Method for tightening VT distribution of 5 volt-only flash EEPROMS

摘要

There is provided an improved method for tightening the distribution of control gate threshold voltages of erase cells in flash EEPROM devices. A relatively low positive voltage is applied to the source regions of the EEPROM devices during an entire erase cycle. The magnitude of a negative constant voltage applied to control gates of the EEPROM devices is lowered to a predetermined voltage level during the entire erase cycle so as to obtain a tighter threshold voltage distribution. The value of a load resistor coupled between the low positive voltage and source regions is reduced simultaneously to a predetermined value so as to compensate for the increased erase time caused by the lowering of the magnitude of the negative constant voltage. As a result, an improved threshold voltage VT distribution after erase is obtained without sacrificing any reduction in the erase speed.