Memory device configured to perform a search operation
A memory device includes a bit line, a source line, a first string in which a plurality of first memory cells are connected in series between the bit line and the source line, and a control circuit. The control circuit performs a sense operation for a search operation to determine if search data is stored in the plurality of first memory cells by supplying voltages to a plurality of word lines respectively corresponding to the plurality of first memory cells based on the search data and determining a similarity between the search data and data actually stored in the plurality of first memory cells based on a change in voltage of the bit line caused by current flowing between the bit line and the source line via the first string.
1 . A memory device comprising:
a bit line;
a source line;
a first string in which a plurality of first memory cells are connected in series between the bit line and the source line; and
a control circuit configured to perform different operations on the first string, including a read operation on a selected first memory cell to determine a value of data stored in the selected first memory cell, and a sense operation for a search operation to determine if search data is stored in the plurality of first memory cells, wherein
the control circuit, in the sense operation,
supplies voltages to a plurality of word lines respectively corresponding to the plurality of first memory cells based on the search data, and
determines a similarity between the search data and data actually stored in the plurality of first memory cells based on a change in voltage of the bit line caused by current flowing between the bit line and the source line via the first string, and
the control circuit, in the read operation,
supplies a source voltage to the source line,
supplies a first read voltage to a word line corresponding to the selected first memory cell, and
supplies a second read voltage to word lines corresponding to the first memory cells other than the selected first memory cell, wherein a difference between the second read voltage and the source voltage is less than a difference between the first read voltage and the source voltage.
2 . The memory device according to claim 1 , wherein the control circuit determines a similarity between the search data and the data actually stored in two or more adjacent memory cells among the plurality of first memory cells by supplying voltages to two or more word lines corresponding to the two or more adjacent first memory cells based on the search data.
3 . The memory device according to claim 1 , wherein the control circuit, in the sense operation,
executes control such that one of at least (2k)-th and (2k+1)-th (where k is a natural number of 1 or more) first memory cells from a side of the source line among the plurality of first memory cells enters an OFF state when data actually stored in (2k)-th and (2k+1)-th first memory cells are the same as the search data, and
executes control such that both the (2k)-th and (2k+1)-th first memory cells from the side of the source line among the plurality of first memory cells enter an ON state when the data actually stored in (2k)-th and (2k+1)-th first memory cells are different from the search data.
4 . The memory device according to claim 1 , wherein
the first string includes a current control memory cell connected in series to the plurality of first memory cells between the bit line and the source line, and
in the sense operation, the control circuit supplies a voltage less than a threshold voltage of the current control memory cell to the word line corresponding to the current control memory cell.
5 . The memory device according to claim 1 , further comprising:
a second string in which a plurality of second memory cells are connected in series between the bit line and the source line, wherein
the first string includes a plurality of first word lines respectively controlling the plurality of first memory cells,
the second string includes a plurality of second word lines respectively controlling the plurality of second memory cells, and
the control circuit, in the sense operation,
individually controls the first and second word lines,
supplies voltages to the plurality of second word lines respectively corresponding to the plurality of second memory cells based on the search data, and
determines a similarity between the search data and the data actually stored in the plurality of first memory cells and the plurality of second memory cells based on a change in voltage of the bit line caused by current flowing between the bit line and the source line via the first and second strings.
6 . The memory device according to claim 5 , wherein
the first string includes a first current control memory cell connected in series to the plurality of first memory cells between the bit line and the source line,
the second string includes a second current control memory cell connected in series to the plurality of second memory cells between the bit line and the source line, and
in the sense operation, the control circuit
controls the first current control memory cell such that first current flows between the bit line and the source line via the first string, and
controls the second current control memory cell such that second current with an amount different from the first current flows between the bit line and the source line via the second string.
7 . The memory device according to claim 6 , wherein a threshold voltage of the first current control memory cell is different from a threshold voltage of the second current control memory cell.
8 . The memory device according to claim 5 , wherein
the first string includes a plurality of first current control memory cells connected in series to the plurality of first memory cells between the bit line and the source line,
the second string includes a plurality of second current control memory cells connected in series to the plurality of second memory cells between the bit line and the source line,
the plurality of first current control memory cells have different threshold voltages,
the plurality of second current control memory cells have different threshold voltages, and
in the sense operation, the control circuit
selects one first current control memory cell among the plurality of first current control memory cells such that first current flows between the bit line and the source line via the first string, and
selects one second current control memory cell among the plurality of second current control memory cells such that second current with an amount different from the first current flows between the bit line and the source line via the second string.
9 . The memory device according to claim 1 , wherein
the first string includes a plurality of current control memory cells connected in series to the plurality of first memory cells between the bit line and the source line, and
threshold voltages of the plurality of current control memory cells are different from each other.
10 . The memory device according to claim 9 , wherein, in the sense operation, the control circuit
supplies one word line among the plurality of word lines corresponding to the plurality of current control memory cells with voltage less than a threshold voltage of the current control memory cell corresponding to the one word line, and
supplies other word lines among the plurality of word lines corresponding to the plurality of current control memory cells with voltages by which the plurality of current control memory cells are controlled to an ON state regardless of data stored therein.
11 . A method of operating a memory device comprising a bit line, a source line, a first string in which a plurality of first memory cells are connected in series between the bit line and the source line, and a control circuit configured to perform different operations on the first string, including a read operation on a selected first memory cell to determine a value of data stored in the selected first memory cell, and a sense operation for a search operation to determine if search data is stored in the plurality of first memory cells, wherein
the sense operation includes the steps of:
supplying voltages to a plurality of word lines respectively corresponding to the plurality of first memory cells based on the search data, and
determining a similarity between the search data and data actually stored in the plurality of first memory cells based on a change in voltage of the bit line caused by current flowing between the bit line and the source line via the first string, and
the read operation includes the steps of:
supplying a source voltage to the source line,
supplying a first read voltage to a word line corresponding to the selected first memory cell, and
supplying a second read voltage to word lines corresponding to the first memory cells other than the selected first memory cell, wherein a difference between the second read voltage and the source voltage is less than a difference between the first read voltage and the source voltage.
12 . The method according to claim 11 , wherein the control circuit determines a similarity between the search data and the data actually stored in two or more adjacent memory cells among the plurality of first memory cells by supplying voltages to two or more word lines corresponding to the two or more adjacent first memory cells based on the search data.
13 . The method according to claim 11 , wherein the control circuit, in the sense operation,
executes control such that one of at least (2k)-th and (2k+1)-th (where k is a natural number of 1 or more) first memory cells from a side of the source line among the plurality of first memory cells enters an OFF state when data actually stored in (2k)-th and (2k+1)-th first memory cells are the same as the search data, and
executes control such that both the (2k)-th and (2k+1)-th first memory cells from the side of the source line among the plurality of first memory cells enter an ON state when the data actually stored in (2k)-th and (2k+1)-th first memory cells are different from the search data.
14 . The method according to claim 11 , wherein
the first string includes a current control memory cell connected in series to the plurality of first memory cells between the bit line and the source line, and
in the sense operation, the control circuit supplies a voltage less than a threshold voltage of the current control memory cell to the word line corresponding to the current control memory cell.
15 . The method according to claim 11 , wherein
the memory device further includes a second string in which a plurality of second memory cells are connected in series between the bit line and the source line,
the first string includes a plurality of first word lines respectively controlling the plurality of first memory cells,
the second string includes a plurality of second word lines respectively controlling the plurality of second memory cells, and
the control circuit, in the sense operation,
individually controls the first and second word lines,
supplies voltages to the plurality of second word lines respectively corresponding to the plurality of second memory cells based on the search data, and
determines a similarity between the search data and the data actually stored in the plurality of first memory cells and the plurality of second memory cells based on a change in voltage of the bit line caused by current flowing between the bit line and the source line via the first and second strings.
16 . The method according to claim 15 , wherein
the first string includes a first current control memory cell connected in series to the plurality of first memory cells between the bit line and the source line,
the second string includes a second current control memory cell connected in series to the plurality of second memory cells between the bit line and the source line, and
in the sense operation, the control circuit
controls the first current control memory cell such that first current flows between the bit line and the source line via the first string, and
controls the second current control memory cell such that second current with an amount different from the first current flows between the bit line and the source line via the second string.
17 . The method according to claim 16 , wherein a threshold voltage of the first current control memory cell is different from a threshold voltage of the second current control memory cell.
18 . The method according to claim 15 , wherein
the first string includes a plurality of first current control memory cells connected in series to the plurality of first memory cells between the bit line and the source line,
the second string includes a plurality of second current control memory cells connected in series to the plurality of second memory cells between the bit line and the source line,
the plurality of first current control memory cells have different threshold voltages,
the plurality of second current control memory cells have different threshold voltages, and
in the sense operation, the control circuit
selects one first current control memory cell among the plurality of first current control memory cells such that first current flows between the bit line and the source line via the first string, and
selects one second current control memory cell among the plurality of second current control memory cells such that second current with an amount different from the first current flows between the bit line and the source line via the second string.
19 . The method according to claim 11 , wherein
the first string includes a plurality of current control memory cells connected in series to the plurality of first memory cells between the bit line and the source line, and
threshold voltages of the plurality of current control memory cells are different from each other.
20 . The method according to claim 19 , wherein, in the sense operation, the control circuit
supplies one word line among the plurality of word lines corresponding to the plurality of current control memory cells with voltage less than a threshold voltage of the current control memory cell corresponding to the one word line, and
supplies other word lines among the plurality of word lines corresponding to the plurality of current control memory cells with voltages by which the plurality of current control memory cells are controlled to an ON state regardless of data stored therein.