Phase Change Memory, P-RAM

Phase change memory of phase change RAM is a form of non-volatile memory that is faster than Flash.

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Phase-change random access memory, P-RAM, is a form of non-volatile memory or computer storage that is faster than the much more commonly used Flash memory technology.

Phase change memory can be referred to by a number of names including P-RAM or PRAM, PC-RAM, phase change RAM, and possibly more.

Phase change memory is based on a technique known as the memresitor that was initially developed by Hewlett Packard.

Now phase change memory has been taken up by a number of other manufacturers and is likely to see increasing use. Phase change memory is seen as a significant advance and one that is likely to become one of the mainstream formats for semiconductor memory for the future.

Phase change memory basics

The phase change memory, PCM or phase change random access memory, P-RAM, exploits the a unique property of a substance called chalcogenide glass.

The P-RAM uses the fact that the chalcogenide glass changes between two states, polycrystalline and amorphous by the passage of current which produces heat as it passes through a cell. This gives rise to the name phase change, as the substance changes between the two states or phases.

In the amorphous state the material demonstrates a high level of resistance and also a low reflectivity.

In the polycrystalline state the material has a regular crystalline structure, and this manifests itself in a change of properties. In this state it has a low resistances as electrons are easily able to move through the crystalline structure, and it also exhibits a high reflectivity.

For phase change storage / phase change RAM, it is the resistance level that is of interest. Circuitry around the cell then detects the change in resistance as the two states have a different resistance and as a result it detects whether a "1" or "0" is stored in that location.

The phase change between the two states of the chalcogenide is brought about through localised heating brought about as a result of injected current for a timed period. The final phase of the material is modulated by the magnitude of the injected current and the time of the operation.

A resistive element provides the heating - it extends from a bottom electrode to the chalcogenide layer. Current passing though the resistive heater element provides heat which is then transferred to the chalcogenide layer.

State Properties
Amorphous     • Short range atomic order
    • High reflectivity
    • High resistance
Polycrystalline     • Long range atomic order
    • Low reflectivity
    • Low resistance

Additionally recent developments of the technology have achieved two additional states, effectively doubling the storage of a given sized device.

The advantage of the phase change technology is that the state remains intact when power is removed from the device, thereby making it a non-volatile form of storage.

Phase change memory advantages & disadvantages

Phase change random access memory, P-RAM offers a number of significant advantages for data storage over its main competitor which is Flash memory:

Advantages of phase change memory:

  • Non-volatile:   Phase change RAM is a non-volatile form of memory, i.e. it does not require power to retain its information. This enables it to compete directly with flash memory.
  • Bit alterable:   Similar to RAM or EEPROM, P-RAM / PCM is what is termed bit-alterable. This means that information can be written directly to it without the need for an erase process. This gives it a significant advantage over flash which requires an erase cycle before new data can be written to it.
  • Fast read performance:   Phase change RAM, P-RAM / PCM features fast random access times. This has the advantage that it enables the execution of code directly from the memory, without the need to copy the data to RAM. The read latency of P-RAM is comparable to single bit per cell NOR flash, while the read bandwidth is similar to that of DRAM
  • Scalability:   For the future, the scalability of P-RAM is another area where it could provide advantages, although this is yet to be realised. The reasoning is that both NOR and NAND flash variants rely on floating gate memory structures, which are difficult to shrink. It is found that as the memory cell size is reduced, the number of electrons stored on the floating gate is reduced and this makes the detection of these smaller charges more difficult to reliably detect. P-RAM does not store charge, but instead it relies on a resistance change. As a result is not susceptible to the same scaling difficulties.
  • Write/erase performance:   The write erase performance of P-Ram is very good having faster speeds and lower latency than NAND flash. As no erase cycle is required this delivers an overall significant improvement over flash.

Disadvantages of phase change memory:

  • Commercial viability:   Despite the many claims about the advantages of P-RAM, few companies have been able to develop chips that have been successfully commercialised.
  • Multiple bit storage per cell of Flash:   The ability of Flash to store and detect multiple bits per cell still gives flash a memory capacity advantage over P-RAM. Although P-RAM / PCM has advantages in possible scalability for the future.

When looking at using phase change memory, both the advantages and disadvantages need to be considered.

Phase change memory has been introduced by a number of manufacturers, yet it is still not widely used as many developers may be cautious of a new technology like this. Nevertheless, phase change memory, PCM has some distinct advantages to offer for a number of occasions.

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