An Analytical Model for Dual Gate Piezoelectrically Sensitive ZnO Thin Film Transistors

We report the experimental realization of piezoelectric ZnO dual-gate thin film transistors (TFTs) as highly sensitive force sensors and discuss the physical origins of its electrically tunable piezoelectric response using a simple analytical model. A dual gate TFT is fabricated on a polyimide substrate using radio-frequency (RF) magnetron sputtering of piezoelectric ZnO thin film as a channel. The ZnO TFTs exhibited a field effect mobility of ~ 5 cm2/Vs, Imax to Imin ratio of 107, and a subthreshold slope of 700 mV/dec. Notably, the TFT exhibited static and transient current changes under external force stimuli, with varying amplitude and polarity for different gate bias regimes. To understand the current modulation of the dual-gate TFT with independently biased top and bottom gates, an analytical model is developed. The model includes accumulation channels at both surfaces and a bulk channel within the film and accounts for the force-induced piezoelectric charge density. The microscopic piezoelectric response that modulates the energy-band edges and correspondent current-voltage characteristics are accurately portrayed by our model. Finally, the field-tunable force response in single TFT is demonstrated as a function of independent bias for the top and bottom gates with a force response range from -0.29 nA/mN to 22.7 nA/mN. This work utilizes intuitive analytical models to shed light on the correlation between the material properties with the force response in piezoelectric TFTs.