| 摘要 |
The flash memory device according to the present invention includes an erase control circuit, used as a state machine, having embodied erase algorithm which can prevent flash memory cells from being over-erased. The erase control circuit, first, checks whether or not threshold voltages of selected cells reach a predetermined pre-verify voltage higher than the maximum value of a target threshold voltage range corresponding to the erased state. When at least one of the selected cells has its threshold voltage higher than the pre-verify voltage, a high voltage generator generates a bulk voltage that is increased step by step by a predetermined voltage level. And, when the selected cells all have threshold voltages equal to or less than the pre-verify voltage, the high voltage generator generates a constant bulk voltage. According to this bulk voltage control scheme, the number of flash memory cells over-erased at the erase operation is reduced reducing the total erase time.
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