RAM vs ROM; EEPROM cell

RAM vs ROM

Working of an EEPROM cell

EEPROM (Electrically Erasable PROM) can be written and erased electrically, byte by byte, in the circuit. Each cell uses a floating-gate MOSFET (FLOTOX) plus a select transistor.

       Control gate
     ==============
       oxide
     --------------  Floating gate (isolated)
       thin tunnel oxide (~10 nm)
  n+ Source  [ p-substrate ]  n+ Drain

1. Structure: a floating gate, fully surrounded by oxide, lies between the control gate and the channel. A small region of very thin oxide lies over the drain.
2. Programming (write): a high voltage (about 12–20 V) is applied to the control gate with the drain grounded. Electrons tunnel through the thin oxide into the floating gate (Fowler–Nordheim tunnelling). The trapped negative charge raises the transistor's threshold voltage.
3. Erasing: the voltage is reversed (high voltage on the drain, control gate grounded), so electrons tunnel back out of the floating gate and the threshold falls to its normal value.
4. Reading: a normal voltage is applied to the control gate. A charged cell does not conduct (read as 0); an uncharged cell conducts (read as 1). The charge stays for 10+ years without power, so the memory is non-volatile.

EEPROM does not need UV light (unlike EPROM), but the cell is larger and the number of write cycles is limited (about $10^5$–$10^6$).