Nonvolatile memory device and control method thereof
View Patent ↗To provide a nonvolatile memory including a word-line drive circuit that supplies a selective voltage to a selective transistor connected in series to a nonvolatile memory device. The word-line drive circuit applies a first selective voltage VDD to a control electrode of the selective transistor in a first period, and applies a second selective voltage VPP higher than the first selective voltage VDD to the control electrode of the selective transistor in a second period that follows the first period. Thereby, a current drive capability of the selective transistor is gradually changed. Thus, it becomes possible to limit the current drive capability of the selective transistor at timing at which snap-back is caused. As a result, an excessive current caused by the snap-back is suppressed, thereby reducing damage inflicted on the nonvolatile memory device.
1. A nonvolatile memory device comprising:
a nonvolatile memory element having a negative resistance characteristic, wherein the nonvolatile memory element is changeable at least to a first state and a second state, each of which differs in electrical resistance;
a selective transistor connected in series to the nonvolatile memory element;
a bit line that supplies the nonvolatile memory element with a current;
a controller that changes a current drive capability of the selective transistor gradually or continuously during a write operation to the nonvolatile memory element; and
a word-line drive circuit that is controlled by the controller, wherein
the word-line drive circuit applies a first selective voltage to the control electrode of the selective transistor in a first period, and applies a second selective voltage higher than the first selective voltage to the control electrode of the selective transistor in a second period that follows the first period.
2. The nonvolatile memory device as claimed in claim 1 , wherein the nonvolatile memory element includes a phase change material.
3. The nonvolatile memory device as claimed in claim 1 , wherein the first selective voltage is lower than at least the second write voltage.
4. The nonvolatile memory device as claimed in claim 1 , further comprising a bit-line drive circuit that is controlled by the controller,
the bit-line drive circuit including a first drive circuit that applies a first write voltage to the bit line and a second drive circuit that applies a second write voltage higher than the first write voltage to the bit line,
the controller activates the first drive circuit at least in the first period when the nonvolatile memory element is changed to the first state,
the controller inactivates the first drive circuit in the first period and activates the second drive circuit in the second period when the nonvolatile memory element is changed to the second state.
5. The nonvolatile memory device as claimed in claim 4 , wherein the controller activates the first drive circuit in the first period and the second period when the nonvolatile memory element is changed to the first state.
6. The nonvolatile memory device as claimed in claim 4 , wherein the first selective voltage is lower than at least the second write voltage.
7. The nonvolatile memory device as claimed in claim 6 , wherein the first selective voltage and the first write voltage are substantially equal, and the second selective voltage and the second write voltage are substantially equal.
8. A nonvolatile memory device, comprising: a plurality of bit lines;
a plurality of nonvolatile memory element having a negative resistance characteristic and a plurality of selective transistors, each of which is connected in series to the plurality of bit lines;
a word line commonly connected to control electrodes of the plurality of selective transistors;
a plurality of bit-line drive circuits that drive each 25 of the plurality of bit lines; and
a word-line drive circuit that drives the word line, wherein
the word-line drive circuit applies a first selective voltage to the word line in a first period, and applies a second selective voltage higher than the first selective voltage to the word line in a second period that follows the first period,
among the plurality of bit-line drive circuits, bit-line drive circuits corresponding to the nonvolatile memory devices to be changed to a relatively low-resistant first state apply a first write voltage to a corresponding bit lines at least in the first period, and
among the plurality of bit-line drive circuits, bit-line drive circuits corresponding to the nonvolatile memory devices to be changed to a relatively high-resistant second state apply a second write voltage higher than the first write voltage to a corresponding bit lines in the second period.
9. The nonvolatile memory device as claimed in claim 8 , wherein the bit-line drive circuits corresponding to the nonvolatile memory devices to be changed to the first state apply the first write voltage to a corresponding bit lines in the first period and the second period.
10. The nonvolatile memory device as claimed in claim 9 , wherein the bit-line drive circuits corresponding to the nonvolatile memory devices to be changed to the second state apply neither the first write voltage nor the second write voltage to a corresponding bit lines in the first period.
11. The nonvolatile memory device as claimed in claim 8 , wherein the bit-line drive circuits corresponding to the nonvolatile memory devices to be changed to the second state apply neither the first write voltage nor the second write voltage to a corresponding bit lines in the first period.
12. The nonvolatile memory device as claimed in claim 8 , wherein the first selective voltage and the first write voltage are substantially equal, and the second selective voltage and the second write voltage are substantially equal.
13. A control method of a nonvolatile memory device including a nonvolatile memory element having a negative resistance characteristic, a selective transistor connected in series to the nonvolatile memory element, and a bit line that supplies the nonvolatile memory element with a current, the control method comprising:
applying a first selective voltage to a control electrode of the selective transistor in a first period; and applying a second selective voltage higher than the first selective voltage to the control electrode of the selective transistor in a second period that follows the first period.
14. The control method of a nonvolatile memory device as claimed in claim 13 , wherein
a first write voltage is applied to the bit line at least in the first period when the nonvolatile memory element to be changed to a first state,
a second write voltage higher than the first write voltage is applied to the bit line in the second period when the nonvolatile memory element to be changed to a second state.
15. The control method of a nonvolatile memory device as claimed in claim 14 , wherein
the first write voltage is applied to the bit line in the first period and the second period when the nonvolatile memory element to be changed to the first state.
16. The control method of a nonvolatile memory device as claimed in claim 14 , wherein
the second write voltage is not applied to the bit line in the first period when the nonvolatile memory element to be changed to the second state.
17. The control method of a nonvolatile memory device as claimed in claim 14 , wherein the first selective voltage and the first write voltage are substantially equal, and the second selective voltage and the second write voltage are substantially equal.
18. The control method of a nonvolatile memory device as claimed in claim 14 , wherein the first state is a relatively low-resistant state and the second state is a relatively high-resistant state.
19. A data processing system comprising a data processor and a nonvolatile memory device, the nonvolatile memory device comprising:
a nonvolatile memory element having a negative resistance characteristic;
a selective transistor connected in series to the nonvolatile memory element;
a bit line that supplies the nonvolatile memory element with a current;
a controller that changes a current drive capability of the selective transistor gradually or continuously during a write operation to the nonvolatile memory element; and
a word-line drive circuit that is controlled by the controller, wherein the word-line drive circuit applies a first selective voltage to the control electrode of the selective transistor in a first period, and applies a second selective voltage higher than the first selective voltage to the control electrode of the selective transistor in a second period that follows the first period.