “5”, and the memory cell whose data is in state “3” is brought into state “4”. Consequently, the number of read operations on the first page is one, the number of read operations on the second page is two, and the number of read operations on the third page is four. Therefore, the number of reads from the first page is reduced as in the first embodiment.
Furthermore, the write operation that brings a memory cell whose data is in state “0” into state “7”, the write operation that brings a memory cell whose data is in state “1” into state “6”, the write operation that brings a memory cell whose data is in state “2” into state “5”, and the write operation that brings a memory cell whose data is in state “3” into state “4” can be carried out simultaneously. Moreover, in these write operations, a higher initial write voltage than that used in a conventional equivalent can be used, enabling a higher-speed write operation.
(Third Embodiment)
In the third embodiment, the threshold voltage of the cell has been raised by the program operation and lowered by the erase operation. The present invention is not limited to this. For instance, the present invention may be applied to a nonvolatile semiconductor memory device where the threshold voltage of the cell is lowered by the program operation and raised by the erase operation.
Furthermore, the first and second latch circuits LAT(A) and LAT(B) have been composed of clocked inverter circuits. The present invention is not restricted to this. For instance, the first and second latch circuits LAT(A) and LAT(B) may be composed of capacitors and transistors that control the charging and discharging of the capacitors.
Additional advantages and modifications will readily occur to those skilled in the art. Therefore, the invention in its broader aspects is not limited to the specific details and representative embodiments shown and described herein. Accordingly, various modifications may be made without departing from the spirit and scope of the general inventive concept as defined by the appended claims and their equivalents.
Tanaka, Tomoharu, Shibata, Noboru
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