We store and transfer all kinds o­f files on our computers — digital photographs, music files, wor­d processing documents, PDFs and countless other forms of media. But sometimes your computer’s hard drive isn’t exactly wher­e you want your information. If you want to make backup copies of files that live off of your systems, then portable storage devices that use a type of electronic memory called FLASH MEMORY is the right solution.

Electronic memory comes in a variety of forms to serve a variety of purposes. Flash memory is used for easy and fast information storage in computers, digital cameras and many more electronic data equipments. It is used more like a hard drive than as RAM. In fact, flash memory is known as a SOLID STATE storage device, meaning there are no moving parts — everything is electronic instead of mechanical.

                    32-Gigabyte NAND memory card and chip

WHAT IS THIS??

Flash memory is a type of EEPROM chip, which stands for Electronically Erasable Programmable Read Only Memory. It has a grid of columns and rows with a cell that has two transistors at each intersection.

The two transistors are separated from each other by a thin oxide layer. One of the transistors is known as a FLOATING GATE, and the other one is the CONTROL GATE.

This kind of structure is used for the storage of any data/memory temporarily for a particular period of time. Implies the data is stored in the form of electrical pulse signals i.e., in low and high voltage levels. This storage is done through a kind of technique called Tunneling and it can be deleted by using another technique called Erasing. It’s more like an electronic device than like an electric circuit.

TUNNELING AND ERASING:

Tunneling is used to alter the placement of electrons in the floating gate. An electrical charge, usually 10 to 13 volts, is applied to the floating gate. The charge comes from the column, enters the floating gate and drains to a ground.

This charge causes the floating-gate transistor to act like an electron gun. The excited electrons are pushed through and trapped on other side of the thin oxide layer, giving it a negative charge. These negatively charged electrons act as a barrier between the control gate and the floating gate. If the flow through the gate is above the 50 percent threshold, it has a value of 1. When the charge passing through drops below the 50-percent threshold, then the value changes to 0. A blank EEPROM has all of the gates fully open, giving each cell a value of 1.

The electrons in the cells of a flash-memory chip can be returned to normal (“1”) by the application of an electric field, a higher-voltage charge. Flash memory uses in-circuit wiring to apply the electric field either to the entire chip or to predetermined sections known as blocks. This erases the targeted area of the chip, which can then be rewritten. Flash memory works much faster than traditional EEPROMs because instead of erasing one byte at a time, it erases a block or the entire chip, and then rewrites it.

FLASH MEMORY TYPES:

In traditional single-level cell flash devices, each cell stores only one bit of information. Later, many developers have developed a new form of flash memory known as multi-level cell flash that can store/hold more than one bits rather than a single bit in each memory cell, thus doubling the capacity of memory.

The two main types of flash memory are the NOR Flash & NAND Flash.

NOR-flash is slower in erase-operation and write-operation compared to NAND-flash. That means the NAND-flash has faster erase and write times. More over NAND has smaller erase units. So fewer erases are only needed. NOR-flash can read data slightly faster than NAND.

NAND-flash occupies smaller chip area per cell. This maker NAND available in greater storage chip densities and at lower costs per bit than NOR-flash. It also has up to ten times the endurance of NOR-flash. NAND is more fit as storage media for large files including video and audio. The USB thumb drives, SD cards and MMC cards are of NAND type.

Posted by

Gopi chand ( Mgit ECE 4th year)

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