Varistor piezotronics: Mechanically tuned conductivity in varistors

Abstract

The piezoelectric effect of ZnO has been investigated recently with the goal to modify metal/semiconductor Schottky-barriers and p-n-junctions by application of mechanical stress. This research area called “piezotronics” is so far focused on nano structured ZnO wires. At the same time, ZnO varistor materials are already widely utilized and may benefit from a piezotronic approach. In this instance, the grain boundary potential barriers in the ceramic can be tuned by mechanical stress. Polycrystalline varistors exhibit huge changes of resistivity upon applied electrical and mechanical fields and therefore offer descriptive model systems to study the piezotronic effect. If the influence of temperature is contemplated, our current mechanistic understanding can be interrogated and corroborated. In this paper, we present a physical model based on parallel conducting pathways. This affords qualitative and semi-quantitative rationalization of temperature dependent electrical properties. The investigations demonstrate that narrow conductive pathways contribute to the overall current, which becomes increasingly conductive with application of mechanical stress due to lowering of the barrier height. Rising temperature increases the thermionic current through the rest of the material with higher average potential barriers, which are hardly affected by the piezoelectric effect. Hence, relative changes in resistance due to application of stress are higher at low temperature.

Piezotronic‐based magnetoelectric sensor: Fabrication and response

“…ZnO, a direct wide band gap (3.37 eV) semiconductor with a non‐centrosymmetric wurtzite structure, not only fulfils the requirements for piezotronics but is also a material that can be easily synthesized in large nanostructural varieties. Using ZnO nanowires, various piezotronic devices have been fabricated which demonstrated interesting applications, such as flexible strain sensors , field effect transistors , memories , and varistors , etc. In spite of the existence of so many reports on 1D ZnO nanostructures‐based electronic devices, to the best of our knowledge, there is at least no such study which clearly describes the limiting factors of piezotronic strain sensors which actually is the main basis for its application in sensors with enhanced sensitivity.…”

Electromechanical properties of CaZrO3 modified (K,Na)NbO3‐based lead‐free piezoceramics under uniaxial stress conditions

“…It should be noted that the slope changing from negative to positive at temperatures above 100°C is possibly ascribed to increased leakage current under the combined effects of high temperature and large uniaxial compressive stress. The former can increase the movement of point defects, for example, oxygen vacancies; whereas the latter lowers the potential barrier height at grain boundaries due to the piezoelectric effect . Thus, the present case does not reflect intrinsic evolution of E C .…”

Segregation and properties at curved vs straight (000) inversion boundaries in piezotronic ZnO bicrystals

“…This has sparked research efforts for the development of new applications such as strain‐triggered transistors, diodes or sensors . In a similar manner, double Schottky barriers at ZnO varistor grain boundaries can be manipulated by stress induced piezoelectric polarization charges . These piezoelectric polarization charges increase or decrease the barrier height depending on the orientation of the polarization vectors and whether tensile or compressive stress is applied.…”

“…These piezoelectric polarization charges increase or decrease the barrier height depending on the orientation of the polarization vectors and whether tensile or compressive stress is applied. The barrier height can be decreased almost entirely turning the ZnO–ZnO interfaces into a very sensitive and effective piezotronic system . In order to systematically study the influence of positive and negative piezoelectric polarization charges on varistor‐type potential barriers, appropriately designed bicrystals provide a well‐suited model system.…”