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Advantages of Phase Change Memory | Disadvantages of Phase Change Memory

This page covers advantages and disadvantages of Phase Change Memory (PRAM). It mentions benefits or advantages of Phase Change Memory (PRAM) and drawbacks or disadvantages of Phase Change Memory (PCM).

What is Phase Change Memory?

This memory uses unique behavior of chalcogenide glass which can be switched between two states viz. crystalline and amorphous with the application of heat. The transition between phases is controlled by heating and cooling. PRAM is much better than flash memory due to its switching time and its inherent scalability.

There are many limitations of flash memory such as its scaling limit, number of write cycles etc. To overcome these limitations, phase change memory has been developed. PCM memory uses reversible structural phase change between amorphous phase and crystalline phase. The small volume of active media in each memory cell acts as fast programmable resistor.

Phase change memory states
Image courtesy : ST Microelectronics

Physical characteristics of phase change memory i.e. PRAM are as follows.
• Chemical formula Gex Sby Tez
• Uses chalcogenide glass
• Varies between two states to represent binary data.
-Crystalline : Low Resistance, represents binary 0
-Amorphous : High Resistance, represents binary 1
• It can switch on the order of nanoseconds.
• Read time is 100-300 nsec and program time is 10-150 µs

PCM memory can either be in amorphous phase or crystalline phase. Amorphous material has following characteristics.
- Short range atomic order
- Low free electron density
- High activation energy
- High resistivity
Crystalline material has following characteristics.
- Long range atomic order
- High free electron density
- Low activation energy
- Low resistivity

Let us understand PCM operating principle.
• PCM cell is programmed by application of current pulse at voltage above switching threshold.
• PCM devices are programmed by electrically altering the structure (amorphous or crystalline) of small volume of chalcogenide alloy.
• The programming pulse drives the memory cell into high or low resistance state (phase transition process), depending on current magnitude or amplitude.
• The phase transition process can be completed in as quickly as 5 ns. Information stored in the cell is read out by measurement of cell's resistance.

Benefits or advantages of Phase Change Memory (PRAM)

Following are the benefits or advantages of Phase Change Memory (PRAM):
➨This memory offers fast switching performance which is in ns (nanoseconds) range.
➨PRAM device offers medium endurance which is about 109 to 1013 range.
➨PRAM uses low voltage ( 0.4 to 2V).
➨It offers good scalability which is better than NVM or flash memory.
➨It offers reduced assembly and test costs.
➨PCM cells can be reprogrammed at least 106 times.

Drawbacks or disadvantages of Phase Change Memory (PCM)

Following are the drawbacks or disadvantages of Phase Change Memory (PCM) :
➨Temperature sensitivity is major concern in the development of PRAM.
➨PRAM devices degrade with use and hence its lifetime is limited like flash memory.
➨The challenge is management of proximity heating with declining cell space.
➨The other challenge is increased set/reset resistance and decreased read current/set current margin with scaling.



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