Analytical and experimental investigation of dual-mode piezo-gated thin film transistor for force sensors

Piezo-gated thin film transistor (PGTFT) capable of modulating charge transport solely relying on piezo-gating effect plays a pivotal role in developing advanced piezotronic devices. However, most previous PGTFTs were reported to show indistinct piezo-gating effect through piezoelectric-induced modulation of Schottky barrier height in detecting only one-dimensional strain for force sensors. Therefore, we first propose a full-functional PGTFT using ZnO thin film as model piezoelectric semiconducting material that works on dual-mode as depletion and accumulation of charges in detecting strain by both analytically and experimentally to exhibit carrier concentration-dependent behavior. Prior to the PGTFT fabrication, the carrier concentrations of the RF-sputtered ZnO thin films are intentionally varied by varying atmosphere conditions. All the ZnO thin films are fully analyzed regarding morphology, structure and electrical properties to validate the high-quality c-axis oriented with Zn⁺ terminated crystalline thin films. Finally, two configurations of the PGTFTs are completed with top and bottom electrodes. The I-V curves of the PGTFTs subjected to external forces exhibit opposite force dependence between the top-electrode and bottom-electrode PGTFTs, in agreement with the simulated data. Further, the effect of free carrier concentration on the depletion and accumulation mode through piezo-gating effect is investigated, where an enhancement of around 44.6% in gauge factor is achieved for an order of reduction in the carrier concentration. We provide new insights into the piezo-gating effect by the novel ohmic-contact-based PGTFT, which can be operated in dual mode for acquiring more information as the basis of a multi-dimensional piezotronic force sensor.