Electrically erasable and programmable non-volatile memory system with write-verify controller using two reference levels
First Claim
1. A non-volatile semiconductor memory system comprising:
- an array of rows and columns of memory cells comprising electrically erasable and programmable memory cell transistors;
charge/discharge control means connected to said array, for causing a sub-array of memory cell transistors selected from said array to change a threshold voltage thereof by changing the amount of electrical carriers being charged therein; and
verify means for verifying a resultant electrical state of said sub-array of memory cell transistors by checking their threshold voltages for variations using a first reference voltage and a second reference voltage potentially greater than the first voltage which voltages define a specific range, and for, when an irregular cell transistor remains among them to have an insufficient threshold voltage which is out of the range, performing an additional charge-amount change operation for a predetermined period of time so as to facilitate the state thereof to come closer to the range.
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Accused Products
Abstract
An EEPROM includes an array of memory cell transistors, which is divided into cell blocks each including NAND cell units of series-connected cell transistors. A sense amplifier is connected to bit lines and a comparator. A data-latch circuit is connected to the comparator, for latching a write-data supplied from a data input buffer. After desired cell transistors selected for programming in a selected block are once programmed, a write-verify operation is performed. The comparator compares the actual data read from one of the programmed cell transistors with the write-data, to verify its written state. The write-verify process checks the resulting threshold voltage for variations using first and second reference voltages defining the lower-limit and upper-limit of an allowable variation range. If the comparison results under employment of the first voltage shows that an irregularly written cell transistor remains with an insufficient threshold voltage which is so low as to fail to fall within the range, the write operation continues for the same cell transistor. If the comparison results under employment of the second voltage shows that an excess-written cell transistor remains, the block is rendered "protected" at least partially.
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Citations
20 Claims
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1. A non-volatile semiconductor memory system comprising:
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an array of rows and columns of memory cells comprising electrically erasable and programmable memory cell transistors; charge/discharge control means connected to said array, for causing a sub-array of memory cell transistors selected from said array to change a threshold voltage thereof by changing the amount of electrical carriers being charged therein; and verify means for verifying a resultant electrical state of said sub-array of memory cell transistors by checking their threshold voltages for variations using a first reference voltage and a second reference voltage potentially greater than the first voltage which voltages define a specific range, and for, when an irregular cell transistor remains among them to have an insufficient threshold voltage which is out of the range, performing an additional charge-amount change operation for a predetermined period of time so as to facilitate the state thereof to come closer to the range. - View Dependent Claims (2, 3, 4, 5)
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6. A method for accessing a non-volatile semiconductor memory device including a plurality of programmable and erasable memory cell transistors which are arranged in rows and columns on a substrate, said method comprising the steps of:
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causing a sub-array of memory cell transistors selected from said array to change a threshold voltage thereof by changing the amount of electrical carriers being charged therein; verifying the resultant electrical state of the selected memory cell transistors by checking their threshold voltages for variations using a first reference voltage and a second reference voltage potentially greater than the first voltage which voltages define a specific range, to determine whether or not an irregular cell transistor remains among them to have an insufficient threshold voltage which is potentially out of the range; performing, when such an irregular cell transistor is found, an additional charge-amount change operation for a predetermined period of time; and repeating the verifying step and the additional charge-amount changing step until the electrical state thereof falls into said range. - View Dependent Claims (7, 8, 9, 10, 11)
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12. An electrically programmable and erasable read-only memory device comprising:
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an array of rows and columns of memory cells each of which includes a field effect transistors having a carrier storage layer and a control gate, said array being divided into a pluraiity of groups each of which includes a plurality of sub-groups which includes a corresponding number of series circuits of memory cell transistors, said sub-arrays having first and second opposite ends; parallel address-control lines coupled to the rows of field effect transistors at their control gates; parallel data-transfer lines insulatively crossed with said address-control lines, and associated with said sub-groups; switch means for electrically connecting selectively said data-transfer lines to said sub-groups at the first ends thereof; write means for electrically writing a sub-group of series-connected memory cell transistors being selected from sub-groups included in a selected one of said groups; and verify means for verifying the electrically written state of the selected memory cell transistors by checking their resulting threshold voltages for variations using a first reference voltage and a second reference voltage potentially greater than the first voltage which voltages define an allowable range, and for, when any irregularly written cell transistor remains among them to have an insufficient threshold voltage which is potentially less than the range, performing an additional write operation for a predetermined period of time so that the written state thereof comes closer to a satisfiable state. - View Dependent Claims (13, 14, 15, 16, 17, 18, 19, 20)
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Specification