Semiconductor memory is a
digital electronic semiconductor device used for
digital data storage, such as
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...
. It typically refers to devices in which data is stored within
metal–oxide–semiconductor (MOS)
memory cells on a
silicon
Silicon is a chemical element with the symbol Si and atomic number 14. It is a hard, brittle crystalline solid with a blue-grey metallic luster, and is a tetravalent metalloid and semiconductor. It is a member of group 14 in the periodic ...
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
memory chip. There are numerous different types using different semiconductor technologies. The two main types of
random-access memory
Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the ...
(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
hard disks and
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
The byte is a unit of digital information that most commonly consists of eight bits. Historically, the byte was the number of bits used to encode a single character of text in a computer and for this reason it is the smallest addressable uni ...
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.
, semiconductor memory chips sell 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
''M'', each containing an ''N'' bit word. Consequently, the amount of data stored in each chip is ''N''2
''M'' bits.
The memory storage capacity for ''M'' number of
address 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 in
kilobit
The kilobit is a multiple of the unit bit for digital information or computer storage. The prefix ''kilo-'' (symbol k) is defined in the International System of Units (SI) as a multiplier of 103 (1 thousand), and therefore,
:1 kilobit = = 1000 ...
s,
megabit
The megabit is a multiple of the unit bit for digital information. The prefix mega (symbol M) is defined in the International System of Units (SI) as a multiplier of 106 (1 million), and therefore
:1 megabit = = = 1000 kilobits.
The megabit ...
s,
gigabits or
terabit
The bit is the most basic unit of information in computing and digital communications. The name is a portmanteau of binary digit. The bit represents a logical state with one of two possible values. These values are most commonly represented ...
s, etc. 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 a
power of two
A power of two is a number of the form where is an integer, that is, the result of exponentiation with number two as the base and integer as the exponent.
In a context where only integers are considered, is restricted to non-negat ...
.
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 as
DDR 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, the
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
chips that run computers contain
cache memory to store instructions awaiting execution.
Types
Volatile memory
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 the
hard disk while the computer is off. Major types are:
RAM (''
Random-access memory
Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the ...
'') 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
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-ox ...
'') This uses
memory cells consisting of one
MOSFET
The metal–oxide–semiconductor field-effect transistor (MOSFET, MOS-FET, or MOS FET) is a type of field-effect transistor (FET), most commonly fabricated by the controlled oxidation of silicon. It has an insulated gate, the voltage of which d ...
(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
Electric charge is the physical property of matter that causes charged matter to experience a force when placed in an electromagnetic field. Electric charge can be ''positive'' or ''negative'' (commonly carried by protons and electrons res ...
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
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-oxide ...
'') 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
A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer moni ...
(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
In computing, a computer bus operating with double data rate (DDR) transfers data on both the rising and falling edges of the clock signal. This is also known as double pumped, dual-pumped, and double transition. The term toggle mode is used in ...
(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
Double Data Rate 3 Synchronous Dynamic Random-Access Memory (DDR3 SDRAM) is a type of synchronous dynamic random-access memory (SDRAM) with a high bandwidth (" double data rate") interface, and has been in use since 2007. It is the higher-spe ...
Transfers 8 consecutive words per internal clock cycle.
****
DDR4 SDRAM Transfers 16 consecutive words per internal clock cycle.
*** RDRAM (''
Rambus DRAM
Rambus DRAM (RDRAM), and its successors Concurrent Rambus DRAM (CRDRAM) and Direct Rambus DRAM (DRDRAM), are types of synchronous dynamic random-access memory (SDRAM) developed by Rambus from the 1990s through to the early 2000s. The third-generati ...
'') 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
Synchronous dynamic random-access memory (synchronous dynamic RAM or SDRAM) is any DRAM where the operation of its external pin interface is coordinated by an externally supplied clock signal.
DRAM integrated circuits (ICs) produced from the ea ...
'') A specialized type of SDRAM made for
graphics adaptor
A graphics card (also called a video card, display card, graphics adapter, VGA card/VGA, video adapter, display adapter, or mistakenly GPU) is an expansion card which generates a feed of output images to a display device, such as a computer moni ...
s (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
GDDR4 SDRAM, an abbreviation for Graphics Double Data Rate 4 Synchronous Dynamic Random-Access Memory, is a type of graphics card memory (SGRAM) specified by the JEDEC Semiconductor Memory Standard. It is a rival medium to Rambus's XDR DRAM. G ...
*****
GDDR5 SDRAM
*****
GDDR6 SDRAM
*** HBM (''
High Bandwidth Memory
High Bandwidth Memory (HBM) is a high-speed computer memory interface for 3D-stacked synchronous dynamic random-access memory (SDRAM) initially from Samsung, AMD and SK Hynix. It is used in conjunction with high-performance graphics accelerators ...
'') 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
The Wii ( ) is a home video game console developed and marketed by Nintendo. It was released on November 19, 2006, in North America and in December 2006 for most other regions of the world. It is Nintendo's fifth major home game console, ...
.
* 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
microprocessor
A microprocessor is a computer processor where the data processing logic and control is included on a single integrated circuit, or a small number of integrated circuits. The microprocessor contains the arithmetic, logic, and control circ ...
s 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:
* ROM (''
Read-only memory
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing ...
'') 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 store
system software which must be immediately accessible to the computer, such as the
BIOS program which starts the computer, and the software (
microcode) for portable devices and embedded computers such as
microcontrollers.
** MROM (
''Mask programmed ROM'' or ''Mask 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 an
ultraviolet light
Ultraviolet (UV) is a form of electromagnetic radiation with wavelength from 10 nm (with a corresponding frequency around 30 PHz) to 400 nm (750 THz), shorter than that of visible light, but longer than X-rays. UV radiatio ...
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
EEPROM (also called E2PROM) stands for electrically erasable programmable read-only memory and is a type of non-volatile memory used in computers, usually integrated in microcontrollers such as smart cards and remote keyless systems, or a ...
'') 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 hold
firmware, the low level microcode which runs hardware devices, such as the
BIOS 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 a
hard disk, to store files. It is used in portable devices such as PDAs,
USB flash drives, and removable
memory card
A memory card is an electronic data storage device used for storing digital information, typically using flash memory. These are commonly used in digital portable electronic devices. They allow adding memory to such devices using a card in a so ...
s used in
digital cameras and
cellphones.
History
Early
computer memory
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...
consisted of
magnetic-core memory
Magnetic-core memory was the predominant form of random-access computer memory for 20 years between about 1955 and 1975.
Such memory is often just called core memory, or, informally, core.
Core memory uses toroids (rings) of a hard magnet ...
, as early
solid-state electronic semiconductors, including
transistors such as the
bipolar 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.
Bipolar semiconductor memory made from
discrete devices was first shipped by
Texas Instruments
Texas Instruments Incorporated (TI) is an American technology company headquartered in Dallas, Texas, that designs and manufactures semiconductors and various integrated circuits, which it sells to electronics designers and manufacturers globa ...
to the
United States Air Force
The United States Air Force (USAF) is the air service branch of the United States Armed Forces, and is one of the eight uniformed services of the United States. Originally created on 1 August 1907, as a part of the United States Army Si ...
in 1961. The same year, the concept of
solid-state
Solid state, or solid matter, is one of the four fundamental states of matter.
Solid state may also refer to:
Electronics
* Solid-state electronics, circuits built of solid materials
* Solid state ionics, study of ionic conductors and their ...
memory on an
integrated circuit
An integrated circuit or monolithic integrated circuit (also referred to as an IC, a chip, or a microchip) is a set of electronic circuits on one small flat piece (or "chip") of semiconductor material, usually silicon. Large numbers of tiny ...
(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.
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.
Bipolar memory failed to replace magnetic-core memory because bipolar
flip-flop circuits were too large and expensive.
MOS memory
The advent of the
metal–oxide–semiconductor field-effect transistor (MOSFET),
invented by
Mohamed M. Atalla and
Dawon Kahng at
Bell Labs
Nokia Bell Labs, originally named Bell Telephone Laboratories (1925–1984),
then AT&T Bell Laboratories (1984–1996)
and Bell Labs Innovations (1996–2007),
is an American industrial research and scientific development company owned by mul ...
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
In computing, memory is a device or system that is used to store information for immediate use in a computer or related computer hardware and digital electronic devices. The term ''memory'' is often synonymous with the term '' primary storag ...
.
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 metal–oxide–semiconductor (CMOS, pronounced "sea-moss", ) is a type of metal–oxide–semiconductor field-effect transistor (MOSFET) fabrication process that uses complementary and symmetrical pairs of p-type and n-type MOSF ...
(complementary MOS) memory cells, in addition to MOSFET
power devices for the
power supply
A power supply is an electrical device that supplies electric power to an electrical load. The main purpose of a power supply is to convert electric current from a source to the correct voltage, current, and frequency to power the load. As ...
, 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.
The term "memory" when used with reference to computers most often refers to volatile
random-access memory
Random-access memory (RAM; ) is a form of computer memory that can be read and changed in any order, typically used to store working data and machine code. A random-access memory device allows data items to be read or written in almost the ...
(RAM). The two main types of volatile RAM are
static random-access memory (SRAM) and
dynamic random-access memory
Dynamic random-access memory (dynamic RAM or DRAM) is a type of random-access semiconductor memory that stores each bit of data in a memory cell, usually consisting of a tiny capacitor and a transistor, both typically based on metal-ox ...
(DRAM). Bipolar SRAM was invented by Robert Norman at Fairchild Semiconductor in 1963,
followed by the development of MOS SRAM by John Schmidt at Fairchild in 1964.
SRAM became an alternative to magnetic-core memory, but required six MOS transistors for each
bit of data.
Commercial use of SRAM began in 1965, when IBM introduced their SP95 SRAM chip for the
System/360 Model 95.
Toshiba introduced bipolar DRAM
memory cells for its Toscal BC-1411
electronic calculator in 1965.
[Toshiba "Toscal" BC-1411 Desktop Calculator](_blank)
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.
In 1967, Dennard filed a patent under IBM for a single-transistor DRAM memory cell, based on MOS technology. This led to the first commercial DRAM IC chip, the
Intel 1103, in October 1970.
[''The DRAM memory of Robert Dennard''](_blank)
history-computer.com. Synchronous dynamic random-access memory (SDRAM) later debuted with the
Samsung
The Samsung Group (or simply Samsung) ( ko, 삼성 ) is a South Korean multinational manufacturing conglomerate headquartered in Samsung Town, Seoul, South Korea. It comprises numerous affiliated businesses, most of them united under the ...
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
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing ...
(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
Read-only memory (ROM) is a type of non-volatile memory used in computers and other electronic devices. Data stored in ROM cannot be electronically modified after the manufacture of the memory device. Read-only memory is useful for storing ...
(ROM), which led to
Dov Frohman of
Intel
Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California. It is the world's largest semiconductor chip manufacturer by revenue, and is one of the developers of the x86 ser ...
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
The was a ministry of the Government of Japan from 1949 to 2001. The MITI was one of the most powerful government agencies in Japan and, at the height of its influence, effectively ran much of Japanese industrial policy, funding research and ...
(MITI)
Electrotechnical Laboratory in 1972. Flash memory was invented by
Fujio Masuoka
is a Japanese engineer, who has worked for Toshiba and Tohoku University, and is currently chief technical officer (CTO) of Unisantis Electronics. He is best known as the inventor of flash memory, including the development of both the NOR fla ...
at
Toshiba in the early 1980s. Masuoka and colleagues presented the invention of
NOR flash in 1984, and then
NAND flash
Flash memory is an Integrated circuit, electronic Non-volatile memory, non-volatile computer memory storage medium that can be electrically erased and reprogrammed. The two main types of flash memory, NOR flash and NAND flash, are named for t ...
in 1987. Toshiba commercialized NAND flash memory in 1987.
Applications
See also
*
List of best-selling electronic devices
*
Semiconductor industry
References
{{DEFAULTSORT:Semiconductor Memory
Digital electronics
Computer memory
Integrated circuits
MOSFETs