Editing Non-volatile random-access memory (section)

==Floating-gate MOSFET==<!-- linked here by redirect [[NOVRAM]] -->

A huge advance in NVRAM technology was the introduction of the [[floating-gate MOSFET]] transistor, which led to the introduction of ''erasable programmable read-only memory'', or [[EPROM]]. EPROM consists of a grid of transistors whose ''gate'' terminal (the switch) is protected by a high-quality insulator. By pushing electrons onto the base with the application of higher-than-normal voltage, the electrons become trapped on the far side of the insulator, thereby permanently switching the transistor on (1). EPROM can be reset to the ''base state'' (all 1s or 0s, depending on the design) by applying [[ultraviolet]] light (UV). The UV [[photon]]s have enough energy to push the electrons through the insulator and return the base to a ground state. At that point the EPROM can be re-written from scratch.

An improvement on EPROM, [[EEPROM]], soon followed. The extra E stands for ''electrically'', referring to the ability to reset EEPROM using electricity instead of UV, making the devices much easier to use in practice. The bits are re-set with the application of even higher power through the other terminals of the transistor (''source'' and ''drain''). This high-power pulse, in effect, sucks the electrons through the insulator, returning it to the ground state. This process has the disadvantage of mechanically degrading the chip, however, so memory systems based on floating-gate transistors in general have short write-lifetimes, on the order of 10<sup>5</sup> writes to any particular bit.

One approach to overcoming the rewrite count limitation is to have a standard [[Static random-access memory|SRAM]] where each bit is backed up by an EEPROM bit. In normal operation the chip functions as a fast SRAM and in case of power failure the content is quickly transferred to the EEPROM part, from where it gets loaded back at the next power up. Such chips were called '''NOVRAM'''s<!-- linked here by redirect [[NOVRAM]] --><ref>{{cite web|url=http://www.intersil.com/data/an/AN1146.pdf|title=X4C105 NOVRAM Features and Applications|first=Peter|last=Chan|date=2005-04-21|website=Intersil|archive-url=https://web.archive.org/web/20070614111904/http://www.intersil.com/data/an/AN1146.pdf|archive-date=2007-06-14}}</ref> by their manufacturers.

The basis of [[flash memory]] is identical to EEPROM and differs largely in internal layout. Flash allows its memory to be written only in blocks, which greatly simplifies the internal wiring and allows for higher densities. [[Memory storage density]] is the main determinant of cost in most computer memory systems, and due to this flash has evolved into one of the lowest-cost solid-state memory devices available. Starting around 2000, demand for ever-greater quantities of flash have driven manufacturers to use only the latest fabrication systems in order to increase density as much as possible. Although fabrication limits are starting to come into play, new [[Multi-level cell|"multi-bit" techniques]] appear to be able to double or quadruple the density even at existing line widths.