emory) A rewritable storage chip that holds its content without power. EEPROMs are byte addressable but must be erased before being re-written. In flash memory, which evolved from EEPROMs and is almost identical in architecture, an entire block of bytes must first be erased. In addition, EEPROMs are typically used on circuit boards to store small amounts of instructions and data, whereas flash memory modules hold gigabytes of data for camera and computer storage (see flash memory
). EEPROMs store location data in LTO tape cartridges, and they are actually used in flash memory-based SSDs; not as data storage but as boot code. See LTO
A Floating Gate Holds the Charge
The bit cells in EEPROM and flash memory are CMOS-based transistors that hold a charge on a "floating gate." With no charge on the floating gate, the transistor acts normally, and a pulse on the control gate causes current to flow. When charged, it blocks the control gate action, and current does not flow. Charging is accomplished by grounding the source and drain terminals and placing sufficient voltage on the control gate tunnel through the oxide to the floating gate. A reverse voltage channeled from another transistor clears the charge by causing it to dissipate into the substrate.
EEPROMs have a lifespan of between 10K and 100K write cycles, which is considerably greater than the EPROMs (single "E") that preceded them. See EPROM
, memory types
and flash memory
The Floating Gate Transistor
EEPROM and flash memory cells use a transistor with a floating gate that holds a charge. When charged, the action of the control gate is impeded, and the charged/uncharged state determines the 0 or 1 content of the bit.
The Erase Circuit
The floating gate transistor stores the charge, and a regular MOS transistor is used to erase it. Most EEPROMs are byte erasable with one MOS transistor for every eight floating gate transistors. Flash memory uses only one MOS transistor to erase an entire block of floating gate transistors.