Specifications

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RAM
RAM stands for “Random Access Memory.” RAM chips use transistors to link the bit and word
lines, and they need a constant energy source to keep the information stored. Once the power
source is removed, the transistors lose whatever information was retained until power is restored
and new information is sent to the chip. This characteristic is described as “volatile” because the
information simply “flies away” once a power supply is removed.
ROM
ROM is “Read Only Memory.” ROM chips use diodes in place of transistors as the links between
bit lines and word lines. Diodes will pass electric current in only one direction once the voltage
reaches a particular threshold (usually 0.6 volts in ROM chips). ROM chips are designed with all
the information programmed in them so that diodes only appear at intersections where the value is
supposed to be a 1. If the value is supposed to be 0, there is no link at all at that intersection. The
information designed into the chip with the presence or absence of diodes is permanent and
unchangeable because the diode links cannot be altered.
PROM
ROM chips are inexpensive to make; but in order to test whether their design is correct in the first
place requires “programmable read only memory, or PROMs. The links in these chips are fuses
that conduct electricity between all the bit and word lines for values of 1 across every intersection
of the entire chip before the chip is programmed. The programming process sends high enough
current
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down every fuse link that is supposed to be zero and “burns” out the fuse so that no link is
left and the intersection will now read 0. Programming or burning the PROM is done only once.
The information is “permanent” unless an accidental burst of electricity from a discharge of static
electricity burns out more fuses.
EPROM
The Erasable Programmable Read Only Memory, or EPROM, allows a chip to be programmed,
erased, and reprogrammed again. In the evolution of memory chips, they are very close to flash
memory chips in the way the function. The link between the bit line and the word line consists of
two transistors separated by a thin layer of oxide. The first transistor is called a “floating gate,” and
the second is a “control gate.” As long as electricity flows through these gates at a value of 50% or
more of the intended current, the intersection is considered a 1. In order to get the value to 0, a
voltage of 10 to 13 volts is applied to the floating gate so that electrons are forced through to the
other side of the oxide layer
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where they block current from flowing to the control gate. If the flow
of current through the floating gate/control gate link is less than 50% of its intended value, that
intersection has a value of 0. Erasing the chip requires the application of an ultraviolet light at a
precise wavelength of 253.7 nanometers through a quartz crystal window in the chip to restore all
the links to 1s, a tricky process that requires removing the chip and erasing everything on it.
EEPROM—(nearly there)
Electrically Erasable Programmable Read Only Memory gets around the erasing problem of
EPROMS by using an electrical field to erase the information and restore each link to a value of 1.
The advantage of EEPROMS is that users do not have to remove the chips to erase them and
erasure does not have to apply to the entire chip, only to the selected links. The disadvantage is
that the erasure is done one byte at a time, a very slow process in computer time.
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”Current” describes the flow of electricity; voltage describes the power pushing it. If electricity were water, a rain shower would be
high current (fast moving) but with low voltage (not a lot of force in each drop). A pinhole leak in Boulder Dam, on the other hand, would
be similar to low current, high voltage—not much flow, but a lot of force behind it. A shock of 50,000 volts is about the same as a static
spark resulting from walking on a rug in a dry room in winter—lots of volts, very little current. A bolt of lightning has about the same
voltage as the static spark—but it is the tremendous current of the lightning bolt that lights up the sky and does the damage.
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This is a process physicists call “Fowler-Nordheim tunneling.”