Phase Change Memory, An IBM-Signed Optical Storage Device

Scientists at the IBM Research Group in Zurich, Switzerland, introduced an experimental optical storage device that is faster than Solid State Drive (SSD) and more efficient than Dynamic Random Access Memory (DRAM). This new technology, called phase-change memory, is an optical memory that takes advantage of the optoelectronic properties of semiconductors composed of germanium, tellurium, and antimony (GST glass).

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In summary, the GST modifies its crystalline structure when it is subjected to a high-intensity current. It passes from the amorphous state (a glass without periodic ordered structure) to the crystalline state (highly ordered structure) and vice versa. This process is called a phase change and hence the phase change memory name. The alternation amorphous and crystalline phases correspond to two states: low and high electrical conductivity when applying an electrical voltage. If the conductivity is high, the bit is 1 and if it is low, the bit is 0. These states remain, even after interruption of the external power supply, which makes this memory PCM non-volatile.

The ease of transformation of the crystal structure of the GST had already been exploited with the aid of a laser source to create memory modules. In fact, this is the technology used in rewritable CDs and DVDs. However, storage density, write speed, and price per GB has always put a brake on this technology compared to DRAMs and, more recently, SSDs.

How does flash and DRAM phase change compare?

Regarding the storage density, flash memories are the best choice. They are relatively slow but inexpensive to produce. The basic memory cells, for the most part, store only one information bit (1 or 0); only the most recent generations (see multilevel cells) give the possibility to store several bits per cell. On the other hand, a PCM cell on GST can store 3 bits of data per cell, while being faster than the best flash memory offered today.

As for the speed of storage, the DRAM and SRAM memories, very expensive, are the most popular. However, although they are very fast, they are volatile and need to be updated frequently to avoid the loss of information. They are found in popular memories RAM and VRAM computers and in the cache memory of processors. The PCM cell launched by IBM is slower than DRAM, of course, but it is not volatile and its cost of production is lower.

In terms of lifetime, the flash memory can support approximately 3000 write cycles. Phase-shift memory, meanwhile, can withstand at least 10 million cycles while maintaining excellent temperature stability. With volatile memories like SRAM, the concept of the lifetime is quite different and difficult to compare.

In conclusion, phase change memory over GST is faster than flash memory, more economical to produce than DRAM and non-volatile. Thus this memory could be used in applications other than rewritable CDs and DVDs. The PCM, used in conjunction with flash memory, would significantly reduce the loading time of mobile devices. In industrial applications, PCM-based database storage would mean more speed in queries and be processing huge amounts of transactions.

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In addition to these applications, IBM is investigating the storage possibilities of this technology for artificial intelligence machines. Supercomputers, like Watson, perform millions of iterations to give a single answer; so it is certainly most desirable to have access to a larger and faster storage possibility.

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