Physically, Flash memory stores information by using a comparatively high voltage (sometimes 10 volts or more) to deposit a charge on the insulated gate of a tiny field effect transistor. Unlike DRAM, the charge in a Flash cell leaks off very slowly unless the cell is deliberately reverse biased in order to erase it. This gives the memory a nominal lifetime of about 10 years. Flash memory has some unusual characteristics -- the most important being that writing one bits is a much different operation than writing zeros. Setting zero bits is called writing and can be done on a bit by bit basis. Writing ones is called erasure and generally is applied to large blocks of memory called Sectors or Blocks. Thus, Flash memory starts off programmed to ones and is rewritten with patterns. It is not convenient to change a one bit of an existing pattern from zero to one (erase then rewrite). So Flash is not an especially convenient replacement for static RAM or DRAM in applications where persistent storage without power is not important. There may be other peculiar restrictions with any specific device or family of devices. e.g. some flash must be set to all zeros before it is erased to all ones. Other units may be damaged if an attempt is made to set a zeroed bit to zero.
Because Flash memory is more easily reprogrammed than other PROMs with comparable cost and holds its content without power, it is very widely used in appliances that require small amounts of user programmable storage -- e.g. phone numbers, what TV channels are available locally, etc. The worldwide market for flash memory is extensive. Moreover modern flash memory can be rewritten hundreds of thousands of times without degradation. However, it is unclear that flash memory will scale to smaller sizes as effectively as alternative persistent RAM technologies such as Magnetic RAM or FerroElectric Memory that share many of Flash's advantages.
One design issue with flash memory is the difficulty in reducing the thickness of the insulating layer much below the current 90 angstroms without encountering increased charge leakage. Another is the difficulty in preventing write pulses from inadvertently toggling bits adjacent to the bits being programmed. Most flash memory currently shipping uses an older technology known as NOR flash. Newer NAND gate flash memory that appears to offer the potential for smaller storage cells is gaining market share. NAND gate technology seems especially well suited to applications perfoming block transfers such as disk emulation.
Some technologies store multiple data bits in a single flash cell.
Last updated 040925
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