PRAM

Phase-change random access memory (PRAM or PCRAM) is another emerging memory solution in competition with MRAM to resolve known issues with DRAM and Flash memory.   PRAM uses the physical state of a chalcogenide material, such as Ge2Sb2Te5 (GST), to represent a 0 or 1 bit value.   GST is useful as a storage element because its properties change when the state changes.   For example, CDs and DVDs use GST and other chalcogenides to store and retrieve information optically.   The amorphous state of GST absorbs light, while the crystalline state reflects the light; thus, with accompanying electronics the phase can be interpreted as a high or low bit.  


Research in the development of PRAM, however, has mainly focused on the change of resistance with the two phases, although an optical approach with an electron beam has also been researched.   GST's amorphous state is highly resistive, and the crystalline state maintains a low resistance.   For the sake of example, consider the amorphous state to be "0" and the crystalline state to be "1" (in reality, the values will depend on the type of current sensing electronics used).   To store a "0" through a write operation, an intense current pulse is sent through the GST.   The current heats the GST into an amorphous state, and since the current is a brief pulse, the GST cools quickly to keep its amorphous state.   To write a "1", the GST is gradually heated to a lower temperature and then cooled.   With lower temperatures and gradual changes, GST forms a crystalline with low resistance.   The diagram, below, shows the PRAM architecture for implementing the GST phase changes.  

The word line acts as a gate of the transistor.   Sending current through a word line will enable a row of bits.   Current can then flow through one of the word's bit lines to the GST to change its state.   Samsung, IBM, and Hewlett-Packard are among top companies researching PRAM.   HP's electron beam optical approach has led to a storage density of 29 gigabits/in2, though HP has its sight set on increasing that density 100 fold.   The resistive approach to PRAM may also lead to an incredibly high density of 1 terabit/in2.   PRAM is high speed and non-volatile since no energy is required to maintain memory state.