Varistor Metal Oxide Varistor Products Page 9 Varistor Metal-Oxide Varistor Products

2017 Littelfuse, Inc. Specifications are subject to change without notice. Revised: 09/14/17 Metal-Oxide Varistors (MOVs) the field lowered barrier, and is thermally activated, at least above about 25C. For semiconductor abrupt P-N junction diodes. The relationship is: Where: ( V b ) = barrier voltage, ( V) = applied voltage, ( q ) = electron charge, (e s ) = semiconductor permittivity and ( N ) = carrier concentration. From this relationship the ZnO carrier concentration, N , was determined to be about 2 x 10 17 per cm 3 . In addition, the width of the depletion layer was calculated to be about 1000 Angstrom units. Single junction studies also support the diode model. Figure 5, shows an energy band diagram for a ZnO-grain boundary-ZnO junction . The left-hand grain is forward biased, V L , and the right side is reverse biased to V R . The depletion layer widths are X L and X R , and the respective barrier heights are f L and f R . The zero biased barrier height is f O . As the voltage bias is increased, f L is decreased and f R is increased, leading to a lowering of the barrier and an increase in conduction. The barrier height f L of a low voltage varistor was measured as a function of applied voltage, and is presented in Figure 6. The rapid decrease in the barrier at high voltage represents the onset of nonlinear conduction. Transport mechanisms in the nonlinear region are very complicated and are still the subject of active research. Most theories draw their inspiration from semiconductor transport theory and is not covered in detail in this docu- ment. 1 C 2 ------ - 2 V b V + ( ) q sN ------------------------- - = (10 14 ) 4 3 2 0 0.4 0.8 1.2 V A PER BOUNDARY 1 c 2 n 2 ------------ - /cm 4 FIGURE 4. CAPACITANCE-VOLTAGE BEHAVIOR OF VARISTOR RESEMBLES A SEMICONDUCTOR ABRUPT-JUNCTION REVERSED BIASED DIODE Nd ~ 2 x 10 17 /cm 3 E V 0 B R V R X L X L L V I E f E C E 0 F FIGURE 5. ENERGY BAND DIAGRAM OF A ZnO-GRAINBOUNDARY-ZnO JUNCTION 1.0 0.8 0.6 0.4 0.2 0 8 16 12 4 NORMALIZED THERMAL BARRIER VOLTAGE (V) L o / FIGURE 6. THERMAL BARRIER vs APPLIED VOLTAGE o 0.59 = Varistor Characteristics, Terms and Consideration Factors (continued)

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