Page 119 - Digital Electronics by harish

Programming

Storing data in the memory requires selecting a given address and applying a higher
voltage to the transistors. This creates an avalanche discharge of electrons, which have
enough energy to pass through the insulating oxide layer and accumulate on the gate
electrode. When the high voltage is removed, the electrons are trapped on the electrode.
Because of the high insulation value of the silicon oxide surrounding the gate, the stored
charge cannot leak away and the data can be retained for many years.
Erasing

To erase the data stored in the array of transistors, ultraviolet light is directed onto the
die. Photons of the UV light cause ionization within the silicon oxide, which allow the stored
charge on the floating gate to dissipate. Since the whole memory array is exposed, all the
memory is erased at the same time. The process takes several minutes for UV lamps of
convenient sizes; sunlight would erase a chip in few weeks.

The erasing window must be kept covered with an opaque label to prevent accidental
erasure by the UV found in sunlight or camera flashes.

4.12 EEPROM (Electrically Erasable Programmable Read Only Memory)

EEPROM (Electrically Erasable Programmable ROM) is a ROM chip which can be
erased and re-programmed for unlimited number of times, without expensive or time-
2
consuming erasing processes. EEPROM is also called E PROM (e-squared PROM) and
EAPROM (Electrically Alterable PROM).To erase the data, a relatively high voltage is
required. Only one external power supply is required since the high voltage for program/erase
is internally generated. In this way the memory device could run from a single supply,
thereby considerably reducing the cost of an overall circuit using an EEPROM and
simplifying the design. Write and erase operations are performed on a byte per byte basis.

EEPROM memory uses the same basic principle that is used by EPROM memory
technology. The memory cell has two field effect transistors. One of these is the floating gate
storage transistor. Electrons can be made to become trapped in this gate and the presence or
absence of electrons is referred as a „0‟or a „1‟. The other transistor is known as the access
transistor and it is required for the operational aspects of the EEPROM memory cell.

Comparison between EPROM and EEPROM

S. No. EPROM EEPROM
1. Erasing by UV light. Electrically erased.
Single byte erase is not
possible. When UV light is
2. Single byte erase is possible.
passed, the complete data
will be erased.
Quartz window is provided
3. Quartz window is not required.
for UV erasing.

Table : EPROM vs EEPROM

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