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AssigneeNEC Corporation
InventorKiyokazu Hashimoto
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For a nonvolatile multi-stage semiconductor memory device, a data reading sequence is given first, second, and third phases. During the first phase, a word line is driven to 2.25 V with a differential amplifier and a bias circuit activated to sense on or off of a selected memory cell. During the second phase, the word line is driven to 3.0 V with a differential amplifier and a bias circuit activated to sense on or off of the selected memory cell. This enables correct read out with a low operating voltage, such as 3.0 V, of data stored in semiconductor memory cells with a selected one of four threshold levels given to each datum. During the third phase, it is possible to use the differential amplifier and the bias circuit which are used during the first phase. Use of one differential amplifier alone is possible with two bias circuits used. Use of only first and second phases is also possible.
A method of reading data from a multi-stage semiconductor memory device comprising a plurality of transistor memory cells, each given a selected threshold level selected from a zeroth threshold level, a first threshold level higher than said zeroth threshold level, a second threshold level higher than said first threshold level, and a third threshold level higher than said second threshold level, said method comprising the steps of:
A multi-stage semicondutor memory device comprising:
A multi-stage semiconductor memory device comprising first through N-th word lines and first through M-th digit lines crossing said first through said N-th word lines at cross points, where N and M represent first and second predetermined integers, a plurality of semiconductor memory cells at said cross points, selecting means responsive to address signals for selecting a selected word line from said first through said N-th word lines and a selected digit line from said first through said M-th digit lines, driving means for driving said selected word line to one of first and second voltages, and sensing means for sensing a cell current flowing through said selected digit line, wherein said sensing means compares said cell current with first and second constant currents while said selected word line is driven to said first and said second voltages, respectively.
Dependencies are inferred from the claim text.