Electrical characteristics and applications

There are a number of situations in which it is valuable to have a resistor which offers a high resistance at low voltages and a low resistance at high voltages as is the case in the current-voltage characteristic shown in Fig. 4.7. 

Ceramics based on SiC and ZnO are two materials in everyday use that have the characteristic shown in Fig. 4.7. In both cases it has been established that the resistance is controlled by the region in which the ceramic grains contact one another. 

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 

The VDR behaviour in ZnO varistors is governed by electron states that are formed on the surfaces of crystals as a consequence of the discontinuity. These surface states act as acceptors for electrons from the n-type semiconductor. Electrons will be withdrawn from the region near the surface and replaced by a positive space charge. In this way oppositely oriented Schottky barriers will be created at the surfaces of neighbouring crystals so that a high resistance will be offered to electron flow in either direction (Fig. 4.10(a)). The situation with an applied field is shown in Fig. 4.10(b). With low applied fields small thermally 

A typical varistor voltage-current characteristic is shown in Fig. 4.11. The linear part can be represented by the relation 

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