Semiconductor memory is adigital electronicsemiconductor device used fordigital data storage, such ascomputer memory. It typically refers to devices in which data is stored withinmetal–oxide–semiconductor (MOS)memory cells on asiliconintegrated circuitmemory chip.^[1][2][3] There are numerous different types using different semiconductor technologies. The two main types ofrandom-access memory (RAM) arestatic RAM (SRAM), which uses severaltransistors per memory cell, anddynamic RAM (DRAM), which uses a transistor and aMOS capacitor per cell.Non-volatile memory (such asEPROM,EEPROM andflash memory) usesfloating-gate memory cells, which consist of a singlefloating-gate transistor per cell.

Most types of semiconductor memory have the property ofrandom access,^[4] 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.^[5] This contrasts with data storage media such asCDs 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 fasteraccess times than other types of data storage; abyte of data can be written to or read from semiconductor memory within a fewnanoseconds, while access time for rotating storage such as hard disks is in the range of milliseconds. For these reasons it is used forprimary storage, to hold the program and data the computer is currently working on, among other uses.

As of 2017^[update], sales of semiconductor memory chips are$124 billion annually, accounting for 30% of thesemiconductor industry.^[6]Shift registers,processor registers,data buffers and other small digital registers that have nomemory address decoding mechanism are typically not referred to asmemory although they also store digital data.

In a semiconductor memory chip, eachbit of binary data is stored in a tiny circuit called amemory cell consisting of one to severaltransistors. 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 calledwords which are accessed together as a single memory address. Memory is manufactured inword length that is usually a power of two, typicallyN=1, 2, 4 or 8 bits.

Data is accessed by means of a binary number called amemory address applied to the chip's address pins, which specifies which word in the chip is to be accessed. If the memory address consists ofM bits, the number of addresses on the chip is 2^M, each containing anN bit word. Consequently, the amount of data stored in each chip isN2^M bits.^[5] The memory storage capacity forM number ofaddress lines is given by 2^M, which is usually in power of two: 2, 4, 8, 16, 32, 64, 128, 256 and 512 and measured inkilobits,megabits,gigabits orterabits, etc. As of 2014^[update] the largest semiconductor memory chips hold a few gigabits of data, but higher capacity memory is constantly being developed. By combining several integrated circuits, memory can be arranged into a larger word length and/or address space than what is offered by each chip, often but not necessarily apower of two.^[5]

The two basic operations performed by a memory chip are "read", in which the data contents of a memory word is read out (nondestructively), and "write" in which data is stored in a memory word, replacing any data that was previously stored there. To increase data rate, in some of the latest types of memory chips such asDDR SDRAM multiple words are accessed with each read or write operation.

In addition to standalone memory chips, blocks of semiconductor memory are integral parts of many computer and data processing integrated circuits. For example, themicroprocessor chips that run computers containcache memory to store instructions awaiting execution.

Volatile memory loses its stored data when the power to the memory chip is turned off. However it can be faster and less expensive than non-volatile memory. This type is used for the main memory in most computers, since data is stored on thehard disk while the computer is off. Major types are:^[7][8]

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 ofrandom access.

• DRAM (Dynamic random-access memory) – This usesmemory cells consisting of oneMOSFET (MOS field-effect transistor) and oneMOS 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, theelectric charge that stores the data in the memory cells slowly leaks out, so the memory cells must be periodicallyrefreshed (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 ofdual-ported memory once used for theframe buffers ofvideo 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'smemory bus. This allowed the chip to process multiple memory requests simultaneously usingpipelining, to increase the speed. The data on the chip is also divided intobanks 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 bydouble 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.
    • 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 forgraphics adaptors (video cards). It can perform graphics-related operations such asbit 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 performmemory 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 fewgame consoles such as theWii.
• SRAM (Static random-access memory) – This stores eachbit of data in a circuit called aflip-flop, made of 4 to 6 transistors. SRAM is less dense and more expensive per bit than DRAM, but faster and does not requirememory refresh. It is used for smallercache 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 asmicroprocessors where it is used forcache 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 removablememory cards among other uses. Major types are:^[7][8]

• ROM (Read-only memory) – This is designed to hold permanent data, and in normal operation is only read from, not written to. Although many types can be written to, the writing process is slow and usually all the data in the chip must be rewritten at once. It is usually used to storesystem software which must be immediately accessible to the computer, such as theBIOS program which starts the computer, and the software (microcode) for portable devices and embedded computers such asmicrocontrollers.
  • MROM (Mask programmed ROM orMask ROM) – In this type the data is programmed into the chip when the chip is manufactured, so it is only used for large production runs. It cannot be rewritten with new data.
  • PROM (Programmable read-only memory) – In this type the data is written into an existing PROM chip before it is installed in the circuit, but it can only be written once. The data is written by plugging the chip into a device called a PROM programmer.
  • EPROM (Erasable programmable read-only memory or UVEPROM) – In this type the data in it can be rewritten by removing the chip from the circuit board, exposing it to anultraviolet light to erase the existing data, and plugging it into a PROM programmer. The IC package has a small transparent "window" in the top to admit the UV light. It is often used for prototypes and small production run devices, where the program in it may have to be changed at the factory.

    

  • EEPROM (Electrically erasable programmable read-only memory) – In this type the data can be rewritten electrically, while the chip is on the circuit board, but the writing process is slow. This type is used to holdfirmware, the low level microcode which runs hardware devices, such as theBIOS program in most computers, so that it can be updated.
• NVRAM (Non-volatile random-access memory)
  • FRAM (Ferroelectric RAM) – One type of nonvolatile RAM.
• Flash memory – In this type the writing process is intermediate in speed between EEPROMS and RAM memory; it can be written to, but not fast enough to serve as main memory. It is often used as a semiconductor version of ahard disk, to store files. It is used in portable devices such as PDAs,USB flash drives, and removablememory cards used indigital cameras andcellphones.

Earlycomputer memory consisted ofmagnetic-core memory, as earlysolid-state electronicsemiconductors, includingtransistors such as thebipolar junction transistor (BJT), were impractical for use as digital storage elements (memory cells). The earliest semiconductor memory dates back to the early 1960s, with bipolar memory, which used bipolar transistors.^[9]Bipolar semiconductor memorymade fromdiscrete devices was first shipped byTexas Instruments to theUnited States Air Force in 1961. The same year, the concept ofsolid-state memory on anintegrated circuit (IC) chip was proposed byapplications engineer Bob Norman atFairchild Semiconductor.^[10] Thefirst single-chip memory IC was the BJT 16-bit IBM SP95 fabricated in December 1965, engineered by Paul Castrucci.^[9][10] While bipolar memory offered improved performance over magnetic-core memory, it could not compete with the lower price of magnetic-core memory, which remained dominant up until the late 1960s.^[9] Bipolar memory failed to replace magnetic-core memory because bipolarflip-flop circuits were too large and expensive.^[11]

The advent of themetal–oxide–semiconductor field-effect transistor (MOSFET),^{[citation needed]} invented byMohamed M. Atalla andDawon Kahng atBell Labs in 1959,^[12] enabled the practical use ofmetal–oxide–semiconductor (MOS) transistors asmemory cell storage elements, a function previously served bymagnetic cores incomputer memory.^{[citation needed]} MOS memory was developed by John Schmidt atFairchild Semiconductor in 1964.^[13][14] In addition to higher performance, MOS memory was cheaper and consumed less power than magnetic-core memory.^[13] This led to MOSFETs eventually replacing magnetic cores as the standard storage elements in computer memory.^{[citation needed]}

In 1965, J. Wood and R. Ball of theRoyal Radar Establishment proposed digital storage systems that useCMOS (complementary MOS) memory cells, in addition to MOSFETpower devices for thepower supply, switched cross-coupling,switches anddelay-line storage.^[15] The development ofsilicon-gateMOS integrated circuit (MOS IC) technology byFederico Faggin at Fairchild in 1968 enabled the production of MOSmemory chips.^[16]NMOS memory was commercialized byIBM in the early 1970s.^[17] MOS memory overtook magnetic core memory as the dominant memory technology in the early 1970s.^[13]

The term "memory" when used with reference to computers most often refers to volatilerandom-access memory (RAM). The two main types of volatile RAM arestatic random-access memory (SRAM) anddynamic random-access memory (DRAM). Bipolar SRAM was invented by Robert Norman at Fairchild Semiconductor in 1963,^[9] followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964.^[13] SRAM became an alternative to magnetic-core memory, but required six MOS transistors for eachbit of data.^[18] Commercial use of SRAM began in 1965, when IBM introduced their SP95 SRAM chip for theSystem/360 Model 95.^[9]

Toshiba introduced bipolar DRAMmemory cells for its Toscal BC-1411electronic calculator in 1965.^[19][20] 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.^[21] MOS technology is the basis for modern DRAM. In 1966, Dr.Robert H. Dennard at theIBM Thomas J. Watson Research Center was working on MOS memory. While examining the characteristics of MOS technology, he found it was capable of buildingcapacitors, 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.^[18] In 1967, Dennard filed a patent under IBM for a single-transistor DRAM memory cell, based on MOS technology.^[22] This led to the first commercial DRAM IC chip, theIntel 1103, in October 1970.^[23][24][25]Synchronous dynamic random-access memory (SDRAM) later debuted with theSamsung KM48SL2000 chip in 1992.^[26][27]

The term "memory" is also often used to refer tonon-volatile memory, specificallyflash memory. It has origins inread-only memory (ROM).Programmable read-only memory (PROM) was invented byWen Tsing Chow in 1956, while working for the Arma Division of the American Bosch Arma Corporation.^[28][29] In 1967, Dawon Kahng andSimon Sze of Bell Labs proposed that thefloating gate of a MOSsemiconductor device could be used for the cell of a reprogrammableread-only memory (ROM), which led toDov Frohman ofIntel inventingEPROM (erasable PROM) in 1971.^[30]EEPROM (electrically erasable PROM) was developed by Yasuo Tarui, Yutaka Hayashi and Kiyoko Naga at Japan'sMinistry of International Trade and Industry (MITI)Electrotechnical Laboratory in 1972.^[31] Flash memory was invented byFujio Masuoka atToshiba in the early 1980s.^[32][33] Masuoka and colleagues presented the invention ofNOR flash in 1984,^[34] and thenNAND flash in 1987.^[35] Toshiba commercialized NAND flash memory in 1987.^[36][37]

• Electronics industry
• Semiconductor industry

• Saecker, Michael; Markl, Volker (2013). "Big Data Analytics on Modern Hardware Architectures: A Technology Survey".Business Intelligence. Lecture Notes in Business Information Processing. Vol. 138. pp. 125–149.doi:10.1007/978-3-642-36318-4_6.ISBN 978-3-642-36317-7.
• Veendrick, Harry J.M. (2017). "Memories".Nanometer CMOS ICs. pp. 249–320.doi:10.1007/978-3-319-47597-4_6.ISBN 978-3-319-47595-0.
• Veendrick, Harry (2000).Deep-Submicron CMOS ICs: From Basics to ASICs. Springer US.ISBN 978-90-440-0111-2.

• Digital electronics
• Computer memory
• Integrated circuits
• MOSFETs

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