# Flash memory-Principle of working



## Digit_Dragon (Mar 13, 2007)

I always wanted to know how things work.....hope everyone would be interested too. 
Here is how these flash memory used in digicams, USB works...

Flash memory stores information in an array of floating gate 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 using more than two levels of electrical charge, placed on the floating gate of a cell.

In NOR gate flash, each cell looks similar to a standard MOSFET, 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 1s and 0s, 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 detecting 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 1s, 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 quantum tunneling. In single-voltage devices (virtually all chips available today), this high voltage is generated by an on-chip charge pump. 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 gate flash uses tunnel injection for writing and tunnel release for erasing. NAND flash memory forms the core of the removable USB interface storage devices known as USB flash drives.

As manufacturers increase the density of flash devices, individual cells shrink and the number of electrons in any cell becomes very small. Coupling between adjacent floating gates can change the cell write characteristics. New designs, such as charge trap flash, attempt to provide better isolation between adjacent cells.


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## Kiran.dks (Mar 13, 2007)

Good info . But source should be mentioned unless untill it is not your own.


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## Digit_Dragon (Mar 13, 2007)

Sources:

*www.intel.com/design/flash/articles/what.htm

*en.wikipedia.org/wiki/Flash_memory

*www.answers.com/topic/flash-memory


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