ideal memory qualities such as non-volatility, fast switching speed, high endurance of more than 1013 read –write cycles, non-destructive read, direct overwriting and long data retention time of more than 10 years. The one advantage that distinguishes it from other next generation non-volatile memory like magnetic random access memory (MRAM) is the unique scaling advantage of having better performance with smaller sizes. The limit to which phase-change memory can be scaled is hence limited by lithography at least until 45 nm. Thus, it offers the biggest potential of achieving ultra-high memory density cells that can be commercialized. Though phase-change memory offers much promise, there are still certain technical problems that need to be solved before it can reach ultra-high density and commercialized. The most important challenge for phase-change memory is to reduce the programming current to the level that is compatible with the minimum MOS transistor drive current for high-density integration. Currently, the programming current in phase-change memory is substantially high. This high current limits the memory density of the phase-change memory cells as the current supplied by the transistor is not sufficient due to their high current requirement. Hence, the unique scaling advantage of phase-change memory cannot be fully utilized. The typical phase-change memory device design is shown. It has layers including the top electrode, GST, the GeSbTe layer, BEC, the bottom electrode and the dielectric layers. The programmable volume is the GeSbTe volume that is in contact with the bottom electrode. This is the part that can be scaled down with lithography. The thermal time constant of the device is also important. The thermal time constant must be fast enough for GeSbTe to cool rapidly into the amorphous state during RESET but slow enough to allow crystallization to occur during SET state. The thermal time constant depends on the design and material the cell is built. To read, a low current pulse is applied to the device. A small current ensures the material does not heat up. Information stored is read out by measuring the resistance of the device. Threshold switching Threshold switching occurs when GeSbTe goes from a high resistive state to a conductive state at the threshold field of about 56 V/um. This can be seen from the current-voltage (IV) plot, where current is very low in the amorphous state at low voltage until threshold voltage is reached. Current increases rapidly after the voltage snapback. The