Piezoelectric contributions to the electrical behavior of ZnO varistors

Abstract

Internal stresses exist in ZnOvaristors as a result of both crystallographic anisotropy in thermal expansion of the constituents and thermal expansion mismatch between phases. Due to the piezoelectric nature of ZnO, these stresses induce a net electric dipole moment that modifies the grain boundarySchottky barriers and causes an alteration of the varistor current–voltage response in the nonlinear regime. We report Raman piezospectroscopic measurements of residual strains in polycrystalline ZnO and develop a stochastic model for the distribution of potential barrier heights based on the distribution of internal stresses. The model provides a physical basis for barrier height distributions used in electrical network simulations of varistor transport behavior.