1. Field of Invention
This invention relates to semiconductor memories, in particular flash memories and providing voltages for erase operations.
2. Description of Related Art
In conventional flash memories both a high negative and a high positive voltage are required to erase data in flash memory cells. The high negative voltage is applied to the gate of the flash cell being erased and the high positive voltage is applied to the source. The drain is usually left floating. This requires that both a positive pump circuit and a negative pump circuit exist on the flash memory chip if the flash memory cells are to be erased after assembly. There are several problems that must be handled when two pump circuits of opposite polarity are on the same chip. Besides increase in chip power, considerable protection circuitry and guarding methodology are required to isolate the pump circuitry and avoid device breakdown.
Using internal pump circuits to perform erasure of flash memory cells is a somewhat slow process since it takes time for the pump circuits to get to the required voltage after being activated. For this reason it is advantageous to programming and erasure during manufacture to provide high negative and high positive voltages to chip pads from external to the chip, thus speeding up the programming process. In the manufacture of flash memory cells providing an external high positive voltage has been known for some time; however, because of the difficulties of handling a high negative voltage, external supplied high negative voltages have not been previously used. By providing an externally applied high negative voltage along with an externally applied high positive voltage to a chip containing a flash memory shorter erase/program, lower power consumption and faster throughput can be realized.