EEPROM stands for Electrically Erasable Programmable Read-Only Memory.
An EEPROM is like an EPROM chip
since it can be written in or programmed more than once.
Unlike the EPROM chip,
however,
an EEPROM chip need not
be taken out of the computer
or electronic device of which
it is part
when a new program
or data needs to be written on it.
Selective programming
can be done
to an EEPROM chip.
The user can alter
the value of certain cells
without needing to erase the programming
on other cells.
Thus, sections of data
can be erased
and
replaced
without needing to alter
the rest of the chip's programming.
Data stored in an EEPROM chip
is permanent,
at least
until the user decides to erase
and replace the information
it contains.
Furthermore,
the data stored
in an EEPROM chip
is not lost
even when power is turned off.
History of the EEPROM
The EEPROM
is
a modification of the EPROM
and
was designed by George Perlegos .
Its development
began
in 1978
while Perlegos was still
employed by Intel.
However
the archetypal EEPROM
still
had to be taken out of the computer
or
electronic device of which
it was part
if any reprogramming was necessary.
When Perlegos left Intel
to form Seeq Technology,
he designed the first fully functional EEPROM.
To eliminate
the necessity of external programming,
Perlegos and
company made the insulating layer thinner
and
integrated an oscillator
and
capacitor circuit
into the memory chip itself.
This charge pump
can produce the necessary programming voltage.
Since it is fully integrated in every EEPROM chip,
there is no need to
take out
the EEPROM chip
for erasing and programming.
To configure an EEPROM chip,
an electric field produced by the charge pump
is applied locally to cells
marked for modification.
EEPROM Structure
The EEPROM chip is physically similar to the EPROM chip.
It is also composed of cells
with two transistors.
The floating gate is separated from the control gate
by a thin oxide layer.
Unlike the EPROM chip, however, the EEPROM chip's oxide layer
is much thinner.
In EEPROM chips,
the insulating layer
is only around 1 nanometre thick whereas in EPROM chips,
the oxide layer is around 3 nanometres thick.
The thinner oxide layer means lower voltage requirements
for initiating changes
in cell value.
Tunneling the electrons of the floating gate towards the oxide layer separating
the floating gate
and the control gate
is
still the method of changing a bit's value from 1 to 0.
To erase EEPROM programming,
the electron barrier
still has to be overcome by the application of enough programming voltage.
EEPROM Limitations
While the EEPROM can be reprogrammed,
the number of times
it can be altered is limited.
This is the main reason
why EEPROM chips are popular
for storing only configuration data such as the computer's BIOS code which doesn't require frequent reprogramming.
The oxide insulating layer can be damaged by frequent rewrite. Modern-day EEPROMs can be rewritten up to a million times.
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