structure. GeSbTe has a glass transition temperature of around 100 °C. GeSbTe also has many vacancy defects in the lattice, of 20 to 25% depending on the specific GeSbTe compound. Hence, Te has an extra lone pair of electrons, which are important for many of the characteristics of GeSbTe. Crystal defects are also common in GeSbTe and due to these defects, an Urbach tail in the band structure is formed in these compounds. GeSbTe is generally p type and there are many electronic states in the band gap accounting for acceptor and donor like traps. GeSbTe has two stable states, crystalline and amorphous. The phase change mechanism from high resistance amorphous phase to low resistance crystalline phase in nano-timescale and threshold switching are two of the most important characteristic of GeSbTe. Applications in phase-change memory The unique characteristic that makes phase-change memory useful as a memory is the ability to effect a reversible phase change when heated or cooled, switching between stable amorphous and crystalline states. These alloys have high resistance in the amorphous state ‘0’ and are semimetals in the crystalline state ‘1’. In amorphous state, the atoms have short-range atomic order and low free electron density. The alloy also has high resistivity and activation energy. This distinguishes it from the crystalline state having low resistivity and activation energy, long-range atomic order and high free electron density. When used in phase-change memory, use of a short, high amplitude electric pulse such that the material reaches melting point and rapidly quenched changes the material from crystalline phase to amorphous phase is widely termed as RESET current and use of a relatively longer, low amplitude electric pulse such that the material reaches only the crystallization point and given time to crystallize allowing phase change from amorphous to crystalline is known as SET current. The early devices were slow, power consuming and broke down easily due to the large currents. Therefore, it did not succeed as SRAM and flash memory took over. In the 1980s though, the discovery of germanium-antimony-tellurium (GeSbTe) meant that phase-change memory now needed less time and power to function. This resulted in the success of the rewriteable optical disk and created renewed interest in the phase-change memory. The advances in lithography also meant that previously excessive programming current has now become much smaller as the volume of GeSbTe that changes phase is reduced. Phase-change memory has many near