Updated 05-19-06
We're going to concentrate on UV-EPROM's in this article as they
are more historical these days.
(however the concept and physics is mostly the same)
An EPROM is a large array of Floating
Gate Transistors.
Inside a floating gate MOSFET, the main components are a control
gate, floating gate, and the thin oxide layer. The EPROM has a
grid of columns and rows and the cell at each intersection has
two transistors. The two transistors are separated from each other
by a thin oxide layer. One of the transistors is known as the
floating gate and the other as the control gate. The floating
gate's only link to the row (wordline) is through the control
gate. As long as this link is in place, the cell has a value of
1. To change the value to 0 requires a process called Fowler-Nordheim
tunneling. Tunneling is used to alter the placement of electrons
in the floating gate. An electrical charge, usually 10 to 28 volts,
is applied to the floating gate. The charge comes from the column
(bitline), enters the floating gate and drains to a ground.
This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on the other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. A device called a cell sensor monitors the level of the charge passing through the floating gate. If the flow through the gate is greater than 50 percent of the charge, it has a value of 1. When the charge passing through drops below the 50-percent threshold, the value changes to 0. A blank EPROM has all of the gates fully open, giving each cell a value of 1.
To rewrite an EPROM, you must erase it first. To erase it, you
must supply a level of energy strong enough to break through the
negative electrons blocking the floating gate. In a standard EPROM,
this is best accomplished with UV light at a wavelength of 253.7
nanometers. Contrary to popular belief, sunlight does not erase
your EPROM. Because this particular frequency will not penetrate
most plastics or glasses, each EPROM chip has a quartz window
on top of it. The EPROM must be very close to the eraser's light
source, within an inch or two, to work properly.
Too much UV-Light can excite the electrons TOO much resulting
in an 'over-erased' condition that cannot be readily be fixed.
OTP EPROM's just lack the window, so there is no way to get UV-Light
in them. Several individuals have gotten around this by using
X-Rays which is just a higher energy wave in the electromagnetic
spectrum. It WILL penetrate the plastic case and erase it, however,
since it is MUCH higher energy, it is easy to over-erase the chip.
EPROM's are packaged in a ceramic package because of the embedded quartz crystal. During normal heating and cooling cycles a Quartz/Plastic package would fail. Ceramic and Quartz expand and contract at the same rate making the ceramic package the only acceptable form. This decreases failures but greatly increases cost. NOTE: Apparently the Soviets solved this, as I have 3 Soviet EPROM's that are in a plastic package. It appears to be all resin, with a small hole that has a resin lens planted in it.
History:
The first EPROM was the Intel C1701 back in 1971 which stored
256 bytes of information. Today's EPROM's hold 8Mb or more.
Below is a listing of most all Parallel EPROM's that were used.
These EPROM's have separate pins for Data and Address (non-multiplexed)
and take the data out in parallel. Modern day EPROM's now either
multiplex the Address/Data ports or take the data out via a serial
connection (often I2C)
UV-EPROM's are at the end of their life. Newer technologies are smaller, cheaper, and faster. These chips served us well into the 21st Century, impressive for a technology that is over 30 years old. They will continue to serve in many control applications until old equipment is replaced.
(image courtesy of MrLaptop)
