Threshold switches current-voltage characteristics

Fig. 1. Current-voltage characteristics for (a) threshold switching and (b) memory switching.

Case (c). The switching device has no stable operating point between the high resistance OFF state and the conductive ON state to which the device switches when the voltage exceeds the threshold voltage V. The device switches to its original OFF state when the current is decreased below the holding current Ij. The characteristic is essentially symmetric. The threshold switch described by Ovshinsky (1967, 1968) is an example of this device. 

The reason for the near identity of the ON voltage in threshold and memory switches is not yet understood as there is no coherent quantitative description of the conductive state at present. One observes that Vj is practically independent of (i) temperature, (ii) electrode separation for 0.4 < d < 5 jLtm, and (iii) electrode area. We conclude from the second point that the potential drop Vj occurs predominantly near one or both electrodes. The third point tells us that the current flow is not uniform over the contact area but instead is restricted to a region whose diameter is independent of the area as predicted by Ridley (1963) for characteristics of the type shown in Figure 6.1 (a-d). Electron microscopic evidence presented below shows that the conducting region in memory switches has a diameter of about 2 jum. 

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