EEPROM (Electrically Erasable Programmable Read Only Memory):
Electrically Erasable Programmable Read Only Memory (EEPROM) also use MOS circuitry very similar to that of EPROM. Data is stored as charge or no charge on an insulated layer or an insulated floating gate in the device.
The insulating layer is made very thin (< 200Å). Therefore, a voltage as low as 20 to 25V can be used to move charges across the thin barrier in either direction for programming or erasing.
EEPROM allows selective erasing at the register level rather than erasing all the information since the information can be changed by using electrical signals.
The Electrically Erasable Programmable Read Only Memory also has a special chip erase mode by which entire chip can be erased in 10 ms. This time is quite small as compared to time required to erase EPROM, and it can be erased and reprogrammed with device right in the circuit. However, EEPROMs are most expensive and the least dense ROMs.