Voltage-dependent resistors

Barriers of the Schottky type control the behaviour of voltage-dependent resistors (VDRs), PTC resistors and barrier-layer capacitors. Their behaviour is by no means as well understood as that occurring in semiconductors such as silicon but, where appropriate in the text, simplified models will be presented to indicate the principles involved. 

The microstructure of a ceramic for a voltage-dependent resistor (VDR) can be visualized as shown in Fig. 4.9(a) with IGLs of varying thickness between grains that differ in size by about a factor of 10. An idealized structure which is... 

Fig. 4.7 Typical current-voltage relation for a voltage-dependent resistor.

Fig. 4.8 Use of a voltage-dependent resistor (VDR) to protect a circuit against transients.

Constant voltage devices in everyday use are avalanche and zener diodes and varistors, or voltage dependent resistors. 

Example 3 GBs in ZnO are processed intentionally to include a glass film. This film allows the ceramic to be used as a varistor (voltage-dependent resistor), a device that protects circuits from high-voltage spikes (Chapter 30). Figure 14.38 illustrates an IGF of varying thickness in ZnO and how this film controls the resistance. 

Lightning Arrester One of the earliest applications of SiC was that of a Ughtning arrester in electrical power systems. The principle of this is based on the varistor (= variable resistor or voltage-dependent resistor, VDR) properties of SiC that is, the dependence of the resistance of the semiconducting SiC on the voltage applied. 

Voltage-dependent resistors (varistors) resistance varies with voltage... 

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