FLASH MEMORY DEVICE CAPABLE OF PREVENTING OVER-ERASE OF FLASH MEMORY CELLS AND ITS ERASE METHOD, ESPECIALLY APPLYING ELECTRIC FIELD BETWEEN SEMICONDUCTOR SUBSTRATE AND CONTROL GATE OF MEMORY CELL

摘要

PURPOSE: A flash memory device capable of preventing over-erase of flash memory cells and its erase method are provided to reduce erase time by reducing the number of flash memory cells which are over-erased during an erase operation. CONSTITUTION: According to the method for erasing flash memory cells, the first electric field is applied between a semiconductor substrate and a control gate of a corresponding memory cell during the first period. The memory cells are verified according to the first verify voltage during the first period. The first electric field is gradually increased during the first period. The second electric field higher than the first electric field is applied between the semiconductor substrate and the control gate of the corresponding memory cell during the second period. The memory cells are verified according to the second verify voltage during the second period. The second electric field is maintained constantly during the second period.