flash memory

Contributor(s): Carol Sliwa, Steve Collins and Julian de Silva

Flash memory is a type of nonvolatile memory that erases data in units called blocks. A block stored on a flash memory chip must be erased before data can be written, or programmed, to the microchip. Flash memory retains data for an extended period of time whether a flash-equipped device is powered on or off.

Dr. Fujio Masuoka is credited with the invention of flash memory when he worked for Toshiba in the 1980s. Masuoka’s colleague, Shoji Ariizumi, coined the term flash because the process of erasing all the data from a semiconductor chip reminded him of the flash of a camera.

Flash memory evolved from erasable programmable read-only memory (EPROM) and electrically erasable programmable read-only memory (EEPROM). Flash is technically a variant of EEPROM, but the industry reserves the term EEPROM for byte-level erasable memory and applies the term flash memory to larger block-level erasable memory. Devices using flash memory erase data at the block level and rewrite data at the byte level (NOR flash) or multiple-byte page level (NAND flash). Flash memory is widely used for storage and data transfer in consumer devices, enterprise systems and industrial applications.

How flash memory works

A basic flash memory cell consists of a storage transistor with a control gate and a floating gate, which is insulated from the rest of the transistor by a thin dielectric material or oxide layer. The floating gate stores the electrical charge and controls the flow of the electrical current.

Floating gate flash cell

Electrons are added to or removed from the floating gate to change the storage transistor’s threshold voltage to program the cell to be a zero or a one. A process called Fowler-Nordheim tunneling removes electrons from the floating gate. Either Fowler-Nordheim tunneling or a phenomenon known as channel hot-electron injection traps the electrons in the floating gate.

When erasing through Fowler-Nordheim tunneling, a strong negative charge on the control gate forces electrons off the floating gate and into the channel, where a strong positive charge exists. The reverse happens when using Fowler-Nordheim tunneling to trap electrons in the floating gate. Electrons are able to forge through the thin oxide layer to the floating gate in the presence of a high electric field, with a strong negative charge on the cell’s source and the drain and a strong positive charge on the control gate.

Fowler-Nordheim tunneling diagram

With channel hot-electron injection (or hot-carrier injection), electrons gain enough energy from the high current in the channel and attracting charge on the control gate to break through the gate oxide and change the threshold voltage of the floating gate.

Channel hot-electron injection diagram

Electrons are trapped in the floating gate, whether a device containing the flash memory cell is powered on or off, because of the electrical isolation created by the oxide layer.

EPROM and EEPROM cells operate similarly to flash memory in writing, or programming, data, but they differ from flash memory in the way they erase data. An EPROM is erased by removing the chip from the system and exposing the array to ultraviolet light to erase data. An EEPROM erases data electronically at the byte level, while flash memory erases data electronically at the block level.

NOR vs. NAND flash memory

There are two types of flash memory: NOR and NAND.

NOR and NAND flash memory differ in architecture and design characteristics. NOR flash uses no shared components and can connect individual memory cells in parallel, enabling random access to data. A NAND flash cell is more compact in size, with fewer bit lines, and strings together floating-gate transistors to achieve greater storage density. NAND is better suited to serial rather than random data access.

NOR flash is fast on data reads, but it is typically slower than NAND on erases and writes. NOR flash programs data at the byte level. NAND flash programs data in pages, which are larger than bytes but smaller than blocks. For instance, a page might be 4 kilobytes (KB), while a block might be 128 KB to 256 KB or megabytes in size. NAND flash uses less power than NOR flash for write-intensive applications.

NOR flash is more expensive to produce than NAND flash and tends to be used primarily in consumer and embedded devices for boot purposes and read-only code-storage applications. NAND flash is more suitable for data storage in consumer devices and enterprise server and storage systems due to its lower cost per bit to store data, greater density, and higher programming and erase speeds.

Devices such as a camera phone may use both NOR and NAND flash in addition to other memory technologies to facilitate code execution and data storage.

Pros and cons of flash memory

Flash is the least expensive form of semiconductor memory. Unlike dynamic random access memory (DRAM) and static RAM (SRAM), flash memory is nonvolatile, offers lower power consumption and can be erased in large blocks. Also on the plus side, NOR flash offers fast random reads, while NAND flash is fast with serial reads and writes

A solid-state drive (SSD) with NAND flash memory chips delivers significantly higher performance than traditional magnetic media such as hard disk drives (HDDs) and tape. Flash drives also consume less power and produce less heat than HDDs. Enterprise storage systems equipped with flash drives are capable of low latency, which is measured in microseconds or milliseconds.

The main disadvantages of flash memory are the wear-out mechanism and cell-to-cell interference as the dies get smaller. Bits can fail with excessively high numbers of program/erase cycles, which eventually break down the oxide layer that traps electrons. The deterioration can distort the manufacturer-set threshold value at which a charge is determined to be a zero or a one. Electrons may escape and get stuck in the oxide insulation layer leading to errors.

Anecdotal evidence suggests NAND flash drives are not wearing out to the degree once feared. Flash drive manufacturers have improved endurance and reliability through error correction code algorithms, wear leveling and other technologies. In addition, SSDs do not wear out without warning. They typically alert users in the same way a sensor might indicate an underinflated tire.

NAND flash memory storage types

NAND flash semiconductor manufacturers have developed different types of memory suitable for a wide range of data storage uses cases. The following chart explains the various NAND flash types.

Types of NAND flash memory storage

  Description Advantages Disadvantages Primary use
Single-level cell (SLC) Stores one bit per cell and two levels of charge Higher performance, endurance and reliability than other types of NAND flash Higher cost than other types of NAND flash Enterprise storage, mission-critical applications
Multilevel cell (MLC) Can store multiple bits per cell and multiple levels of charge. The term MLC equates to two bits per cell. Cheaper than SLC and enterprise MLC (eMLC), high density Lower endurance than SLC and eMLC, slower than SLC Consumer devices, enterprise storage
Enterprise MLC (eMLC) Typically stores two bits per cell and multiple levels of charge; uses special algorithms to extend write endurance. Less expensive than SLC flash, greater endurance than MLC flash More expensive than MLC, slower than SLC Enterprise applications with high write workloads
Triple-level cell (TLC) Stores three bits per cell and multiple levels of charge. Also referred to as MLC-3, X3 or 3-bit MLC. Lower cost and higher density than MLC and SLC Lower performance and endurance than MLC and SLC Mass storage consumer applications, such as USB drives, flash memory cards, smartphones, and client SSDs and datacenter SSDs for read-intensive workloads
Vertical/3D NAND Stacks memory cells on top of each other in three dimensions vs. traditional planar NAND technology Higher density, higher write performance and lower cost per bit vs. planar NAND Higher manufacturing cost than planar NAND, difficulty in manufacturing using production planar NAND processes, potentially lower data retention Consumer and enterprise storage
Note: NAND flash wear-out is less of a problem in SLC flash than it is in less expensive types of flash, such as MLC and TLC, for which the manufacturers may set multiple threshold values for a charge. The commonly cited industry wear-out figures are 100,000 program/erase (write/erase) cycles for SLC NAND flash, 30,000 for eMLC, 10,000 or fewer for MLC, and 3,000 or fewer for TLC. Actual endurance figures may be higher.

NOR flash memory types

The two main types of NOR flash memory are parallel and serial (also known as serial peripheral interface). NOR flash originally was available only with a parallel interface. Parallel NOR offers high performance, security and additional features; its primary uses include industrial, automotive, networking, and telecom systems and equipment. Serial NOR flash has lower pin counts and smaller packaging and is less expensive than parallel NOR. Use cases for serial NOR include personal and ultra-thin computers, servers, HDDs, printers, digital cameras, modems and routers.

Flash memory producers and products

Major manufacturers of NAND flash memory chips include Intel, Micron, Samsung, SanDisk, SK Hynix and Toshiba. Major manufacturers of NOR flash memory include Macronix, Microchip, Micron, Spansion and Winbond.

Flash memory is used in enterprise server, storage and networking technology as well as a wide range of consumer devices, including USB drives, mobile phones, digital cameras, tablet computers, PC cards in notebook computers and embedded controllers. For instance, NAND flash-based SSDs are often used to accelerate the performance of I/O-intensive applications. NOR flash memory is often used to hold control code such as the basic input/output system (BIOS) in a PC.

Flash memory is seeing growing use for in-memory computing to help speed performance and increase the scalability of systems that manage and analyze enormous amounts of data.

This was last updated in March 2015

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How does NAND flash memory differ from NOR flash memory?
NAND has more capacity over NOR but NOR has code execution while NAND does not. NOR performance is extremely slow with fast read and slow write while NAND has fast write and read capabilities. NOR reliability is standard that has bit-flipping issues whereas NAND has low performance and requires one bit for error management. NOR has full memory interface while NAND is I/O only. NOR’s hardware is easy to use while NAND is complicated.
Thanks a lot for the explanation.


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