Investigation On Fabrication And Threshold Voltage Modulation Of Low-voltage Electric-double-layer Thin Film Transistors Gated By Sodium Alginate

Oxide-based thin-film transistors(TFTs)are attracting considerable attentions because of their distinctive advantages,such as high mobility,high optical transmittance,low processing temperature,etc.Due to forming the interface capacitance,the operation voltages of oxide TFTs gated by conventional dense dielectrics are uaually ten-odd and even a few decades volt,which greatly limits the application of oxide-based TFTs in portable electronics such as chemical and biological sensors.However,because a huge electric-double-layer(EDL)capacitance of more than 1.0 ?F/cm2 is formed at the interface of channel/oxide proton conducting films,the operation voltages of Electric Double Layer Transistors(EDLTs)gated by such films are less than 3.0 V.Therefore,EDLTs gated by oxide proton conducting films are attaching more and more attention.Among them,solution-processed sodium alginate(SA)proton conducting films,which have been widely concerned,have several advantages such as with no need for large vacuum coating equipments,simple preparation process and easy process applications in print electronics,etc.However,up to now,there are few studies on the electrical properties,threshold voltage regulation and operation mechanism of TFTs with varying device structures.Based on above,the TFTs with three types of device structures were fabricated.The threshold voltages of such TFTs were effectively modulated by different channel thicknesses.In addition,the mechanism of proton vertical coupling and lateral coupling was proposed and demonstrated.The main conclusions in this paper were summarized as follows:(1)The SA proton conducting films were fabricated by solution-processed methord.The results of scanning electron microscope showed that the SA proton conducting films were composed of compact nanoparticles with a mean size of less than 10 nm and a globular morphology with close packed structure.The nano-granular feature of the SA films produced indispensable nanochannel/nanoporous for proton conduction.The SA films with the thickness of?3 ?m facilitated the transportion and coupling of proton in three-dimensional space.In addition,the number of total water molecules(chemisorbed+physisorbed)per unit of specific surface area of such SA films with a large number of hydroxyl and carboxyl groups providing a large number of protons was calculated to be 10420 molecules per nm2,which would account for the origin of proton conduction.So it was calculated that the proton conductivity of the SA films was up to 4.8×10-4 S/cm.At room temperature,the specific gate capacitances were estimated to be 2.07 ?F/cm2,0.82 ?F/cm2,and 2.01 ?F/cm2 for the vertical IZO/SA/ITO sandwich structure and the lateral coplanar IZO/SA/IZO structure with the bottom ITO layer and without the bottom ITO layer,respectively.In the wavelength range of 200 nm to 1000 nm,the transmittance of the whole device was as high as 81%,which indicated that the whole device was fully transparent in the visible range.(2)The proton conducting SA gated IZO EDLTs with varying device structure were fabricated.Such EDLTs have a large current on/off ratio of 3.7×106?2.9?107,a low subthreshold swing of up to 75 mV/decade and a high field-effect mobility of 13:3?40.5 cm2/V·s for both vertical operation and lateral operation modes.The operation voltages of the EDLTs were all under 1 volt.The three-dimensional evolution from vertical coupling to lateral coupling by induced protons was demonstrated.Such TFTs facilitate applications in low-voltage operating multifunctional electronics.The results and mechanisms of low-voltage IZO EDLTs with varying device structure would enrich the knowledge of electronics and be used as the experimental and theoretical basis for the applications in portable electronics and low-cost biosensors.(3)The vertically/laterally coupled IZO EDLTs,with different IZO channel thicknesses,gated by SA proton conducting films and self-assembled by the one-step mask process exhibited a low-voltage operation voltages of 1.0 V and 1.5 V,respectively.The threshold voltages of IZO EDLTs were effectively modulated by various thicknesses of IZO channel.With the IZO channel thicknesses decreasing from 45 nm to 8 nm,the threshold voltages of the vertically/laterally coupling IZO EDLTs ranged from-0.25 V to 0.12 V and shifted from-0.07 V to 0.25 V,respectively.Both depletion mode and enhancement mode on the same chip were obtained,which would be of fundamental significance for the further applications in logic circuit.Such IZO EDLTs had a high-performance with a large current on/off ratio of 1.5×106?2.9÷107,a low subthreshold swing of up to 70 mV/decade and a high field-effect mobility of up to 34.3 cm2/V·s.Our results demonstrate that SA films are promising gate dielectric candidates for portable biosensor and synaptic electronics.