metal oxides to chalcogenides. Silicon dioxide was shown to exhibit resistive switching as early as May 1966, and has recently been revisited. In 1963 and 1964, a thin-film resistive memory array was first proposed by members of the University of Nebraska–Lincoln. Further work on this new thin-film resistive memory was reported by J.G. Simmons in 1967. In 1970, members of the Atomic Energy Research Establishment and University of Leeds attempted to explain the mechanism theoretically. In May 1997, a research team from the University of Florida and Honeywell reported a manufacturing method for "magneto-resistive random access memory" by utilizing electron cyclotron resonance plasma etching. Leon Chua argued that all two-terminal non-volatile memory devices including ReRAM should be considered memristors. Stan Williams of HP Labs also argued that ReRAM was a memristor. However, others challenged this terminology and the applicability of memristor theory to any physically realizable device is open to question. Whether redox-based resistively switching elements (ReRAM) are covered by the current memristor theory is disputed. Silicon oxide presents an interesting case of resistance switching. Two distinct modes of intrinsic switching have been reported - surface-based, in which conductive silicon filaments are generated at exposed edges (which may be internal—within pores—or external—on the surface of mesa structures), and bulk switching, in which oxygen vacancy filaments are generated within the bulk of the oxide. The former mode suffers from oxidation of the filaments in air, requiring hermetic sealing to enable switching. The latter requires no sealing. In 2014 researchers from Rice University announced a silicon filament-based device that used a porous silicon oxide dielectric with no external edge structure - rather, filaments were formed at internal edges within pores. Devices can be manufactured at room temperature and have a sub-2V forming voltage, high on-off ratio, low power consumption, nine-bit capacity per cell, high switching speeds and good endurance. Problems with their inoperability in air can be overcome by hermetic sealing of devices. Bulk switching in silicon oxide, pioneered by researchers at UCL (University College London) since 2012, offers low electroforming voltages (2.5V), switching voltages around 1V, switching times in the nanoseconds regime, and more than 10,000,000 cycles without device failure - all in ambient conditions. Forming The basic idea is that a dielectric, which is normally insulating, can be made to conduct through a filament or conduction path formed after application of a sufficiently high voltage. The conduction path can arise