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QuickStudy: Phase-Change Memory

May 7, 2007 10:00 AM ET

Computerworld - Phase-change random access memory (PRAM) is a new form of nonvolatile memory based on using electrical charges to change areas on a glassy material from crystalline to random. PRAM promises, in time, to be faster and cheaper, and consume less power, than other forms of memory.

There's a new contender coming to the realm of nonvolatile memory and storage, which enables data to remain intact when the power has been shut off.

For decades, the chief medium here has been magnetic disk. But as computers get smaller and require more and quicker storage, disk drives are lagging behind in satisfying many users??? needs.

The most recent technology to gain widespread acceptance is flash memory. USB flash drives and memory cards the size of a thumbnail that can hold several gigabytes have become important, especially for newer multimegapixel digital cameras. In 2005, consumers worldwide bought nearly $12billion worth of flash products, and the market should top $20 billion this year.

But as storage and speed requirements increase, seemingly with each new product generation, flash memory is reaching the end of its ability to keep pace. The technology can scale up only so far as the processes used to make these chips reach both practical and theoretical limits.


The new kid on the block is another solid-state technology, phase-change random access memory. Known as PRAM or PCM, it uses a medium called chalcogenide, a glassy substance containing sulphur, selenium or tellurium. These silvery semiconductors, as soft as lead, have the unique property that their physical state (i.e., the arrangement of their atoms) can be changed from crystalline to amorphous through the application of heat. The two states have very different electrical resistance properties that can be easily measured, making chalcogenide ideal for data storage.

PRAM is not the first use of chalcogenide for storage. The same material is used in rewritable optical media (CD-RW and DVD-RW), in which a laser heats up a small spot on the disk's inner layer to between 300 and 600 degrees Celsius for an instant. That alters the arrangement of atoms in that spot and changes the material's refractive index in a way that can be optically measured.

PRAM uses electrical current instead of laser light to trigger the structural change. An electrical charge just a few nanoseconds in duration melts the chalcogenide in a given spot; when the charge ends, the spot's temperature drops so quickly that the disorganized atoms freeze in place before they can rearrange themselves back into their regular, crystalline order.

Going in the other direction, the process applies a longer, less-intense current that warms the amorphous patch without melting it. This energizes the atoms just enough that they rearrange themselves into a crystalline lattice, which is characterized by lower energy or electrical resistance.

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