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Concept
Phase-change memory
Summary
Phase-change memory (also known as PCM, PCME, PRAM, PCRAM, OUM (ovonic unified memory) and C-RAM or CRAM (chalcogenide RAM)) is a type of non-volatile random-access memory. PRAMs exploit the unique behaviour of chalcogenide glass. In PCM, heat produced by the passage of an electric current through a heating element generally made of titanium nitride is used to either quickly heat and quench the glass, making it amorphous, or to hold it in its crystallization temperature range for some time, thereby switching it to a crystalline state. PCM also has the ability to achieve a number of distinct intermediary states, thereby having the ability to hold multiple bits in a single cell, but the difficulties in programming cells in this way has prevented these capabilities from being implemented in other technologies (most notably flash memory) with the same capability.
Recent research on PCM has been directed towards attempting to find viable material alternatives to the phase-change material Ge2Sb2Te5 (GST), with mixed success. Other research has focused on the development of a GeTe–Sb2Te3 superlattice to achieve non-thermal phase changes by changing the co-ordination state of the germanium atoms with a laser pulse. This new Interfacial Phase-Change Memory (IPCM) has had many successes and continues to be the site of much active research.
Leon Chua has argued that all two-terminal non-volatile-memory devices, including PCM, should be considered memristors. Stan Williams of HP Labs has also argued that PCM should be considered a memristor. However, this terminology has been challenged, and the potential applicability of memristor theory to any physically realizable device is open to question.
In the 1960s, Stanford R. Ovshinsky of Energy Conversion Devices first explored the properties of chalcogenide glasses as a potential memory technology. In 1969, Charles Sie published a dissertation at Iowa State University that both described and demonstrated the feasibility of a phase-change-memory device by integrating chalcogenide film with a diode array.
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