Phase change memories (PCM) are non-volatile memory devices that use reversible phase change in amorphous materials like Germanium, Antimony and Tellurium (Ge,Sb,Te,) alloy to store information. The phase reversal results in the change in the electrical resistivity of the material used in the memory operation. These materials are broadly classified as chalcogenide materials. The phase change is induced by heating the material through a resistive element using a current pulse of a fixed duration during the two modes of operation. These are called as SET and RESET operations. During RESET operation, the current pulse of During RESET operation, a current pulse of high magnitude is applied to the materials so as to melt a critical volume of the material. The pulse produces a temperature more than the melting point of the material. The material melts and starts cooling down immediately when the voltage across the material is removed after a few nanoseconds. The molten volume of the material cools rapidly and the atoms fail to reorganize and loose crystalline behavior and an amorphous state of very high resistance is formed. On the other hand, during SET operation, a moderate voltage is applied across the material for a relatively longer duration than the RESET pulse. The temperature of the material rises above the crystallization temperature, but below the melting temperature. This mode results in the transformation of the amorphous state to the crystalline state. This state is also called low resistive state.
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