Abstract: The invention provides for a write-once nonvolatile memory array, the memory cells comprising a phase change material, such as a chalcogenide. Phase change is achieved in chalcogenide memories by thermal means. The initial, unprogrammed state of each memory cell is a crystalline, low-resistance state, while the programmed state is an amorphous, high-resistance state. Optimizing the circuitry for a write-only memory array, the wordlines or bitlines can be long, with at least 256 cells on a wordline or bitline, and in some embodiments, having thousands of cells on a wordline or bitline. In a preferred embodiment, such an array can be a monolithic three dimensional memory array comprising stacked memory levels.

Abstract: A memory array having memory cells comprising a diode and a phase change material is reliably programmed by maintaining all unselected memory cells in a reverse biased state. Thus leakage is low and assurance is high that no unselected memory cells are disturbed. In order to avoid disturbing unselected memory cells during sequential writing, previously selected word and bit lines are brought to their unselected voltages before new bit lines and word lines are selected. A modified current mirror structure controls state switching of the phase change material.