COA – Dept of ECE Page 31
The different memory types or memory technologies are detailed below:
DRAM: Dynamic RAM is a form of random access memory. DRAM uses a capacitor to store each bit
of data, and the level of charge on each capacitor determines whether that bit is a logical 1 or 0.
However these capacitors do not hold their charge indefinitely, and therefore the data needs to be
refreshed periodically. As a result of this dynamic refreshing it gains its name of being a dynamic RAM.
DRAM is the form of semiconductor memory that is often used in equipment including personal
computers and workstations where it forms the main RAM for the computer.
EEPROM: This is an Electrically Erasable Programmable Read Only Memory. Data can be written to
it and it can be erased using an electrical voltage. This is typically applied to an erase pin on the chip.
Like other types of PROM, EEPROM retains the contents of the memory even when the power is turned
off. Also like other types of ROM, EEPROM is not as fast as RAM.
EPROM: This is an Erasable Programmable Read Only Memory. This form of semiconductor
memory can be programmed and then erased at a later time. This is normally achieved by exposing the
silicon to ultraviolet light. To enable this to happen there is a circular window in the package of the
EPROM to enable the light to reach the silicon of the chip. When the PROM is in use, this window is
normally covered by a label, especially when the data may need to be preserved for an extended period.
The PROM stores its data as a charge on a capacitor. There is a charge storage capacitor for each cell
and this can be read repeatedly as required. However it is found that after many years the charge may
leak away and the data may be lost. Nevertheless, this type of semiconductor memory used to be widely
used in applications where a form of ROM was required, but where the data needed to be changed
periodically, as in a development environment, or where quantities were low.
FLASH MEMORY: Flash memory may be considered as a development of EEPROM technology.
Data can be written to it and it can be erased, although only in blocks, but data can be read on an
individual cell basis. To erase and re-programme areas of the chip, programming voltages at levels that
are available within electronic equipment are used. It is also non-volatile, and this makes it particularly
useful. As a result Flash memory is widely used in many applications including memory cards for digital
cameras, mobile phones, computer memory sticks and many other applications.
F-RAM: Ferroelectric RAM is a random-access memory technology that has many similarities to the
standard DRAM technology. The major difference is that it incorporates a ferroelectric layer instead of
the more usual dielectric layer and this provides its non-volatile capability. As it offers a non-volatile
capability, F-RAM is a direct competitor to Flash.
MRAM: This is Magneto-resistive RAM, or Magnetic RAM. It is a non-volatile RAM memory
technology that uses magnetic charges to store data instead of electric charges. Unlike technologies
including DRAM, which require a constant flow of electricity to maintain the integrity of the data,
MRAM retains data even when the power is removed. An additional advantage is that it only requires
low power for active operation. As a result this technology could become a major player in the
electronics industry now that production processes have been developed to enable it to be produced.
P-RAM / PCM: This type of semiconductor memory is known as Phase change Random Access
Memory, P-RAM or just Phase Change memory, PCM. It is based around a phenomenon where a form
of chalcogenide glass changes is state or phase between an amorphous state (high resistance) and a
polycrystalline state (low resistance). It is possible to detect the state of an individual cell and hence use
this for data storage. Currently this type of memory has not been widely commercialized, but it is
expected to be a competitor for flash memory.