Phase-change memory (PCM) is a form of computer RAM (random-access memory) that stores data by altering the state of the matter from which the device is fabricated. The structure of the material can change rapidly back and forth between amorphous and crystalline on a microscopic scale. In the amorphous or disordered phase, the material has high electrical resistance ; in the crystalline or ordered phase, its resistance is reduced. This allows electrical current s to be switched on and off, representing digital high and low states. Working prototype s of PCM chips have been tested by IBM, Infineon, Samsung, Macronix and others.
According to its proponents, PCM technology has the potential to provide inexpensive, high-speed, high-density, high-volume nonvolatile storage on an unprecedented scale. The physical structure is three-dimensional, maximizing the number of transistor s that can exist in a chip of fixed size. PCM is sometimes called "perfect RAM" (PRAM) because data can be overwritten without having to erase it first. This makes it possible for PCM to function many times faster than conventional flash memory while using less power. In addition, PCM chips are expected to last several times as long as currently available flash memory chips and may prove cheaper to mass-produce.