Fundamental theories of piezotronics and piezo-phototronics

Wurtzite structured mateirals such as ZnO, GaN, InN and CdS simultaneously exhibit piezoelectric, semiconducting and photoexcitation properties. The piezotronic effect is to use the inner crystal potential generated by piezoelectric polarization charges for controlling/tuning the charge carrier transport characteristics in these materials. The piezo-phototronic effect is about the use of piezoelectric charges to tube the generation, transport, separation and/or recombination of charge carriers at p-n junction. This article reviews the fundamental theories of piezotronics and piezo-phototronics, forming their basis for electromechancial devices, sensors and energy sciences. Starting from the basic equations for piezoelectricity, semiconductor and photoexcitation, analytical equations for describing the strain-tuned device current were derived. Through analytical calculations and numerical simulations, it was confirmed that the piezoelectric polarization charges can act in the form of inner-crystal charges in the depletion region, resulting in a change in Schottky barrier height, depletion region shift and/or formation of a charge channel, which can be used effectively to enhance the efficiency of LED, solar cell and photon detectors.