(Electrically Erasable Programmable ROM) A rewritable memory chip that holds its content without power. Although EEPROMs spawned flash memory, EEPROMs are byte addressable at the write level, whereas flash chips must erase a block of bytes before rewriting.
A Floating Gate Holds the Charge
EEPROMs use a transistor with a floating gate underneath a control gate. To program the bit, a voltage is applied to the control gate. This causes electrons to tunnel through the insulating oxide layer into the floating gate, which impedes subsequent operation of the control gate. The 0 or 1 is determined by whether the voltage on the control gate is blocked or not.
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, SEEPROM, memory types and flash memory.
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| EEPROM Floating Gate |
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| The storage part of an EEPROM cell acts like a permanently-open or closed transistor. Charging is accomplished by grounding the source and drain terminals and placing a voltage on the control gate. When the "floating gate" is charged, it impedes the flow of electrons from the control gate to the silicon, and the 0 or 1 is determined by whether the voltage on the control gate is blocked or not. A reverse voltage from another transistor causes the charge to dissipate into the substrate. |
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| EEPROM Cell |
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| A single EEPROM bit is made up of two transistors: the MOS transistor for erasure and the floating gate transistor for storage. (Image courtesy of TechBites Interactive Inc, www.techbites.com.) |