A permanent electronic memory (keeping memorised data without a power supply) using the phase change of a particular material that exists in two forms: vitreous and crystalline. The phase change occurs with a rise in temperature, representing a 0 or 1. The material used is a chalcogenide glass.
This process is also used for rewritable disks, CD-RW or DVD-RW, where information is coded by phase changes of the material. Here the heat is provided by a laser. The disk is read optically: the crystalline form reflects the laser beam better than the vitreous form.
In a PRAM, writing is carried out by the heat from electric pulses. Reading uses a weak electric current. The resistivity (the specific resistance of the material) is much higher for the vitreous form than for the crystalline form, and the two states are differentiated by the output current.
Intel, Samsung and IBM have presented PRAM models. Its field of application is flash memory, with the advantage of a writing speed thirty times higher and a service life ten times longer (figures announced by Samsung). In February 2008, Intel and STMicroelectronics presented a four-state PRAM.
The possibility of using this phenomenon as an electronic memory was proposed in 1970 by R.G. Neale, D.L Nelson and G.E. Moore (Gordon Edward Moore, the co-founder of Intel) in the review Electronics (Electronics 43 (20): 56-&; Amorphous semiconductors. 1. Non-volatile and reprogrammable, read-mostly memory is here).)