Study Of Flexible Oxide-Based Thin-Film Transistors With Poly(Vinyl Alcohol)/Graphene Oxide Composite Gate Dielectrics

Flexible electronics have provided innovational guidance for the intelligent technology revolution in the post-Moore era.As the fundamental building blocks of flexible electronic products,flexible oxide-based thin-film transistors（TFTs）have attracted wide attention due to their high mobility,good flexibility,low-temperature process and low-cost fabrication.With the rapid development of flexible applications,higher requirements to reduce operating voltage and improve flexibility of TFTs have been put forward.On the one hand,the electric-double-layer（EDL）effect at the interface of channel layer/dielectric layer could provide extremely large specific gate capacitance and realize low-power operation.On the other hand,it is meaningful to explore novel device configurations to meet increasing demands of mechanical flexibility.In addition,it is great challenge to enrich the functionalities of TFTs and reduce the integration complexity for future development.In view of this,in this paper,flexible low-voltage electric-double-layer oxide-based thin-film transistors with graphene oxide enhanced poly（vinyl alcohol）composite electrolyte as the gate dielectric layer are fabricated at room temperature.Besides,a novel freestanding device configuration without substrate is explored,and multifunctional integration is successfully realized in a single transistor with multi-gate cooperative modulation.The main content can be summarized from the following aspects:（1）The flexible oxide-based thin-film transistors with indium zinc oxide（IZO）as the active channel layer are prepared on PET substrates by magnetron sputtering and thermal evaporation.The maximum leakage current of the poly（vinyl alcohol）/graphene oxide composite electrolyte film is 8.3 n A,and the electric-double-layer capacitance can reach 2.6μF/cm².The basic electrical properties of the TFTs are measured using the bottom-gate structure.Good transistor performances are achieved with current on/off ratio of 3.3×10⁶,threshold voltage of 0.31 V,field-effect mobility of 19.6 cm²V^-1s^-1,subthreshold swing of 102 m V/decade.Besides,inverter and Schmitt trigger are successfully demonstrated.In addition,the test results using the in-plane-gate structure show the effective coordinated modulation with in-plane gates,which lays a theoretical and practical foundation for the further research of freestanding laterally coupled multi-gate TFTs.（2）The freestanding multi-gate TFTs are fabricated through a straightforward coating-peeling process and vacuum technique.All the electrical tests are based on the in-plane-gate structure.The laterally coupled EDL capacitance is measured to be1.36μF/cm²,and the leakage current of the freestanding film is less than 2 n A.The current on/off ratio,threshold voltage,field-effect mobility and subthreshold swing are 4.6×10⁶,0.54 V,24.3 cm²V^-1s^-1and 106 m V/decade,respectively.In addition,no obvious electrical degradation is observed under 1500 repeated mechanical stimuli and continuous switching due to the reinforcement effect of graphene oxide.（3）The multifunctional applications of freestanding multi-gate TFTs are explored.The resistor-loaded inverter is successfully demonstrated and the maximum voltage gain reaches 13.4,exhibiting a stable dynamic response.Schmitt trigger with adjustable hysteresis window is realized.Under the dual-gate operation mode,the NAND logic is successfully implemented.Especially,“OR”and“AND”logic behavior can be effectively transformed by a third modulatory terminal.Finally,synaptic bionic simulations are performed,including excitatory postsynaptic current（EPSC）,paired-pulse facilitation（PPF）and accumulation characteristic of EPSC under multi-pulse stimulation sequences.In summary,the two types of flexible electric-double-layer thin-film transistors exhibit high electrical properties as well as low power consumption.Especially,freestanding multi-gate TFTs could meet increasing demands of mechanical flexibility and multifunctional integration,which provides a rational guide to new-concept portable and low-cost flexible electronics.