Definition :
Flash Memory is a non-volatile memory (it doesn't lose data when power is turned off) with a fast read time but relatively long write time. It is the optimum solution for many devices because of its low cost, low power, high speed and fairly high density (currently 32-64MB). Wireless devices such as cell phones (the primary flash memory customer) usually require a set of protocols to be written to the device once, and then repeatedly used to make calls, which is perfect for flash. I believe the word "flash" came from the fact that the read cycle is very fast. Flash memory isn't studied much in an academic setting, much of Flash technology is owned by various companies.

Applications :
Flash memory is primarily made by Intel and AMD. Its fast read time but slow write time make it useful in many wireless devices. Flash memory is currently used in cell phones, MP3 players, digital cameras, handheld computing devices, and sometimes in desktop computers.

Functionality :
The flash cell consists of a "floating gate" that can store charge and keep it there when power is turned off. These are arranged in large arrays which make reading or writing large blocks easy but writing individual cells time consuming.

  The Flash Memory Cell
           
           |
      _____|_____
     |___________| <- Gate
      ___________
     |___________| <- Floating 
  ___________________     Gate
 |     /       \     |
 |____/_________\____|
 Source          Drain

The flash memory cell can store charge on the floating gate. Putting electrons on the floating gate is called programming and removing electrons from the floating gate is erasing, (aka. writing and erasing).

Writing Data :
Data is programmed into a flash cell from Hot Electron Injection. This is when 0V is put on the Source, 10V on the Gate, and 5V on the Drain. The direct path from source to Drain with a 5V difference will cause electrons to flow from the source to the gate extremely fast. The actual voltages may vary depending on the memory, but the principle is the same. Once in a while one of the electrons will gain enough energy to jump from the source/substrate into the floating gate, thereby causing the floating gate to build up charge. The actual voltages are pulsed on and off and a read cycle is used to test the cell and make sure it was programmed. This causes programming to take a relatively long time.

Erasing Data :
Erasing data off of the flash cell is done through the quantum effect known as Fowler Nordheim Tunneling. The Drain is left floating while the gate is put at -10V and the source is put at 5V. This causes the electrons to be strongly attracted to the source. The electrons will then tunnel through the oxide barrier and effectively de-program the cell. Occasionally too many electrons will leave causing an over-erase state, the cell then needs to be programmed back up to counteract it. This also causes the erase stage to take a fairly long time, which is why flash memory is meant to be read a lot and written infrequently. (NAND technology uses Tunneling for program and erase).

New Advances :
Recently, the floating cell in the flash device could be programmed and erased with such accuracy that they could have four levels of charge stored on the floating gate instead of just 2. This causes each cell to hold two bits (00, 01, 10, or 11) instead of just one bit (0 or 1). It's called multi-level cell (MLC) devices which currently have 2 bits per cell. Additionally a feature called read while write is in some devices which allows the user to read part of the array while programming a different portion.

Variations :
There are two main flavors of flash memory, NOR and NAND. Both behave essentially the same, but basic architectural differences give them the following qualities.
NOR
   Pros : Fast random access, easier to make, multi-levels per cell capable, & simple user interface.
   Cons : Slow program/erase, & large blocks

NAND
   Pros :Cheap, fast program/erase, & smaller size
   Cons :Slow random access, poor reliability, & complex user interface

Because of these characteristics, the different technologies are used in different applications. Small flash cards used in many digital cameras and other devices that need fast write are NAND, while NOR flash are chips with spider-like pins used in cell phones and other places that require fast read time.


Resources :
http://www.howstuffworks.com/flash-memory.htm

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