Semiconductor memory is a digital electronic semiconductor device used for digital data storage, such as computer memory. It typically refers to devices in which data is stored within metal–oxide–semiconductor (MOS) memory cells on a silicon integrated circuit memory chip. There are numerous different types using different semiconductor technologies. The two main types of random-access memory (RAM) are static RAM (SRAM), which uses several transistors per memory cell, and dynamic RAM (DRAM), which uses a transistor and a MOS capacitor per cell. Non-volatile memory (such as EPROM, EEPROM and flash memory) uses floating-gate memory cells, which consist of a single floating-gate transistor per cell.

Most types of semiconductor memory have the property of random access, which means that it takes the same amount of time to access any memory location, so data can be efficiently accessed in any random order. This contrasts with data storage media such as CDs which read and write data consecutively and therefore the data can only be accessed in the same sequence it was written. Semiconductor memory also has much faster access times than other types of data storage; a byte of data can be written to or read from semiconductor memory within a few nanoseconds, while access time for rotating storage such as hard disks is in the range of milliseconds. For these reasons it is used for primary storage, to hold the program and data the computer is currently working on, among other uses.

, sales of semiconductor memory chips are annually, accounting for % of the semiconductor industry. Shift registers, processor registers, data buffers and other small digital registers that have no memory address decoding mechanism are typically not referred to as memory although they also store digital data.

Description

In a semiconductor memory chip, each bit of binary data is stored in a tiny circuit called a memory cell consisting of one to several transistors. The memory cells are laid out in rectangular arrays on the surface of the chip. The 1-bit memory cells are grouped in small units called "words," which are accessed together as a single memory address. Memory is manufactured in word length that is usually a power of two, typically N=1, 2, 4 or 8 bits.

Data is accessed by means of a binary number called a memory address applied to the chip's address pins, which specifies which word in the chip is to be accessed. If the memory address consists of M bits, the number of addresses on the chip is 2<sup>M</sup>, each containing an N bit word. Consequently, the amount of data stored in each chip is N2<sup>M</sup> bits.

RAM (Random-access memory) This has become a generic term for any semiconductor memory that can be written to, as well as read from, in contrast to ROM (below), which can only be read. All semiconductor memory, not just RAM, has the property of random access.

  • DRAM (Dynamic random-access memory) This uses memory cells consisting of one MOSFET (MOS field-effect transistor) and one MOS capacitor to store each bit. This type of RAM is the cheapest and highest in density, so it is used for the main memory in computers. However, the electric charge that stores the data in the memory cells slowly leaks out, so the memory cells must be periodically refreshed (rewritten) which requires additional circuitry. The refresh process is handled internally by the computer and is transparent to its user.
  • FPM DRAM (Fast page mode DRAM) An older type of asynchronous DRAM that improved on previous types by allowing repeated accesses to a single "page" of memory to occur at a faster rate. Used in the mid-1990s.
  • EDO DRAM (Extended data out DRAM) An older type of asynchronous DRAM which had faster access time than earlier types by being able to initiate a new memory access while data from the previous access was still being transferred. Used in the later part of the 1990s.
  • VRAM (Video random access memory) An older type of dual-ported memory once used for the frame buffers of video adapters (video cards).
  • SDRAM (Synchronous dynamic random-access memory) This added circuitry to the DRAM chip which synchronizes all operations with a clock signal added to the computer's memory bus. This allowed the chip to process multiple memory requests simultaneously using pipelining, to increase the speed. The data on the chip is also divided into banks which can each work on a memory operation simultaneously. This became the dominant type of computer memory by about the year 2000.
  • DDR SDRAM (Double data rate SDRAM) This could transfer twice the data (two consecutive words) on each clock cycle by double pumping (transferring data on both the rising and falling edges of the clock pulse). Extensions of this idea are the current (2012) technique being used to increase memory access rate and throughput. Since it is proving difficult to further increase the internal clock speed of memory chips, these chips increase the transfer rate by transferring more data words on each clock cycle
  • DDR2 SDRAM Transfers 4 consecutive words per internal clock cycle
  • DDR3 SDRAM Transfers 8 consecutive words per internal clock cycle.
  • DDR4 SDRAM Transfers 16 consecutive words per internal clock cycle.
  • DDR5 SDRAM
  • RDRAM (Rambus DRAM) An alternate double data rate memory standard that was used on some Intel systems but ultimately lost out to DDR SDRAM.
  • XDR DRAM (Extreme data rate DRAM)
  • SGRAM (Synchronous graphics RAM) A specialized type of SDRAM made for graphics adaptors (video cards). It can perform graphics-related operations such as bit masking and block write, and can open two pages of memory at once.
  • GDDR SDRAM (Graphics DDR SDRAM)
  • GDDR2
  • GDDR3 SDRAM
  • GDDR4 SDRAM
  • GDDR5 SDRAM
  • GDDR6 SDRAM
  • HBM (High Bandwidth Memory) A development of SDRAM used in graphics cards that can transfer data at a faster rate. It consists of multiple memory chips stacked on top of one another, with a wider data bus.
  • PSRAM (Pseudostatic RAM) This is DRAM which has circuitry to perform memory refresh on the chip, so that it acts like SRAM, allowing the external memory controller to be shut down to save energy. It is used in a few game consoles such as the Wii.
  • SRAM (Static random-access memory) This stores each bit of data in a circuit called a flip-flop, made of 4 to 6 transistors. SRAM is less dense and more expensive per bit than DRAM, but faster and does not require memory refresh. It is used for smaller cache memories in computers.
  • CAM (Content-addressable memory) This is a specialized type in which, instead of accessing data using an address, a data word is applied and the memory returns the location if the word is stored in the memory. It is mostly incorporated in other chips such as microprocessors where it is used for cache memory.

Non-volatile memory

Non-volatile memory (NVM) preserves the data stored in it during periods when the power to the chip is turned off. Therefore, it is used for the memory in portable devices, which don't have disks, and for removable memory cards among other uses. Major types are: Bipolar semiconductor memory made from discrete devices was first shipped by Texas Instruments to the United States Air Force in 1961. The same year, the concept of solid-state memory on an integrated circuit (IC) chip was proposed by applications engineer Bob Norman at Fairchild Semiconductor. The first single-chip memory IC was the BJT 16-bit IBM SP95 fabricated in December 1965, engineered by Paul Castrucci.

MOS memory

The advent of the metal–oxide–semiconductor field-effect transistor (MOSFET), invented by Mohamed M. Atalla and Dawon Kahng at Bell Labs in 1959, enabled the practical use of metal–oxide–semiconductor (MOS) transistors as memory cell storage elements, a function previously served by magnetic cores in computer memory. MOS memory was developed by John Schmidt at Fairchild Semiconductor in 1964. In addition to higher performance, MOS memory was cheaper and consumed less power than magnetic-core memory. This led to MOSFETs eventually replacing magnetic cores as the standard storage elements in computer memory.

In 1965, J. Wood and R. Ball of the Royal Radar Establishment proposed digital storage systems that use CMOS (complementary MOS) memory cells, in addition to MOSFET power devices for the power supply, switched cross-coupling, switches and delay-line storage. The development of silicon-gate MOS integrated circuit (MOS IC) technology by Federico Faggin at Fairchild in 1968 enabled the production of MOS memory chips. NMOS memory was commercialized by IBM in the early 1970s. MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s. Commercial use of SRAM began in 1965, when IBM introduced their SP95 SRAM chip for the System/360 Model 95. While it offered improved performance over magnetic-core memory, bipolar DRAM could not compete with the lower price of the then dominant magnetic-core memory. MOS technology is the basis for modern DRAM. In 1966, Dr. Robert H. Dennard at the IBM Thomas J. Watson Research Center was working on MOS memory. While examining the characteristics of MOS technology, he found it was capable of building capacitors, and that storing a charge or no charge on the MOS capacitor could represent the 1 and 0 of a bit, while the MOS transistor could control writing the charge to the capacitor. This led to his development of a single-transistor DRAM memory cell. This led to the first commercial DRAM IC chip, the Intel 1103, in October 1970. Synchronous dynamic random-access memory (SDRAM) later debuted with the Samsung KM48SL2000 chip in 1992.

The term "memory" is also often used to refer to non-volatile memory, specifically flash memory. It has origins in read-only memory (ROM). Programmable read-only memory (PROM) was invented by Wen Tsing Chow in 1956, while working for the Arma Division of the American Bosch Arma Corporation. In 1967, Dawon Kahng and Simon Sze of Bell Labs proposed that the floating gate of a MOS semiconductor device could be used for the cell of a reprogrammable read-only memory (ROM), which led to Dov Frohman of Intel inventing EPROM (erasable PROM) in 1971. EEPROM (electrically erasable PROM) was developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at Japan's Ministry of International Trade and Industry (MITI) Electrotechnical Laboratory in 1972. Flash memory was invented by Fujio Masuoka at Toshiba in the early 1980s. Masuoka and colleagues presented the invention of NOR flash in 1984, and then NAND flash in 1987. Toshiba commercialized NAND flash memory in 1987.

Applications

{| class="wikitable sortable"

|+ MOS memory applications

! style="width:20%" | MOS memory type

!

! style="width:10%" | MOS memory cell

! Applications

|-

|Static random-access memory

|SRAM

|MOSFETs

|Cache memory, cell phones, eSRAM, mainframes, multimedia computers, networking, personal computers, servers, supercomputers, telecommunications, workstations, DVD disk buffer, data buffer, nonvolatile BIOS memory

|-

|Dynamic random-access memory

|DRAM

|MOSFET, MOS capacitor

|Camcorders, embedded logic, eDRAM, graphics card, hard disk drive (HDD), networks, personal computers, personal digital assistants, printers, main computer memory, desktop computers, servers, solid-state drives, video memory, framebuffer memory

|-

|Ferroelectric random-access memory

|FRAM

|MOSFET, Ferroelectric capacitor

|Non-volatile memory, radio-frequency identification (RF identification), smart cards

|-

|Read-only memory

|ROM

|MOSFET

|Character generators, electronic musical instruments, laser printer fonts, video game ROM cartridges, word processor dictionary data

|-

|Erasable programmable read-only memory

|EPROM

|Floating-gate MOSFET

|CD-ROM drives, embedded memory, code storage, modems

|-

|Electrically erasable programmable read-only memory

|EEPROM

|Floating-gate MOSFET

|Anti-lock braking systems, air bags, car radios, cell phones, consumer electronics, cordless telephones, disk drives, embedded memory, flight controllers, military technology, modems, pagers, printers, set-top box, smart cards

|-

|Flash memory

|Flash

|Floating-gate MOSFET

|ATA controllers, battery-powered applications, telecommunications, code storage, digital cameras, MP3 players, portable media players, BIOS memory, USB flash drive, digital TV, e-books, memory cards, mobile devices, set-top box, smartphones, solid-state drives, tablet computers

|-

|Non-volatile random-access memory

|NVRAM

|Floating-gate MOSFETs

|Medical equipment, spacecraft

|}

See also

  • Electronics industry
  • Semiconductor industry

References

Sources