Threshold switches

Fig. 5.53 Schematic of FeRAM matrix (a) a matrix of memory cells (b) each memory cell coupled with a switching transistor to more closely define the threshold switching voltage.

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

The threshold switching in melanins and melanosomes, a rather exotic property of amorphous semiconductors, was studied by McGuiness et al.,... 

If the structural state arrived at Vth is a metastable structural state which is also metallic, it corresponds to threshold switching. But if at Vth the field induced rearrangement is such that the structure tumbles over into a deeper minimum corresponding to a crystalline state, it results in a permanent ON state. Superlinear state corresponds to an unstable structural state in this approach. The mechanism considers heating as incidental rather than as a cause for 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. 

Fig. 6.2. Sketch of the time response to a voltage pulse Vp > and to interrogating voltage pulses V < for (a) a threshold switch and (b) a memory switch. Switching from OFF to ON occurs after a delay time tj. The memory is SET after a lock-on time interval LO after switching. The threshold switch (a) returns to OFF after the end of the Vp pulse. The memory switch (b) requires a RESET current pulse to return to the high resistance OFF state.

Fig. 6.6. Frequency dependence of a threshold switch with carbon electrodes separated by 1 nm After Sieja (1966).

Furthermore, Henisch et aL (1972) showed that it is possible to initiate switching by a light pulse directed at the crystalline semiconductor electrode when the threshold switch is suitably biased. 

Fig. 6.7. Oscilloscope pictures of voltage drop across threshold switch operated at 1 M Hz and 20 percent overvoltage. Rectifier in series with device eliminates negative voltage signal (courtesy of Energy Conversion Devices, Inc.).

Threshold switch made with 1 iim thick Te-As-Ge glass and Ta electrodes. 

As shown in Figure 6.1(c) the conducting branch of the threshold switch is nearly vertical. The precise I—V relationship of the ON-state is obtained by subtracting the spreading resistance in the electrodes adjacent to the current filament and, in thin film devices, by subtracting the additional strip resistance of the thin metal films. Lee (1972) reports that he obtained 1 OL for the ON-state characteristic. 

The electrical response of a typical memory switch is shown in Figures 6.1(d) and 6.2(b). The switching process and filamentary conduction of a memory switch is essentially the same as tha of the threshold switch. In fact, when the voltage is removed immediately after switching, the memory switch reverts to the OFF state. On the other hand, the memory switch retains its conductive state after it has been kept in this state (5-10 mA current level) for about 10" sec, the lock-on time LO, to SET the memory state. Once in the ON-state, the device is turned off by the application of a short, intense RESET current pulse, typically of 5 /x sec duration and 120 mA current. The impedance of the device can be ascertained by a low level (0.5 V, 5 ju sec) read pulse as sketched in Figure 2(b). A typical SET — RESET sequence with interrogating READ pulses is shown in Figure 6.13 (Bunton and Quilliam (1972)). 18 traces are superimposed in this... 

Fig. 6.15. Electrode configurations of threshold switch, (a) vertical structure, (b) gap structure.

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