EPROM (rarely EROM), or erasable programmable read-only memory, is
a type of memory chip that retains its data when its power supply is
Computer memory that can retrieve stored data after a
power supply has been turned off and back on is called non-volatile.
It is an array of floating-gate transistors individually programmed by
an electronic device that supplies higher voltages than those normally
used in digital circuits. Once programmed, an
EPROM can be erased by
exposing it to strong ultraviolet light source (such as from a
mercury-vapor light). EPROMs are easily recognizable by the
transparent fused quartz window in the top of the package, through
which the silicon chip is visible, and which permits exposure to
ultraviolet light during erasing.
EPROM generations, sizes and types
6 See also
10 External links
Intel 1702A EPROM, one of the earliest
EPROM types (1971), 256 by 8
bit. The small quartz window admits UV light for erasure.
Development of the
EPROM memory cell started with investigation of
faulty integrated circuits where the gate connections of transistors
had broken. Stored charge on these isolated gates changed their
EPROM was invented by
Dov Frohman of
Intel in 1971,
who was awarded US patent 3660819 in 1968.
Each storage location of an
EPROM consists of a single field-effect
transistor. Each field-effect transistor consists of a channel in the
semiconductor body of the device. Source and drain contacts are made
to regions at the end of the channel. An insulating layer of oxide is
grown over the channel, then a conductive (silicon or aluminum) gate
electrode is deposited, and a further thick layer of oxide is
deposited over the gate electrode. The floating-gate electrode has no
connections to other parts of the integrated circuit and is completely
insulated by the surrounding layers of oxide. A control gate electrode
is deposited and further oxide covers it.
To retrieve data from the EPROM, the address represented by the values
at the address pins of the
EPROM is decoded and used to connect one
word (usually an 8-bit byte) of storage to the output buffer
amplifiers. Each bit of the word is a 1 or 0, depending on the storage
transistor being switched on or off, conducting or non-conducting.
A cross-section of a floating-gate transistor
The switching state of the field-effect transistor is controlled by
the voltage on the control gate of the transistor. Presence of a
voltage on this gate creates a conductive channel in the transistor,
switching it on. In effect, the stored charge on the floating gate
allows the threshold voltage of the transistor to be programmed.
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 readily
leak away and the data can be retained for decades.
The programming process is not electrically reversible. 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 weeks, and
indoor fluorescent lighting over several years. Generally, the
EPROMs must be removed from equipment to be erased, since it is not
usually practical to build in a UV lamp to erase parts in-circuit. The
Electrically Erasable Programmable Read-Only Memory (EEPROM) was
developed to provide an electrical erase function and has now mostly
displaced ultraviolet-erased parts.
Atmel AT27C010 - an OTP EPROM
As the quartz window is expensive to make, OTP (one-time programmable)
chips were introduced; here, the die is mounted in an opaque package
so it cannot be erased after programming – this also eliminates the
need to test the erase function, further reducing cost. OTP versions
of both EPROMs and EPROM-based microcontrollers are manufactured.
EPROM (whether separate or part of a larger chip) is
being increasingly replaced by
EEPROM for small sizes, where the cell
cost isn't too important, and flash for larger sizes.
EPROM retains its data for a minimum of ten to twenty
years, with many still retaining data after 35 or more years, and
can be read an unlimited number of times without affecting the
lifetime. The erasing window must be kept covered with an opaque label
to prevent accidental erasure by the UV found in sunlight or camera
flashes. Old PC
BIOS chips were often EPROMs, and the erasing window
was often covered with an adhesive label containing the BIOS
publisher's name, the
BIOS revision, and a copyright notice. Often
this label was foil-backed to ensure its opacity to UV.
Erasure of the
EPROM begins to occur with wavelengths shorter than 400
nm. Exposure time for sunlight of one week or three years for room
fluorescent lighting may cause erasure. The recommended erasure
procedure is exposure to UV light at 253.7 nm of at least 15
W-sec/cm2 for 20 to 30 minutes, with the lamp at a distance of about
Erasure can also be accomplished with X-rays:
Erasure, however, has to be accomplished by non-electrical methods,
since the gate electrode is not accessible electrically. Shining
ultraviolet light on any part of an unpackaged device causes a
photocurrent to flow from the floating gate back to the silicon
substrate, thereby discharging the gate to its initial, uncharged
condition (photoelectric effect). This method of erasure allows
complete testing and correction of a complex memory array before the
package is finally sealed. Once the package is sealed, information can
still be erased by exposing it to X radiation in excess of 5*104
rads,[a] a dose which is easily attained with commercial X-ray
In other words, to erase your EPROM, you would first have to
and then put it in an oven at about 600 degrees Celsius (to anneal
semiconductor alterations caused by the X-rays). The effects of this
process on the reliability of the part would have required extensive
testing so they decided on the window instead.
EPROMs had a limited but large number of erase cycles; the silicon
dioxide around the gates would accumulate damage from each cycle,
making the chip unreliable after several thousand cycles. EPROM
programming is slow compared to other forms of memory. Because
higher-density parts have little exposed oxide between the layers of
interconnects and gate, ultraviolet erasing becomes less practical for
very large memories. Even dust inside the package can prevent some
cells from being erased.
For large volumes of parts (thousands of pieces or more),
mask-programmed ROMs are the lowest cost devices to produce. However,
these require many weeks lead time to make, since the artwork for an
IC mask layer must be altered to store data on the ROMs. Initially, it
was thought that the
EPROM would be too expensive for mass production
use and that it would be confined to development only. It was soon
found that small-volume production was economical with
particularly when the advantage of rapid upgrades of firmware was
Some microcontrollers, from before the era of EEPROMs and flash
memory, use an on-chip
EPROM to store their program. Such
microcontrollers include some versions of the
Intel 8048, the
Freescale 68HC11, and the "C" versions of the PIC microcontroller.
EPROM chips, such microcontrollers came in windowed (expensive)
versions that were used for debugging and program development. The
same chip came in (somewhat cheaper) opaque OTP packages for
production. Leaving the die of such a chip exposed to light can also
change behavior in unexpected ways when moving from a windowed part
used for development to a non-windowed part for production.
EPROM generations, sizes and types
The first generation 1702 devices were fabricated with the p-MOS
techology. They were powered with VCC = VBB = +5 V and VDD = VGG
= -9 V in Read mode, and with VDD = VGG = -47 V in
The second generation 2704/2708 devices switched to n-MOS technology
and to three-rail VCC = +5 V, VBB = -5 V, VDD = +12 V power
supply with VPP = 12 V and a +25 V pulse in Programming
The n-MOS technology evolution introduced single-rail VCC = +5 V
power supply and single VPP = +12 V programming voltage without
pulse in the third generation. The unneeded VBB and VDD pins were
reused for additional address bits allowing larger capacities
(2716/2732) in the same 24-pin package, and even larger capacities
with larger packages. Later the decreased cost of the CMOS technology
allowed same devices to be fabricated using it, adding the letter "C"
to the device numbers (27xx(x) are n-MOS and 27Cxx(x) are CMOS).
While parts of the same size from different manufacturers are
compatible in read mode, different manufacturers added different and
sometimes multiple programming modes leading to subtle differences in
the programming process. This prompted larger capacity devices to
introduce a "signature mode", allowing the manufacturer and device to
be identified by the
EPROM programmer. It was implemented by forcing
+12 V on pin A9 and reading out two bytes of data. However, as
this was not universal, programmer software also would allow manual
setting of the manufacturer and device type of the chip to ensure
Size — bits
Size — bytes
Last address (hex)
2716, 27C16, TMS2716, 2516
2732, 27C32, 2532
2764, 27C64, 2564
27C040, 27C400, 27C4001
A 32 KB (256 Kbit) EPROM
Microcontroller stores its program in internal EPROM
NEC 02716, 16 K
Programmable read-only memory
Intel HEX -
^ 500 J/kg
EPROM patent, Google .
^ Sah 1991, p. 639.
^ Oklobdzija, Vojin G. (2008). Digital Design and Fabrication. CRC
Press. pp. 5–17. ISBN 0-8493-8602-0.
^ Ayers, John E (2004), Digital integrated circuits: analysis and
design, CRC Press, p. 591, ISBN 0-8493-1951-X .
^ Horowitz, Paul; Hill, Winfield (1989), The Art of Electronics (2nd
ed.), Cambridge: Cambridge University Press, p. 817,
ISBN 0-521-37095-7 .
^ "M27C512 Datasheet". Missing or empty url=
^ Frohman, Dov (May 10, 1971), Electronics Magazine (article) .
^ Margolin, J (May 8, 2009). "EPROM". .
^ Sah 1991, p. 640.
Intel 1702A 2K (256 x 8) UV Erasable PROM
^ AMD Am1702A 256-Word by 8-
Bit Programmable Read Only Memory
^ U.S. International Trade Commission, ed. (October 1998). Certain
EPROM, EEPROM, Flash Memory and Flash
Devices and Products Containing Same, Inv. 337-TA-395. Diane
Publishing. pp. 51–72. ISBN 1-4289-5721-9. The
details of SEEQ's
Silicon Signature method of a device programmer
reading an EPROM's ID.
Sah, Chih-Tang (1991), Fundamentals of solid-state electronics, World
Scientific, ISBN 981-02-0637-2
Wikimedia Commons has media related to EPROM.
Detailed information about
EPROM types and
Video of the
Intel 1702 EPROM
Intel Data Book, includes 1702, 2704, 2708 datasheets