Novel modelling for high-field current-voltage characteristics of semiconducting varistors

In this work, the current-voltage property at a high electric field region (1000V/cm, 1000A/cm²) is described. From the relationship among average power dissipation (P), upturn nonlinear coefficient (α_up) and ZnO grain resistivity (ρ_gr, a new current-voltage equation is derived (upturn equation) and four constants are found. A generic form of this equation is α = SR_up+ c_i where S is the upturn power constant, R_up is the upturn resistance at 1000A/cm₂ and c_i is the ideal constant. In this high-field region, also known as upturn region, the current-voltage characteristic is almost ohmic whereby the α_up values are less than 5. Due to the diminished Schottky barrier in the upturn region, ZnO grains play the key role in determining the I-V characteristics. SEM micrograph photos of the starting ZnO powder show presence of intrinsic defects (ID) in the form of irregularly-shaped microclusters of nanocrystallites 40nm-300nm in size. A new model is introduced to describe the impact of these ID on the power dissipation in the I-V upturn region. It is believed that charge carriers are trapped in the potential wells created by ID, and when they are annihilated by opposite charges, thermal energy is dissipated. The fabricated ZnO varistors, categorised as low-voltage models (≤17V_rms), can be used as external surge protection devices (SPDs) for microprocessor-controlled equipment.