Phase-change memory (PCM) is a term used to describe a class of nonvolatile memory devices that exploit the ability of certain materials to rapidly change phase between two stable physical states. Phase-change memory blends the attributes commonly associated with NOR-type flash memory, NAND-type flash memory, EEPROM memory, and DRAM (Figure 1) and is a leading candidate for the next generation of nonvolatile memory (NVM), expanding the use of NVM in computing and storage systems. As traditional electron storage-based memories such as NOR and NAND flash begin to encounter scaling difficulties, PCM is considered to be the best candidate to continue the scaling of NVM.
Click on image to enlarge.
Figure 1: PCM attributes: This new class of nonvolatile memory brings together the best attributes of NOR, NAND, and RAM.
Here are some of the unique capabilities of phase-change memory:
Nonvolatile: Like NOR flash and NAND flash, PCM is nonvolatile. DRAM, of course, requires a constant power supply, such as a battery backup system, to retain information, resulting in higher power consumption. DRAM technologies also suffer from susceptibility to so-called "soft errors" or random bit corruption caused by alpha particles or cosmic radiation, an effect not observed in PCM.
Scalability: Existing memories, NOR, NAND, EEPROM, and RAM all rely on charge (electron) storage as their memory mechanism. As scaling progresses to ever smaller dimensions, the number of stored electrons reduces to the point where reliability is degraded and continued scaling becomes very difficult. This, then, requires the introduction of increasingly complex structures with diminishing returns. PCM does not use electrons, but instead a physical phase change as a storage mechanism. Stability of the stored phase has been demonstrated to be at least as small as 5 nm. Scalability is one of the major motivations for the development of PCM.
Bit-alterable/erase/software: Like RAM, PCM is bit alterable. Unlike RAM and PCM, flash technology requires a separate erase step involving the manipulation of a large block of data in order to change a small amount of information. Bit alterability can greatly simplify the use of the memory and, combined with the lack of a need to erase, makes software management of the memory much easier. In some usage environments, PCM can be as easy to use as RAM.
Write speed: PCM is capable of achieving write bandwidth comparable to NAND, but with 100x lower initial latency (time to write the first byte) and with no separate (and slow) erase step required. The lack of a slow erase step before write can greatly increase the overall effective write bandwidth, and the low latency can allow PCM to be used, in many cases, like a direct write memory as opposed to the mass storage "disk" usage model for NAND. The write speed (bandwidth and latency) of PCM does not match the capability of DRAM, although with proper management some DRAM usage can be displaced for infrequent or managed write applications.
Read speed: Like RAM and NOR-type flash, PCM technology features fast random read access times. This enables the execution of code directly from the memory without an intermediate copy to RAM. In contrast, NAND flash suffers from long random access times on the order of 10s of microseconds that prevents direct code execution.
The combination of these attributes positions PCM uniquely with an opportunity to provide the next generation of nonvolatile memory with an expanded set of performance capabilities, sitting solidly between today's DRAM and NAND.