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Flash memory stores information on a silicon chip in a way that does not need power to maintain the information in the chip. This means that if you turn off the power to the chip, the information is retained without consuming any power. In addition, flash offers fast read access times and solid-state shock resistance. These characteristics are why flash is popular for applications such as storage on battery-powered devices like cellular phones and PDAs.
Flash memory is made in two forms: NOR flash and NAND flash. The names refer to the type of logic gate used in each storage cell.
Flash memory is often used in MP3 players and digital cameras.
Flash memory stores information in an array of transistors, called "cells," each of which traditionally stores one bit of information. Newer flash memory devices, sometimes referred to as multi-level cell devices, can store more than 1 bit per cell, by varying the number of electrons placed on the FG of a cell.
In NOR flash, each cell looks similar to a standard MOSFET transistor, except that it has two gates instead of just one. One gate is the control gate (CG) like in other MOS transistors, but the second is a floating gate (FG) that is insulated all around by an oxide layer. The FG is between the CG and the substrate. Because the FG is isolated by its insulating oxide layer, any electrons placed on it get trapped there and thus store the information. When electrons are on the FG, they modify (partially cancel out) the electric field coming from the CG, which modifies the threshold voltage (Vt) of the cell. Thus, when the cell is "read" by placing a specific voltage on the CG, electrical current will either flow or not flow, depending on the Vt of the cell, which is controlled by the number of electrons on the FG. This presence or absence of current is sensed and translated into 1's and 0's, reproducing the stored data. In a multi-level cell device, which stores more than 1 bit of information per cell, the amount of current flow will be sensed, rather than simply the presence or absence of current, in order to determine the number of electrons stored on the FG.
A NOR flash cell is programmed (set to a specified data value) by starting up electrons flowing from the source to the drain, then a large voltage placed on the CG provides a strong enough electric field to suck them up onto the FG, a process called hot-electron injection. To erase (reset to all 1's, in preparation for reprogramming) a NOR flash cell, a large voltage differential is placed between the CG and source, which pulls the electrons off through Fowler-Nordheim tunneling, a quantum mechanical tunneling process. Most modern NOR flash memory components are divided into erase segments, usually called either blocks or sectors. All of the memory cells in a block must be erased at the same time. NOR programming, however, can generally be performed one byte or word at a time.
NAND Flash uses tunnel injection for writing and tunnel release for erasing.
NOR flash was the first type to be developed, invented by Intel in 1988. It has long erase and write times, but has a full address/data (memory) interface that allows random access to any location. This makes it suitable for storage of program code that needs to be infrequently updated, such as a computer's BIOSIn computing, the Basic Input-Output System or BIOS is computer interface code that locates and loads the operating system into RAM. It provides low-level communication, operation and configuration to the hardware of a system, which at a minimum drives th or the firmwareIn computing, firmware is software that is embedded in a hardware device, that allows reading and executing the software, but does not allow modification, e. writing or deleting data by an end user. Examples of firmware include: the BIOS found in personal of set-top boxes. Its endurance is 10,000 to 100,000 erase cycles. NOR-based flash is the basis of early flash-based removable media; Compact Flash was originally based on it, though later cards moved to the cheaper NAND flash.
NAND flash from SamsungSamsung Korean Name Revised RomanizationSamseong McCune-ReischauerSamsong Hangul Hanja Samsung is one of the largest electronics companies in the world. Founded in March 1938 in Daegu, Korea, it operates in around 58 countries and it has over 201,000 work and ToshibaToshiba ( Japanese language) is a high technology electrical and electronics manufacturing firm, headquartered in Tokyo, Japan. It is the 7th largest integrated manufacturer of electric and electronic equipment in the world. The first kanji, to means "eas followed in 19891989 is a common year starting on Sunday (click on link for calendar). Events January January 7 Akihito becomes Emperor of Japan following the death of Hirohito. The Heisei period begins January 8 the Kegworth Air Disaster A British Midland Boeing 737 cra. It has faster erase and write times, higher density, and lower cost per bit than NOR flash, and ten times the endurance. However its I/O interface allows only sequential accessIn computer science sequential access means that a group of elements (e. data in a memory array or a disk file or on a tape) is accessed in a predetermined, ordered sequence. Sequential access is sometimes the only way of accessing the data, for example i to data. This makes it suitable for mass-storage devices such as PC cardThe PCMCIA is the Personal Computer Memory Card International Association an industry trade association that creates standards for notebook computer peripheral devices. The best known such devices are known as PC Cards (formerly PCMCIA cards . A later revs and various memory cards, and somewhat less useful for computer memory. The first NAND-based removable media format was SmartMedia, and numerous others have followed: MMC, Secure Digital, Memory Stick and xD-Picture Cards.
Flash memory forms the core of the removable USB interface storage devices known as keydrives.