Current sensing method and apparatus particularly useful for a memory array of cells having diode-like characteristics
View Patent ↗A memory array includes a sensing circuit for sensing bit line current while keeping the voltage of the selected bit line substantially unchanged. The word lines and bit lines are biased so that essentially no bias voltage is impressed across half-selected memory cells, which substantially eliminates leakage current through half-selected memory cells. The bit line current which is sensed arises largely from only the current through the selected memory cell. A noise detection line in the memory array reduces the effect of coupling from unselected word lines to the selected bit line. In a preferred embodiment, a three-dimensional memory array having a plurality of rail-stacks forming bit lines on more than one layer, includes at least one noise detection line associated with each layer of bit lines. A sensing circuit is connected to a selected bit line and to its associated noise detection line.
1. In a memory array having at least two memory planes of memory cells with diode-like conduction characteristics, for at least one of two memory cell data states, each memory cell within a memory plane coupled between a word line and a bit line associated with the memory plane and having first and second nominal current levels in accordance with its data state when forward biased, a method of sensing the data state of a selected memory cell comprising the steps of:
driving a selected word line from an unselected word line bias voltage to a selected word line bias voltage;
driving a selected bit line from an unselected bit line bias voltage to a selected bit line bias voltage;
sensing current flow on the selected bit line while the selected bit line remains substantially at the selected bit line bias voltage to determine which of the first or second nominal current levels flows through the selected memory cell;
wherein the sensing current flow step comprises:
subtracting a reference current having a magnitude between the first and second nominal current levels from the bit line current, resulting in a net bit line current; and
sensing whether the net bit line current is positive or negative.
2. In a memory array having memory cells with diode-like conduction characteristics, for at least one of two memory cell data states, each memory cell coupled between a word line and a bit line and having first and second nominal current levels in accordance with its data state when forward biased, a method of sensing the data state of a selected memory cell comprising the steps of:
driving a selected word line from an unselected word line bias voltage to a selected word line bias voltage;
driving a selected bit line from an unselected bit line bias voltage to a selected bit line bias voltage: and
sensing current flow on the selected bit line while the selected bit line remains substantially at the selected bit line bias vo 1 tage to determine which of the first or second nominal current levels flows through the selected memory cell;
wherein the sensing current flow step comprises:
providing a noise detection line in the memory array associated with a group of bit lines, said noise detection line having capacitive coupling characteristics to adjoining structures matching those of each bit line of the group;
developing a reference current within the sensing circuit;
coupling any noise current received from the noise detection line into the reference current, resulting in a noise-adjusted reference current; and
comparing the bit line current against the noise-adjusted reference current to determine the data state of the selected memory cell.
3. The invention defined by claim 2 wherein:
the reference current has a magnitude between the first and second nominal current levels;
the coupling step comprises subtracting the reference current from the bit line current, resulting in a net bit line current; and
the comparing step comprises sensing whether the net bit line current is positive or negative.
4. The invention defined by claim 1 wherein:
the selected bit line bias voltage differs from the unselected bit line bias voltage by at least about 1 volt.
5. The invention defined by claim 1 further comprising:
the unselected word line bias voltage is close enough in magnitude to the selected bit line bias voltage so that current between the selected bit line and the unselected word lines is negligible relative to a selected memory cell current.
6. The invention defined by claim 5 wherein:
the selected bit line bias voltage is about equal to the unselected word line bias voltage.
7. The invention defined by claim 1 further comprising:
the unselected bit line bias voltage is close enough in magnitude to the selected word line bias voltage so that current between the selected word line and the unselected bit lines is negligible relative to a selected memory cell current.
8. The invention defined by claim 7 wherein:
the unselected bit line bias voltage is about equal to the selected word line bias voltage.
9. The invention defined by claim 1 wherein:
the selected word line bias voltage differs from the selected bit line bias voltage by at least about 1 volt.
10. The invention defined by claim 1 wherein:
the memory cells comprise antifuse memory cells.
11. The invention defined by claim 1 wherein:
the selected word line bias voltage differs from the selected bit line bias voltage by at least about 1 volt;
the selected bit line bias voltage is about equal to the unselected word line bias voltage; and
the selected word line bias voltage is about equal to the unselected bit line bias voltage.
12. The invention defined by claim 11 wherein:
the memory cells comprise antifuse memory cells.
13. The invention defined by claim 2 wherein:
the selected bit line bias voltage differs from the unselected bit line bias voltage by at least about 1 volt.
14. The invention defined by claim 2 further comprising:
the unselected word line bias voltage is close enough in magnitude to the selected bit line bias voltage so that current between the selected bit line and the unselected word lines is negligible relative to a selected memory cell current.
15. The invention defined by claim 14 wherein:
the selected bit line bias voltage is about equal to the unselected word line bias voltage.
16. The invention defined by claim 2 further comprising:
the unselected bit line bias voltage is close enough in magnitude to the selected word line bias voltage so that current between the selected word line and the unselected bit lines is negligible relative to a selected memory cell current.
17. The invention defined by claim 16 wherein:
the unselected bit line bias voltage is about equal to the selected word line bias voltage.
18. The invention defined by claim 2 wherein:
the selected word line bias voltage differs from the selected bit line bias voltage by at least about 1 volt.
19. The invention defined by claim 2 wherein:
the selected word line bias voltage differs from the selected bit line bias voltage by at least about 1 volt;
the selected bit line bias voltage is about equal to the unselected word line bias voltage; and
the selected word line bias voltage is about equal to the unselected bit line bias voltage.
20. The invention defined by claim 19 wherein:
the memory cells comprise antifuse memory cells.