Semiconductor random-access memories (RAMs) are available in a wide range of speeds. Their cycle times range from 100 ns to less than 10 ns. Semiconductor memory is used in any electronics assembly that uses computer processing technology. The use of semiconductor memory has grown, and the size of these memory cards has increased as the need for larger and larger amounts of storage is needed.
There are two main types or categories that can be used for semiconductor technology.
RAM - Random Access Memory: As the names suggest, the RAM or random access memory is a form of semiconductor memory technology that is used for reading and writing data in any order - in other words as it is required by the processor. 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.
ROM - Read Only Memory: A ROM is a form of semiconductor memory technology used where the data is written once and then not changed. In view of this it is used where data needs to be stored permanently, even when the power is removed - many memory technologies lose the data once the power is removed. As a result, this type of semiconductor memory technology 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.
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.
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.
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.
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.
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-program 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.
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.