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electronics Electronics is a scientific and engineering discipline that studies and applies the principles of physics to design, create, and operate devices that manipulate electrons and other Electric charge, electrically charged particles. It is a subfield ...
, a multi-level cell (MLC) is a memory cell capable of storing more than a single bit of information, compared to a single-level cell (SLC), which can store only one bit per memory cell. A memory cell typically consists of a single floating-gate MOSFET (metal–oxide–semiconductor field-effect transistor), thus multi-level cells reduce the number of MOSFETs required to store the same amount of data as single-level cells. Triple-level cells (TLC) and quad-level cells (QLC) are versions of MLC memory, which can store three and four bits per cell respectively. The name "''multi''-level cell" is sometimes used specifically to refer to the "''two''-level cell". Overall, the memories are named as follows: # Single-level cell or SLC (1 bit per cell) # Multi-level cell or MLC (2 bits per cell), alternatively double-level cell or DLC # Triple-level cell or TLC (3 bits per cell) or 3-Bit MLC # Quad-level cell or QLC (4 bits per cell) # Penta-level cell or PLC (5 bits per cell) – currently in development Notice that this nomenclature can be misleading, since an "''n''-level cell" in fact uses 2''n'' levels of charge to store ''n'' bits (see below). Typically, as the "level" count increases, performance (speed and reliability) and consumer cost decrease; however, this correlation can vary between manufacturers. Examples of MLC memories are MLC
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 ...
, MLC PCM ( phase-change memory), etc. For example, in SLC NAND flash technology, each cell can exist in one of the two states, storing one bit of information per cell. Most MLC NAND
flash memory 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 ...
has four possible states per cell, so it can store two bits of information per cell. This reduces the amount of margin separating the states and results in the possibility of more errors. Multi-level cells that are designed for low error rates are sometimes called enterprise MLC (eMLC). New technologies, such as multi-level cells and 3D Flash, and increased production volumes will continue to bring prices down.


Single-level cell

Flash memory 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 ...
stores data in individual memory cells, which are made of floating-gate MOSFET transistors. Traditionally, each cell had two possible states (each with one voltage level), with each state representing either a one or a zero, so one bit of data was stored in each cell in so-called ''single-level cells'', or SLC flash memory. SLC memory has the advantage of higher write speeds, lower power consumption and higher cell endurance. However, because SLC memory stores less data per cell than MLC memory, it costs more per megabyte of storage to manufacture. Due to higher transfer speeds and expected longer life, SLC flash technology is used in high-performance
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, such as digital cameras as well as in many early games conso ...
s. In February 2016, a study was published that showed little difference in practice between the reliability of SLC and MLC. A single-level cell (SLC) flash memory may have a lifetime of about 50,000 to 100,000 program/erase cycles. A single-level cell represents a 1 when almost empty and a 0 when almost full. There is a region of uncertainty (a read margin) between the two possible states at which the data stored in the cell cannot be precisely read.


Multi-level cell

The primary benefit of MLC flash memory is its lower cost per unit of storage due to the higher data density, and memory-reading software can compensate for a larger bit error rate. The higher error rate necessitates an
error-correcting code In computing, telecommunication, information theory, and coding theory, forward error correction (FEC) or channel coding is a technique used for controlling errors in data transmission over unreliable or noisy communication channels. The centra ...
(ECC) that can correct multiple bit errors; for example, the SandForce SF-2500 flash controller can correct up to 55 bits per 512-byte sector with an unrecoverable read error rate of less than one sector per 1017 bits read. The most commonly used algorithm is Bose–Chaudhuri–Hocquenghem (
BCH code In coding theory, the Bose–Chaudhuri–Hocquenghem codes (BCH codes) form a class of cyclic error-correcting codes that are constructed using polynomials over a finite field (also called a '' Galois field''). BCH codes were invented in ...
). Other drawbacks of MLC NAND are lower write speeds, lower number of program/erase cycles and higher power consumption compared to SLC flash memory. Read speeds can also be lower for MLC NAND than SLC due to the need to read the same data at a second threshold voltage to help resolve errors. TLC and QLC devices may need to read the same data up to 4 and 8 times respectively to obtain values that are correctable by ECC. MLC flash may have a lifetime of about 1,000 to 10,000 program/erase cycles. This typically necessitates the use of a flash file system, which is designed around the limitations of flash memory, such as using wear leveling to extend the useful lifetime of the flash device. The
Intel 8087 The Intel 8087, announced in 1980, was the first floating-point coprocessor for the 8086 line of microprocessors. The purpose of the chip was to speed up floating-point arithmetic operations, such as addition, subtraction, multiplication, div ...
used two-bits-per-cell technology for its
microcode In processor design, microcode serves as an intermediary layer situated between the central processing unit (CPU) hardware and the programmer-visible instruction set architecture of a computer. It consists of a set of hardware-level instructions ...
ROM Rom, or ROM may refer to: Biomechanics and medicine * Risk of mortality, a medical classification to estimate the likelihood of death for a patient * Rupture of membranes, a term used during pregnancy to describe a rupture of the amniotic sac * ...
, and in 1980 was one of the first devices on the market to use multi-level ROM cells.
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
later demonstrated 2-bit multi-level cell (MLC) NOR flash in 1997.
NEC is a Japanese multinational information technology and electronics corporation, headquartered at the NEC Supertower in Minato, Tokyo, Japan. It provides IT and network solutions, including cloud computing, artificial intelligence (AI), Inte ...
demonstrated quad-level cells in 1996, with a 64 Mbit
flash memory 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 ...
chip storing 2 bits per cell. In 1997, NEC demonstrated a
dynamic random-access memory Dynamics (from Greek language, Greek δυναμικός ''dynamikos'' "powerful", from δύναμις ''dynamis'' "power (disambiguation), power") or dynamic may refer to: Physics and engineering * Dynamics (mechanics), the study of forces and t ...
(DRAM) chip with quad-level cells, holding a capacity of 4Gbit.
STMicroelectronics STMicroelectronics Naamloze vennootschap, NV (commonly referred to as ST or STMicro) is a European multinational corporation, multinational semiconductor contract manufacturing and design company. It is the largest of such companies in Europe. ...
also demonstrated quad-level cells in 2000, with a 64Mbit NOR flash memory chip. MLC is used to refer to cells that store 2 bits per cell, using 4 charge values or levels. A 2-bit MLC has a single charge level assigned to every possible combination of ones and zeros, as follows: When close to 25% full, the cell represents a binary value of 11; when close to 50%, the cell represents a 01; when close to 75%, the cell represents a 00; and when close to 100%, the cell represents a 10. Once again, there is a region of uncertainty (read margin) between values, at which the data stored in the cell cannot be precisely read. some solid-state drives use part of an MLC NAND die as if it were single-bit SLC NAND, giving higher write speeds. nearly all commercial MLCs are planar-based (i.e. cells are built on silicon surface) and so subject to scaling limitations. To address this potential problem, the industry is already looking at technologies that can guarantee storage density increases beyond today’s limitations. One of the most promising is 3D Flash, where cells are stacked vertically, thereby avoiding the limitations of planar scaling. In the past, a few memory devices went the other direction and used two cells per bit to give even lower bit error rates. Enterprise MLC (eMLC) is a more expensive variant of MLC that is optimized for commercial use. It claims to last longer and be more reliable than normal MLCs while providing cost savings over traditional SLC drives. Although many SSD manufacturers have produced MLC drives intended for enterprise use, only Micron sells raw NAND Flash chips under this designation.


Triple-level cell

A triple-level cell (TLC) is a type of
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 ...
memory that stores 3 bits of information per cell.
Toshiba is a Japanese multinational electronics company headquartered in Minato, Tokyo. Its diversified products and services include power, industrial and social infrastructure systems, elevators and escalators, electronic components, semiconductors ...
introduced memory with triple-level cells in 2009. With current technology a maximum lifetime of up to 3,000 program/erase cycles is achievable.
Samsung Samsung Group (; stylised as SΛMSUNG) is a South Korean Multinational corporation, multinational manufacturing Conglomerate (company), conglomerate headquartered in the Samsung Town office complex in Seoul. The group consists of numerous a ...
announced a type of NAND flash that stores 3 bits of information per cell, with 8 total voltage states (values or levels), coining the term "triple-level cell" ("TLC").
Samsung Electronics Samsung Electronics Co., Ltd. (SEC; stylized as SΛMSUNG; ) is a South Korean multinational major appliance and consumer electronics corporation founded on 13 January 1969 and headquartered in Yeongtong District, Suwon, South Korea. It is curr ...
began mass-producing it in 2010, and it was first seen in Samsung's 840 Series SSDs. Samsung refers to this technology as 3-bit MLC. The negative aspects of MLC are amplified with TLC, but TLC benefits from still higher storage density and lower cost. In 2013, Samsung introduced V-NAND (Vertical NAND, also known as 3D NAND) with triple-level cells, which had a memory capacity of 128 Gbit. They expanded their TLC V-NAND technology to 256Gbit memory in 2015, and 512Gbit in 2017. Enterprise TLC (eTLC) is a more expensive variant of TLC that is optimized for commercial use.


Quad-level cell

Memory that stores 4 bits per cell is commonly referred to as quad-level cell (QLC), following the convention set by TLC. Prior to its invention, the term "QLC" was synonymous with MLC in referring to cells that can have 4 voltage states, i.e. ones that store 2 bits per cell – what is now unambiguously referred to as DLC. Due to the exponentially increasing number of required voltage stages for higher level flash the lifetime of QLC is further reduced to a maximum of 1,000 program/erase cycles. In 2009, Toshiba and SanDisk introduced
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 ...
memory chips with quad-level cells, storing 4 bits per cell and holding a capacity of 64Gbit. SanDisk X4 flash memory cards, introduced in 2009, was one of the first products based on NAND memory that stores 4 bits per cell, commonly referred to as quad-level-cell (QLC), using 16 discrete charge levels (states) in each individual transistor. The QLC chips used in these memory cards were manufactured by Toshiba, SanDisk and
SK Hynix SK Hynix Inc. () is a South Korean supplier of dynamic random-access memory (DRAM) chips and flash memory chips. SK Hynix is one of the world's largest semiconductor vendors. Founded as Hyundai Electronics in 1983, SK Hynix was integrated into ...
. In 2017, Toshiba introduced V-NAND memory chips with quad-level cells, which have a storage capacity of up to 768Gbit. In 2018, ADATA,
Intel Intel Corporation is an American multinational corporation and technology company headquartered in Santa Clara, California, and Delaware General Corporation Law, incorporated in Delaware. Intel designs, manufactures, and sells computer compo ...
,
Micron The micrometre (English in the Commonwealth of Nations, Commonwealth English as used by the International Bureau of Weights and Measures; SI symbol: μm) or micrometer (American English), also commonly known by the non-SI term micron, is a uni ...
and Samsung have launched some SSD products using QLC NAND memory. In 2020, Samsung released a QLC SSD with storage space up to 8 TB for customers. It is the SATA SSD with the largest storage capacity for consumers as of 2020. Enterprise QLC (eQLC) is a more expensive variant of QLC that is optimized for commercial use.


See also

* StrataFlash


References

{{Reflist


External links


Linux Memory Technology Devices - NAND

Open NAND Flash Interface
Computer memory Non-volatile memory Solid-state computer storage media