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Perfect RAM is a type of non-volatile random access memory. It stores data by changing the state of the material used; chalcogenide glass switches between states when it is subjected to the heat produced by the passage of an electric current. On a microscopic level, the data changes back and forth between two states: amorphous and crystalline.
Perfect RAM is one of several memory technologies competing to replace flash memory, which has a number of problems.
This term is also known as PRAM, PCRAM, Ovonic Unified Memory, Chalcogenide RAM, C-RAM, and Phase-Change Memory (PCM).
In the amorphous state (or disordered phase), the material has high electrical resistance. In the crystalline state (or ordered phase), it has less resistance. Thus, the electric current is allowed to be turned on and off, which represent digital high and low states corresponding to the 1 and 0 values of a binary code. Recent research has discovered two additional states, effectively doubling storage capacity.
Write times for a flash memory are about one millisecond for a block of data, 100,000 times greater than the typical 10 nanosecond (ns) read time for a byte using static random access memory (SRAM). Perfect RAM can offer much higher performance when rapid writing is important. Flash memory degrades with each burst of voltage. Perfect RAM devices also degrade, but at a much slower rate. These devices can endure about 100 million write cycles. Perfect RAM lifetime is limited by thermal expansion during programming, metal migration, and unknown mechanisms.
Some of the challenges for perfect RAM technology involve the requirement for high programming current density (precise control of current), long-term resistance (sustained resistance to electric current), and threshold voltage drift (precise control of electric voltage) - all at the microscopic level. Hewlett-Packard, Samsung, STMicroelectronics, and Numonyx, among others, are researching these challenges.