POLARIZATION CURRENTS IN ZINC-OXIDE VARISTORS FROM 77-K TO 450-K

The time and temperature dependencies of polarization currents are inv estigated to higher temperatures and with greater temperature resoluti on than has been reported previously for zinc oxide varistors. Arrheni us plots of these transient polarization currents yield two thermal ac tivation energies near 140 and 8 meV, which are;in reasonable agreemen t with values reported recently as well as an additional thermally act ivated level at 0.66 eV, which becomes evident in the extension of the measurements to T>300 K. Conductance data in this higher temperature range yield the nearly identical energy of 0.68 eV for the intergranul ar (Schottky) barrier height. The temperature dependence of the expone nt m in the power-law expression for the current-time dependence I=I(0 )t(-m) was carefully examined and found to change twice from 0.5 to 1. 0. The nonmonotonic behavior of m(T) can be interpreted as due to reso lution of polarization currents from two different traps, each of whic h evolves from diffusive to dispersive transport as temperature is low ered rather than, for example, a complicated distribution of relaxatio n times. (C) 1994 American Institute of Physics.