Study On Electric Double Layer At Solid-liquid Interface And Its Working Mechanism In Ionic-liquid-gated Field Effect Transistor

Field effect transistors(FETs),which have attracted great research interests in the field of transistors in recent decades,have been widely used in integrated circuit,display and sensor,etc.With the miniaturization of the devices,the traditional gate dielectrics are no longer sufficient for device performance.Electric double layers(EDLs)with high capacitance,which could induce ultrahigh carrier density at semiconductor surface,have been adopted in FETs to overcome the drawbacks of traditional gate dielectrics.Among different gate dielectrics in EDL transistors,ionic liquids(ILs)could afford ultrahigh capacitance with high thermal and chemical stabilities,which makes them potential gate dielectrics in FETs.Therefore,revealing the underlying mechanism of the IL-gated FETs is of great significance for design and optimization of the performance of such devices.Because the EDL regulates the carrier transportation in semiconductors via its microstructure,the characterization of the EDL structure is vital for addressing working mechanism.However,since the EDL structures are difficult to be detected by experiments,there is still a lack of studies for revealing the underlying working mechanism of the IL-gated FETs.In this dissertation,molecular simulation and theoretical analysis are combined to investigate the effect of EDL structure on the gating performance of IL-gated FETs and to propose the direction for designing and optimizing the gating performance.Besides,the results between simulation and experiment are quantitatively compared via a hierarchical simulation approach,which provides a new technique for screening out ILs for better device performance.The main results of this dissertation are summarized as follows:(1)In order to explore the relationship between the EDL structure and the conductivity of semiconductor,we characterize the EDL structures in IL-gated FETs by molecular dynamics(MD)simulation.With the applied gate voltage,the EDL structures alter via the cation reorientation,and the electronic structures at semiconductor surface change as well,which consequently realizes the insulator-metal transition.Meanwhile,it reveals that the insulator-metal transition in semiconductor is achieved by the expansion and connection of metallic regions on the semiconductor surface.This study provides a basic theory for improving the performance of IL-gated FETs by regulating the EDL structure.(2)To obtain the relationship between different EDL structures and the gating performance of IL-gated FETs,the effect of ion size on the performance of FETs is investigated by MD simulation.The different EDL structures could not only induce different EDL capacitance,but also produce carrier distribution at semiconductor surface with different heterogeneity and a more homogeneous charge distribution would enhance the conductivity of the semiconductor.This provides ideas and directions for synthesis and screening of ILs for better device performance.Besides,we successfully scale up the simulation results to macroscopic device via a new hierarchical simulation approach that could be used to evaluate the gating performance for more practical devices.(3)Based on the analyses of the cation size and anion type effects on the gating performance of IL-gated FETs,we study the cation type effect.The simulation results confirm that ILs with larger ion sizes would produce lower EDL capacitances and more inhomogeneous carrier distributions at semiconductor surface,which results in reduced semiconductor conductance.By comparing ILs with different cation types,it shows that the structure of charged group in cation has significantly impact on the homogeneity of carrier distribution,and both the EDL capacitance and the homogeneity of carrier distribution should be taken into account when evaluating the gating performance of IL-gated FETs.(4)We study the dynamic properties of IL-gated FETs.By adding acetonitrile,the ionic conductivity of IL/solvent mixture is significantly improved,and the switching speed of device is accelerated correspondingly.Moreover,the EDL structures show that the organic solvent could homogenize the carrier distribution at semiconductor surface,which eventually results in higher semiconductor conductivity.Surprisingly,the optimal ion concentration for increased switching speed is nearly the same as that for achieving highest semiconductor conductivity.Thus,the addition of organic solvent could simultaneously improve the static and dynamical performance of IL-gated FETs,which demonstrates a new direction for the optimization of device.