Study On Humidity Sensing Characteristics Of Electric-Double-Layer Thin Film Transistors

Humidity is an important parameter to characterize the water content in the air,and its measurement is critical for many industrial and domestic applications.In recent years,a new type of ion-electrolyte gated field-effect transistor has attracted much attention due to its ultra-strong electric double layer（EDL）gating effect.In such devices,the carrier concentration of the semiconductor channel can be efficiently modulated by the ion-induced EDL capacitance at the semiconductor/electrolyte interface,hence the term electric double-layer thin-film transistors（EDLTs）.Due to its significant advantages such as large specific capacitance,low operating voltage,interface sensitivity and low power consumption,it has received more and more attention in the field of sensing.Based on the characteristic that EDLTs own gate dielectrics have moisture-sensitive properties,their application in the field of humidity sensors is gradually emerging.Therefore,this paper takes the humidity sensing characteristics of EDLTs as the research topic.Based on the work of the device gate dielectric material and process,device structure and humidity sensing test method,the following researches are carried out:（1）Improvement of EDLTs device materials and processes.In this paper,the indium gallium zinc oxide（IGZO）conductive channel and the indium zinc oxide（IZO）source-drain electrodes are first prepared by magnetron sputtering technology.Al₂O₃ gate dielectric films were prepared by three different methods,including DC magnetron sputtering,intermediate frequency magnetron sputtering and atomic layer deposition（ALD）;Nafion precursor solution and chitosan solution were prepared as the other two top gates Gate dielectric for top-contact EDLTs.The transistor electrical properties of three gate dielectric materials were tested respectively,and the experimental results showed that the devices with DC magnetron sputtering Al₂O₃,Nafion and chitosan solution as gate dielectrics exhibited good quasi-static electrical properties.Then,under different ambient humidity,the devices with Nafion and chitosan solution as gate dielectrics were tested for their humidity-sensitive properties.The results show that the chitosan-EDLTs device has no obvious response to humidity,while the Nafion-EDLTs device responds to humidity but has a limited humidity test range（30%RH-70%RH）.（2）A new type of device structure is designed.The top-gate structure,which is easily affected by the gate dielectric,is changed to a planar side-gate structure.The electrodes will not be affected by the change of the gate dielectric,and the moisture-sensitive gate dielectric material can also better respond to the humidity of the environment.In addition,the structure protects the conductive channel to a certain extent.In order to realize the planar-gate structured EDLTs with polyelectrolyte as the gate dielectric,this work fabricated devices with three kinds of polyelectrolytes using magnetron sputtered indium gallium zinc oxide（IGZO）channel and indium zinc oxide（IZO）electrodes.It is found that device with sodium polystyrene sulfonate（PSSNa）as the gate dielectric works properly.With its capacitance reaching 2.9μF/cm² at the low frequency end,the working voltage is within 1 V,the mobility is 0.23 cm²/Vs and current on-off ratio is 4.6×10³.Whereas,devices with chitosan or Nafion as the gate dielectric are restrained by their low ion concentration and weak controllability of the planar-gate.Further study of humidity response with PSSNa-EDLTs has shown a dependence of the transfer curve with the increase of humidity and it is directly related to the amount of water molecules in the ion-based dielectric.The results provide a basis for the subsequent optimization of planar side-gate devices,and demonstrate the potential of PSSNa as a gate dielectric for EDLTs devices in related sensing.（3）Humidity sensing properties of PSSNa-EDLTs with top-gate structure.Usually,the humidity-sensitive characteristics of transistors are obtained under static signals.In this paper,it is found that the humidity response of EDLTs under static signals is not ideal.Therefore,the dynamic response of EDLTs is studied by inputting pulsed square waves with different frequencies and duty cycles to the gate.properties,an increase in humidity-sensitive response was obtained.First,the PSSNa-EDLTs device with top-gate structure was prepared for testing.In the static test,the response current under 80%RH was only 10.2 times that under 20%RH,but it was increased to 1.08×10³ times under the dynamic test.Next,the humidity sensing properties of chitosan-EDLTs and Nafion-EDLTs were tested,which were also improved over static tests.（4）The top-gate structure PSSNa-EDLTs are used in the testing of voltage output circuits and two-terminal connected devices.First,in order to convert the current output into the voltage output signal of the sensor,the follower circuit is used to realize the voltage response under different humidity,and the humidity sensing performance of the device at low frequency is defined as a linear relationship"V_out=3.3RH+A".The response to a single pulse under different humidity was tested in the device two-terminal connection mode,the current of high humidity（80%RH）was 2.4 times that of low humidity（20%RH）,and the energy consumption was about 105 n J;the input was continuous For two-terminal devices under square wave voltage,the current at high humidity（80%RH）is 4.13 times higher than that at low humidity（20%RH）.The test method in this mode confirms the self-modulating behavior of the device,and the test of transient channel current also shows its application potential in neurosynaptic devices.