Memory device and operation thereof
A memory device includes an array of memory cells in a plurality of memory strings and arranged in a plurality of rows of memory cells. The memory device also includes a plurality of word lines respectively coupled to the plurality of rows of memory cells, and a peripheral circuit coupled to the plurality of word lines and configured to perform a read operation on a selected row of memory cells of the plurality of rows of memory cells. The selected row of memory cells is coupled to a selected word line, wherein the peripheral circuit is configured to apply a word line voltage on each of the plurality of word lines and determine a highest threshold voltage of the plurality of rows of memory cells based on a change of a word line capacitance loading in response to the word line voltage.
1. A memory device, comprising:
an array of memory cells in a plurality of memory strings and arranged in a plurality of rows of memory cells;
a plurality of word lines respectively coupled to the plurality of rows of memory cells; and
a peripheral circuit coupled to the plurality of word lines and configured to perform a read operation on a selected row of memory cells of the plurality of rows of memory cells, the selected row of memory cells being coupled to a selected word line, wherein the peripheral circuit is configured to:
apply a word line voltage on each of the plurality of word lines; and
determine a highest threshold voltage of the plurality of rows of memory cells based on a change of a word line capacitance loading in response to the word line voltage.
2. The memory device of claim 1 , wherein the word line capacitance loading comprises a variable capacitance, and determining the highest threshold voltage of the plurality of rows of memory cells comprises:
determining a maximum capacitance of the variable capacitance; and
determining the highest threshold voltage based on the maximum capacitance.
3. The memory device of claim 1 , wherein the peripheral circuit comprises a control logic configured to perform a least one of:
determining a time at which the highest threshold voltage is reached;
determining a pass voltage applied on an unselected word line of the plurality of word lines in the read operation based on the highest threshold voltage; or
determining a read reference voltage applied on the selected word line based on the highest threshold voltage.
4. The memory device of claim 3 , wherein the peripheral circuit further comprises a driver configured to, based on operations of the control logic, perform a least one of:
applying a turn-off voltage on a select-gate (SG) transistor coupled to an unselected memory string of the plurality of memory strings at the time the highest threshold voltage is reached;
applying the pass voltage on the unselected word line of the plurality of word lines in the read operation; or
applying the read reference voltage on the selected word line.
5. The memory device of claim 1 , wherein the peripheral circuit further comprises a voltage generator configured to output the word line voltage, a control logic configured to control the voltage generator, and a driver configured to apply the word line voltage on the plurality of rows of memory cells, wherein:
the control logic controls the voltage generator to increase the word line voltage from a first voltage value to a second voltage value, and from the second voltage value to a third voltage value; and
the driver applies the word line voltage on the plurality of word lines.
6. The memory device of claim 5 , wherein the control logic is further configured to:
obtain a first value associated with a first capacitance charged to the word line capacitance loading between the first voltage value and the second voltage value, and a second value associated with a second capacitance charged to the word line capacitance loading between the second voltage value and the third voltage value;
determine a difference between the first value and the second value; and
in response to the difference reaching a maximum value, determine a capacitance of the word line capacitance loading to be the maximum capacitance of the word line capacitance loading.
7. The memory device of claim 6 , wherein the control logic is further configured to determine the third voltage value to be the highest threshold voltage of the plurality of rows of memory cells.
8. The memory device of claim 6 , wherein the peripheral circuit further comprises a sensing device coupled to the voltage generator and the control logic, the sensing device being configured to:
generate the first value associated with the first capacitance; and
generate the second value associated with the second capacitance.
9. The memory device of claim 8 , wherein the voltage generator further comprises a comparing circuit having an output of a flag signal, the flag signal being coupled to an input of the voltage generator, and wherein:
in response to the flag signal being equal to 1, the voltage generator continues to increase the word line voltage; and
in response to the flag signal being equal to 0, the voltage generator stops increasing the word line voltage.
10. The memory device of claim 9 , wherein
the flag signal is equal to 1 in a first time period and a second time period, respectively; and
the flag signal is equal to 0 between the first time period and the second time period.
11. The memory device of claim 10 , wherein the sensing device comprises a counter that counts a time period in response to the flag being 1 and generates a value corresponding to the time period, such that the first time period corresponds to the first value and the second time period corresponds to the second value.
12. The memory device of claim 11 , wherein the sensing device outputs the first value at an end of the first time period and outputs the second value at an end of the second time period.
13. The memory device of claim 9 , wherein the sensing device comprises an input and a plurality of flip flops, the input comprising the flag signal, each of the flip flops generating a digit of an output of the sensing device.
14. The memory device of claim 9 , wherein the comparing circuit comprises a first input of a voltage portion proportional to the word line voltage and a second input of a reference signal coupled to the control logic, the flag signal being a difference between the voltage portion and the reference signal, wherein the control logic is configured to:
apply a first step voltage on the second input of the comparator, the first step voltage being equal to the second voltage;
obtain the first value from the sensing device;
in response to the flag signal turning from 1 to 0, apply a second step voltage on the second input of the comparator, the second step voltage being equal to the third voltage and greater than the first step voltage; and
obtain the second value from the sensing device.
15. The memory device of claim 1 , wherein:
the plurality of memory cells comprises at least one of a plurality of triple-level cells (TLCs) or a plurality of quad-level cells (QLCs); and
the highest threshold voltage of the plurality of rows of memory cells comprises at least one of a highest threshold voltage in the TLCs or one of a highest threshold voltage in the QLCs.
16. A system, comprising:
a memory device configured to store data, the memory device comprising:
an array of memory cells in a plurality of memory strings and arranged in a plurality of rows of memory cells;
a plurality of word lines respectively coupled to the plurality of rows of memory cells; and
a peripheral circuit coupled to the plurality of word lines and configured to perform a read operation on a selected row of memory cells of the plurality of rows of memory cells, the selected row of memory cells being coupled to a selected word line, wherein the peripheral circuit is configured to:
apply a word line voltage on each of the plurality of word lines; and
determine a highest threshold voltage of the plurality of rows of memory cells based on a change of a word line capacitance loading in response to the word line voltage, and
a memory controller coupled to the memory device and configured to control the memory device.
17. A method for operating a memory device, the memory device comprising an array of memory cells in a plurality of memory strings and arranged in a plurality of rows of memory cells; and a plurality of word lines respectively coupled to the plurality of rows of memory cells, the method comprising performing a read operation on a selected row of memory cells of the plurality of rows of memory cells, the selected row of memory cells being coupled to a selected word line, wherein the performing the read operation comprises:
applying a word line voltage on each of the plurality of word lines; and
determining a highest threshold voltage of the plurality of rows of memory cells based on a change of a word line capacitance loading in response to the word line voltage.
18. The method of claim 17 , wherein the word line capacitance loading comprises a variable capacitance, and determining the highest threshold voltage of the plurality of rows of memory cells comprises:
determining a maximum capacitance of the variable capacitance; and
determining the highest threshold voltage based on the maximum capacitance.
19. The method of claim 17 , wherein method further comprises:
determining a time at which the highest threshold voltage is reached;
determining a pass voltage applied on an unselected word line of the plurality of word lines in the read operation based on the highest threshold voltage; or
determining a read reference voltage applied on the selected word line based on the highest threshold voltage.
20. The method of claim 19 , wherein the method further comprises performing a least one of:
applying a turn-off voltage on a select-gate (SG) transistor coupled to an unselected memory string of the plurality of memory strings at the time the highest threshold voltage is reached;
applying the pass voltage on the unselected word line of the plurality of word lines in the read operation; or
applying the read reference voltage on the selected word line.