Studies On Polyelectrolyte-Gated Thin Film Transistors And Their Artificial Synapses

Oxide semiconductor thin film transistors?TFTs?have wide application prospects in flat panel display due to their advantages of high mobility,low-temperature and transparency in the visible region.However,the operating voltage of TFTs is generally above 10 V due to the low capacitance of the conventional gate dielectric,which limits its application in portable low-power electronics.To solve this issue,this thesis investigates TFTs using polyelectrolyte as the gate dielectric layer.Such transistors can operate at lower voltages owing to the huge ouble layer capacitance effect of polyelectrolyte.Main contents in this thesis can be summarized as follows:?1?By using the inorganic-organic hybrid electrolyte as the gating dielectric,ZnO electric double layer transistors?EDLTs?have been successfully fabricated and have been demonstrated with a large On/Off drain current ratio above 10⁷,a low positive threshold voltage below 3 V.To improve the performance of ZnO EDLTs,different active layer thicknesses,oxygen?O₂?/argon?Ar?flow ratios and active layer deposition temperature are investigated.Superior properties,such as a large On/Off drain current ratio(I_on/I_off)of 4.66×10⁸,a positive threshold voltage(V_th)of 2.4 V,a steep subthreshold swing?SS?of 61 mV dec^-1,and a transconductance?g_m?of 8.40 mS are obtained on the device fabricated with an O₂/Ar flow ratio of 5/25 sccm at room temperature.Furthermore,by increasing the deposition temperature,the device can exhibit a lower V_th and a more stable cyclic performance,but may result in an increased Off-state current(I_off).?2?By using the inorganic-organic hybrid electrolyte as the gating dielectric,WO₃electrochemical transistors?ECTs?which exhibit simultaneous optical and electrical modulation have been successfully fabricated.When V_G exceeds the reduction potential(V_red),Li⁺can be injected into the channel layer,enabling effective electrochemical doping.WO₃ is reduced to Li_xWO₃ and becomes metallic,resulting in very high current modulations.It is demonstrated that the incorporation of O₂ in the WO₃ deposition atmosphere and Li⁺concentration of the polyelectrolyte play important roles in improving the electrical performance of the ECTs.Moreover,the amorphous WO₃ ECT exhibits improved performance over polycrystalline WO₃ ECTs,allowing to obtain the transistor with a large Ion/Ioff of2.82×10⁸,a low Vth of 0.8 V,a steep SS of 86.4 mV dec^-1,a g_m of 8.48 mS and excellent cyclic transfer characteristics.?3?Based on the WO₃ ECTs,some basic functions of synapses are emulated.When applying a positive gate pulse,the channel current response behavior is quite similar to the excitatory postsynaptic current?EPSC?process of biological synapses.It is demonstrated that the EPSC is determined by the width and amplitude of the pulse.In addition,the synaptic plasticity can be simulated using successive pulse stimulations.The short-term synaptic plasticity?STP?of thin film transistors is caused by pure EDL electrostatic modulation process,while the long-term memory plasticity?LTP?is caused by the electrochemical doping during the stimulus trains.A STP to LTP transition can be realized by increasing the amplitude of the gate pulse.Furthermore,the long-term plasticity of the transistor can be greatly enhanced by reducing the V_DS.