ABSTRACT
Phase Change Memory (PCM) is probably the closest to the commercialization stage. Data storage, however, is based on switching between the amorphous phase with high electrical resistivity and the crystalline phases with low electrical resistivity. PCM is categorized as a so-called emerging memory technology, the history of the phase changing concept may be longer than commonly thought. A multilevel cell (MLC) is an effective technology to boost memory density without increasing the memory cell area. MLC technology can be applied to PCM designs using a diode or bipolar transistor as the select device. In a design using bipolar transistors as the select device, the sense amplifier must be designed to compensate for the effect of the bipolar transistor along the sensing path. The structural relaxation process is the main factor affecting the bit retention time of a PCM cell, which is caused by the nature that chalcogenide material tends to minimize its free energy.
