On-state behaviour and degradation effects in thin-film amorphous switching devices

The on-state behaviour of thin-film chalcogenide threshold switches was studied with special reference to the influence of electrode materials and the progressive effects of device degradation. For operations during the forming period, the on-state current-voltage characteristics remain identical for both rising and falling current. The on-state minimum voltage is found to be closely proportional to the square root of the electrode thermal conductivity and, in addition, shows some polarity dependence with dissimilar electrode combinations. This behaviour is interpreted in terms of a model in which the voltage drop across the on-state current channel is dominated by constrictions and associated hot spots near the channel/electrode boundaries. After forming and after a further number of operations depending both on electrode material and ambient temperature, one or more stable intermediate steps or “branches” develop in the I–V characteristic of the transition from off-state to on-state. The exact pattern shows some polarity dependence. Branching is identified with the progressive precipitation of tellurium-rich “islands” in the glass as the number of device operations increases and as a result of which the growth of the on-state channel proceeds via a number of “island-hopping” stages.