| Semiconductor Memory Technologies |
Semiconductor memory technologies
- an overview of the different memory technologies used in electronic
circuits including PROM, EPROM, EEPROM, RAM, DRAM,SRAM, SDRAM, Flash memory,
MRAM, etc.
Memory is an essential element of today's electronics.
Normally based around semiconductor technology, memory is used in any equipment
that uses a processor of one form or another. Indeed as processors have become
more popular and the number of microprocessor controlled items has increased so
has the requirement for semiconductor memory. An additional driver has been the
fact that the software associated with the processors and computers has become
more sophisticated and much larger, and this too has greatly increased the
requirement for semiconductor memory. In addition to this new applications such
as digital cameras, PDAs and many more applications have given rise to the need
to memories. Accordingly it is not uncommon to see semiconductor memories of 4
Gbyte and more required for various applications.
With the rapid growth in the requirement for semiconductor
memories there have been a number of technologies and types of memory that have
emerged. Names such as ROM, RAM, EPROM, EEPROM, Flash memory, DRAM, SRAM, SDRAM,
and the very new MRAM can now be seen in the electronics literature. Each one
has its own advantages and area in which it may be used.
Types of semiconductor memory
Electronic semiconductor memory can be split into two main categories, according
to the way in which they operate:
- RAM - Random Access Memory
- ROM - Read Only Memory
As the names suggest, the RAM or random access memory is used
or reading and writing data in any order as required. It is used for such
applications as the computer or processor memory where variables and other
stored and are required on a random basis. Data is stored and read many times to
and from this type of memory.
A ROM is a form of semiconductor memory to which data is
written once and then not changed. In view of this technologies can be used that
retain the data once the power is removed. As a result, this type of memory is
widely used for storing programs and data that must survive when a computer or
processor is powered down. For example the BIOS of a computer will be stored in
ROM. As the name implies, data cannot be easily written to ROM. Depending on the
technology used in the ROM, writing the data into the ROM initially may require
special hardware. Although it is often possible to change the data, this gain
requires special hardware to erase the data ready for new data to be written in.
Overview of memory technologies
There is a large variety of types of ROM and RAM that are available. These arise
from the variety of applications and also the number of technologies available.
This means that there is a large number of abbreviations or acronyms and
categories for memories ranging from Flash to MRAM, PROM to EEPROM, and many
more:
- PROM This stands for Programmable Read Only Memory. It is a
semiconductor memory which can only have data written to it once - the data
written to it is permanent. These memories are bought in a blank format and
they are programmed using a special PROM programmer. Typically a PROM will
consist of an array of fuseable links some of which are "blown" during the
programming process to provide the required data pattern.
- 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. These 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.
- 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.
- 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.
- 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.
- SRAM Static Random Access Memory. This form of semiconductor
memory gains its name from the fact that, unlike DRAM, the data does not
need to be refreshed dynamically. It is able to support faster read and
write times than DRAM (typically 10 ns against 60 ns for DRAM), and in
addition its cycle time is much shorter because it does not need to pause
between accesses. However it consumes more power, is less dense and more
expensive than DRAM. As a result of this it is normally used for caches,
while DRAM is used as the main semiconductor memory technology.
- SDRAM Synchronous DRAM. This form of semiconductor memory can
run at faster speeds than conventional DRAM. It is synchronised to the clock
of the processor and is capable of keeping two sets of memory addresses open
simultaneously. By transferring data alternately from one set of addresses,
and then the other, SDRAM cuts down on the delays associated with
non-synchronous RAM, which must close one address bank before opening the
next.
- 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.
Summary
Semiconductor memory technology is moving forward apace. Not only are the sizes
of memories being increased, and the data densities improving, but new forms of
semiconductor memory such as MRAM are being introduced. With the requirements
for ever larger, faster and lower power memories always increasing, this area of
technology will remain one of the most dynamic in the electronics industry.
|
