phase change memory

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phase change memory

A non-volatile, random access memory (RAM) technology that is designed to initially replace flash memory and, eventually, DRAM memory. Invented by Stanford Ovshinsky, who founded Energy Conversion Devices (ECD) in 1960, the vendors of phase change memory products are licensees of Ovonyx Unified Memory (OUM) from Ovonyx, Inc. (, an ECD spin-off. "Phase change RAM" (PRAM and PCRAM) and "chalcogenide RAM" (C-RAM) are other names for phase change memory (PCM).

Electrical Vs. Optical Phase Change
Phase change memory employs the same principle as rewritable optical discs (CD-RWs, DVD-RWs, etc.), in which the bit cell is either in an unstructured "amorphous" state or highly structured "crystalline" state, both of which are extremely stable. However, phase change memory uses electrical pulses to change the bit rather than the heat from a laser as with optical discs. In addition, the bit in phase change memory is read by measuring the electrical resistance through the cell, not the reflection of the laser light (see phase change disc).

In addition, phase change memory cells are considerably denser than optical disc cells, and they can be made to hold more than one bit. In fact, prototypes with several dozen bits per cell have been demonstrated.

Phase Change Vs. Flash
Phase change memory eliminates many of the disadvantages of flash memory. Like DRAM and SRAM memory, any byte in phase change memory can be written; whereas, flash requires an entire block to be written. As the flash cell's elements (feature sizes) become smaller, its floating gate architecture becomes more problematic. However, the smaller the phase change memory cell, the denser and faster the phase change chip becomes. In addition, phase change memory handles millions of rewrites compared to hundreds of thousands for flash. See PCMS, phase change disc, chalcogenide glass and future memory chips.

Change the Phase of the Bit
A long, medium-amplitude pulse creates a highly conductive crystalline bit in the memory cell. A short, high-amplitude pulse resets the bit back to an amorphous state, which is a poor conductor.

Automatically Radiation Hardened
Based on the Ovonyx phase change memory cell, this 4 megabit C-RAM (Chalcogenide RAM) memory chip from BAE Systems is shown in its package before it is covered and the leads are cut. Due to the huge resistance difference between a 1 and 0 in the memory cell (5K and 100K ohms), the chip is automatically "rad hard." External radiation cannot change the phase sufficiently enough to alter the value of the cell. (Image courtesy of BAE Systems,
References in periodicals archive ?
In theory, phase change memory could eventually present a solution to the so-called memory wall, or memory gap.
Surprisingly, the technology behind phase change memory is not expensive, and can be commercially used for a variety of devices requiring storage and memory, from servers to tablets.
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introduced a set of innovative products today based on the new class of memory technology called phase change memory (PCM).
Common applications include non-volatile memory devices (NVM) such as Flash and phase change memory, isothermal characterization on silicon-on-insulator (SOI) devices and compound semiconductors, and transient characteristics of new materials such as high-K dielectrics.
Phase Change Memory, a type of non-volatile computer memory also known as PRAM, PCM, and PCRAM, is a new and promising technology.
Various technologies including phase change memory, charge trap memory, nanocrystal memory, PMC, RRAM and 3D memory are potential candidates to replace flash memory.
researchers will present their latest findings on phase change memory (PCM) next week at the 2009 International Electron Devices Meeting (IEDM), the world's main forum for reporting breakthroughs in technology, physics and the modeling of semiconductors and other electronic devices.