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a semiconducting resistor whose electrical resistance (conduction) varies nonlinearly and equally under the influence of both positive and negative voltage. Varistors are made from powdered silicon carbide (a semiconductor) and a binding substance (such as clay, water glass, lacquers, or resins), which are pressed into a mold and baked at a temperature of about 1700° C. The surface of the model is then metallized and leads are soldered to it. The change in electrical conduction of the varistor with voltage buildup across the leads is associated with complex phenomena on the contacts or on the surface of the crystals (the closing of the gaps between the grains of the semiconductor, an increase in the conduction of the surface oxide films in strong electrical fields and the breakdown of the oxides, and an increase in current through p-n junctions that form between grain). Low-voltage varistors are made with an operating voltage of 3 to 200 volts and a current of from 0.1 milliamperes to 1 ampere. High-voltage varistors are made with an operating voltage of up to 20 kilovolts. Varistors have a negative temperature coefficient of resistance. They can tolerate significant electrical overloads; are simple, cheap, and highly reliable; and have low inertia (a maximum operating frequency of up to 500 kilohertz). But they have considerable low-frequency noise and change their parameters with time and with changes in temperature. They are used to stabilize and regulate low-frequency currents and voltages, to perform mathematical operations on known quantities (such as raising to a degree and extracting a root), and to protect contacts from destruction as a result of overloading in electrical circuits (for example, in high-voltage electrical transmission lines, communication lines, and electrical appliances).
REFERENCEPasynkov, V. V., L. K. Chirkin, and A. D. Shinkov. Poluprovod-nikovye pribory. Moscow, 1966.
Any two-terminal solid-state device in which the electric current I increases considerably faster than the voltage V. This nonlinear effect may occur over all, or only part, of the current-voltage characteristic. It is generally specified as I ∝ Vn, where n is a number ranging from 3 to 35 depending on the type of varistor. The main use of varistors is to protect electrical and electronic equipment against high-voltage surges by shunting them to ground. See Electric protective devices
One type of varistor comprises a sintered compact of silicon carbide particles with electrical terminals at each end. It has symmetrical characteristics (the same for either polarity of voltage) with n ranging from 3 to 7. These devices are capable of application to very high power levels, for example, lightning arresters. See Lightning and surge protection
Another symmetrical device, the metal-oxide varistor, is made of a ceramiclike material comprising zinc oxide grains and a complex amorphous intergranular material. It has a high resistance (about 109 ohms) at low voltage due to the high resistance of the intergranular phase, which becomes nonlinearly conducting in its control range (100–1000 V) with n > 25.
Semiconductor rectifiers, of either the pn-junction or Schottky barrier (hot carrier) types, are commonly utilized for varistors. A single rectifier has a nonsymmetrical characteristic which makes it useful as a low-voltage varistor when biased in the low-resistance (forward) polarity, and as a high-voltage varistor when biased in the high-resistance (reverse) polarity. Symmetrical rectifier varistors are made by utilizing two rectifiers connected with opposing polarity, in parallel (illus. a) for low-voltage operation and in series (illus. b) for high-voltage use. For the high-voltage semiconductor varistor, n is approximately 35 in its control range, which can be designed to be anywhere from a few volts to several hundred. See Semiconductor rectifier