Phase-change materials exhibit at least two different states. The states of phase-change material may be referenced to as amorphous and crystalline states. The states may be distinguished because the amorphous state generally exhibits higher resistivity than does the crystalline state. Generally, the amorphous state involves a more disordered atomic structure, white the crystalline state is an ordered lattice. Some phase-change materials exhibit two crystalline states, e.g. a face-centered cubic (FCC) state and a hexagonal closest packing (HCP) state. These two crystalline states have different resistivities. In the following description, the amorphous state generally refers to the state having the higher resistivity, and the crystalline state generally refers to the state having the lower resistivity.
Phase change in the phase-change materials may be induced reversibly. In this way, the phase-change material may change from the amorphous state to the crystalline state, and from the crystalline state to the amorphous state, in response to temperature changes. The temperature changes to the phase-change material may be achieved in a variety of ways. For example, a laser can be directed to the phase-change material, current may be driven through the phase-change material, or current can be fed through a resistive heater adjacent the phase-change material. With any of these methods, controllable heating of the phase-change material causes controllable phase change within the phase-change material.
An array of phase-change memory cells includes bit line and ground line wiring. Since the bit line and ground line wiring have a parasitic resistance, the set and reset current fed through a phase-change memory cell during the application of a voltage write pulse depends on the location of the phase-change memory cell in the memory array. This location dependent set and reset current contributes to a broadening of the phase-change memory cell resistance distribution and thereby affects the reliability of the phase-change memory cell set and reset operations. Typical solutions to this problem attempt to minimize the resistance or adjust the set and reset current based on the location of the phase-change memory cell within the memory array to compensate for the voltage drop over the bit line. These methods increase the costs of the phase-change memory due to increased process complexity and chip complexity.