Electric Double Layer Field-effect Transistors Research Articles

Ionic gating in metallic superconductors: A brief review

Ionic gating is a very popular tool to investigate and control the electric charge transport and electronic ground state in a wide variety of different materials. This is due to its capability to induce large modulations of the surface charge density by means of the electric-double-layer field-effect transistor (EDL-FET) architecture, and has been proven to be capable of tuning even the properties of metallic systems. In this short review, I summarize the main results which have been achieved so far in controlling the superconducting (SC) properties of thin films of conventional metallic superconductors by means of the ionic gating technique. I discuss how the gate-induced charge doping, despite being confined to a thin surface layer by electrostatic screening, results in a long-range ‘bulk’ modulation of the SC properties by the coherent nature of the SC condensate, as evidenced by the observation of suppressions in the critical temperature of films much thicker than the electrostatic screening length, and by the pronounced thickness-dependence of their magnitude. I review how this behavior can be modelled in terms of proximity effect between the charge-doped surface layer and the unperturbed bulk with different degrees of approximation, and how first-principles calculations have been employed to determine the origin of an anomalous increase in the electrostatic screening length at ultrahigh electric fields, thus fully confirming the validity of the proximity effect model. Finally, I discuss a general framework—based on the combination of ab-initio Density Functional Theory and the Migdal-Eliashberg theory of superconductivity—by which the properties of any gated thin film of a conventional metallic superconductor can be determined purely from first principles.

Development of the EDL-FET Based Cell Culture Platform for Electrical Cell Proliferation Monitoring

The pathophysiological changes in cells are mainly focusing on the ionic regulations like hyperpolarization, depolarization etc. The real-time monitoring of these changes in ionic behavior is one of the challenging areas in the biosensor field. A field-effect transistor (FET) based sensor system shows an exceptional advantage in noninvasive real-time monitoring of cellular changes. In this study, we monitored cell proliferation in real-time by using the FET biosensor. Cells were cultured on the FET sensing surface and the potential difference was monitored using the Electric-double-layer field-effect transistor (EDL-FET) biosensor system. Cell proliferation in the sensor surface was electrically measured as drain current change. The impedance measurement changes revealed the capacitive equivalent model of the FETs system. The most important factor in a cell-based system is to overcome the limited life span of cells in sensor platforms. This sensor platform was capable of monitoring cell growth and proliferation, compared with other conventional and current cell sensor platforms. Rapid electrical response corresponding to the cell growth and other functional changes associated with the cell can be used in many fields, including medicine, environmental monitoring and drug screening.

Electric Double Layer Field-Effect Transistors Using Two-Dimensional (2D) Layers of Copper Indium Selenide (CuIn7Se11)

Innovations in the design of field-effect transistor (FET) devices will be the key to future application development related to ultrathin and low-power device technologies. In order to boost the current semiconductor device industry, new device architectures based on novel materials and system need to be envisioned. Here we report the fabrication of electric double layer field-effect transistors (EDL-FET) with two-dimensional (2D) layers of copper indium selenide (CuIn7Se11) as the channel material and an ionic liquid electrolyte (1-Butyl-3-methylimidazolium hexafluorophosphate (BMIM-PF6)) as the gate terminal. We found one order of magnitude improvement in the on-off ratio, a five- to six-times increase in the field-effect mobility, and two orders of magnitude in the improvement in the subthreshold swing for ionic liquid gated devices as compared to silicon dioxide (SiO2) back gates. We also show that the performance of EDL-FETs can be enhanced by operating them under dual (top and back) gate conditions. Our investigations suggest that the performance of CuIn7Se11 FETs can be significantly improved when BMIM-PF6 is used as a top gate material (in both single and dual gate geometry) instead of the conventional dielectric layer of the SiO2 gate. These investigations show the potential of 2D material-based EDL-FETs in developing active components of future electronics needed for low-power applications.

Triboiontronic Transistor of MoS2

Electric double layers (EDLs) formed in electrolyte-gated field-effect transistors (FETs) induce an extremely large local electric field that gives a highly efficient charge carrier control in the semiconductor channel. To achieve highly efficient triboelectric potential gating on the FET and explore diversified applications of electric double layer FETs (EDL-FETs), a triboiontronic transistor is proposed to bridge triboelectric potential modulation and ion-controlled semiconductor devices. Utilizing the triboelectric potential instead of applying an external gate voltage, the triboiontronic MoS2 transistor is efficiently operated owing to the formation of EDLs in the ion-gel dielectric layer. The operation mechanism of the triboiontronic transistor is proposed, and high current on/off ratio over 107 , low threshold value (75 μm), and steep switching properties (20 µm dec-1 ) are achieved. A triboiontronic logic inverter with desirable gain (8.3 V mm-1 ), low power consumption, and high stability is also demonstrated. This work presents a low-power-consuming, active, and a general approach to efficiently modulate semiconductor devices through mechanical instructions, which has great potential in human-machine interaction, electronic skin, and intelligent wearable devices. The proposed triboiontronics utilize ion migration and arrangement triggered by mechanical stimuli to control electronic properties, which are ready to deliver new interdisciplinary research directions.

Electric Double Layer Doping of WSe2 Field-Effect Transistors Using a Monolayer Electrolyte

Electric double layer (EDL) doping using a polymer electrolyte or ionic liquid is a common technique to explore transport in two-dimensional (2D) materials. The doping relies on the formation of an EDL due to the accumulation of mobile ions at the interface between an electrolyte and a 2D semiconductor. The induced image charge yields large capacitance densities (1-10 μF/cm2) and sheet carrier densities (~1014 cm-2) in 2D crystal field-effect transistors (FETs).1 Our group has developed a monolayer electrolyte that is a single molecule thick and can electrostatically dope the surface of 2D semiconductors.2 The monolayer electrolyte has several advantages over liquid-phase electrolytes because it is solid state, bistability has been demonstrated,2 and DFT calculations predict a low switching voltages (sub-volt) at fast speeds (sub-ns).3 We have demonstrated EDL doping on graphene2 and MoS2 using the monolayer electrolyte. Bistability, state retention and a sheet carrier density of 3.2×1012 cm-2 have been demonstrated. In this study, WSe2 is selected as the 2D channel because it is ambipolar with a sizable bandgap (1.4 eV), large on/off ratio (> 106) and high carrier mobility (~140 cm2V-1s-1).4 The monolayer electrolyte consists of cobalt crown ether phthalocyanine (CoCrPc) and lithium perchlorate (LiClO4) (Fig.1 (a)). CoCrPc is an atomically thin and electrically insulating molecule (band gap ~1.34 eV) that can lie flat on a 2D surface.5 It includes four crown ethers, and each of them can solvate one Li+.6 An ordered array of CoCrPc can be deposited on a 2D material by drop-casting and annealing.5 Back-gated WSe2 FETs were fabricated by electron beam lithography (EBL), and the monolayer electrolyte was deposited on the surface of the WSe2 FETs by drop-casting and annealing. The WSe2 channel surface was cleaned by contact-mode AFM to remove e-beam resist residue before electrolyte deposition to avoid interference to the packing arrangement of the monolayer electrolyte. The quality of the cleaning is confirmed by AFM topography and surface roughness measurements; the roughness after the cleaning equals the value of freshly exfoliated flakes (Rq ~0.22 nm) (Fig.1 (c) and (d)). The cleaning does not deteriorate the back-gated device performance, as indicated by transfer characteristics after the AFM cleaning (Fig.1 (b), no monolayer electrolyte). The device transfer characteristics before and after doping with the monolayer electrolyte will be presented, along with our work on top-gating the devices using h-BN and graphene to create an all-2D EDL FET. References Kim, Se Hyun, et al. Advanced Materials 25.13 (2013): 1822-1846.Xu, Ke, et al. ACS nano (2017).Wang, Wei-Hua, et al. Solid State Ionics 301 (2017): 176-181.Fang, Hui, et al. Nano letters 12.7 (2012): 3788-3792.Lu, H., et al. The Journal of Physical Chemistry C 119.38 (2015): 21992-22000.De, S., et al. Journal of Molecular Structure: THEOCHEM 941.1 (2010): 90-101. ACKNOWLEDGEMENTS: This work was supported by NSF-ECCS/GOALI 1408425 and by the Center for Low Energy Systems Technology (LEAST), a STARnet Semiconductor Research Corporation program sponsored by MARCO and DARPA. Figure 1

Low-power logic computing realized in a single electric-double-layer MoS2 transistor gated with polymer electrolyte

Due to its mechanical flexibility, large bandgap and carrier mobility, atomically thin molybdenum disulphide (MoS2) has attracted widespread attention. However, it still lacks a facile route to fabricate a low-power high-performance logic gates/circuits before it gets the real application. Herein, we reported a facile and environment-friendly method to establish the low-power logic function in a single MoS2 field-effect transistor (FET) configuration gated with a polymer electrolyte. Such low-power and high-performance MoS2 FET can be implemented by using water-soluble polyvinyl alcohol (PVA) polymer as proton-conducting electric-double-layer (EDL) dielectric layer. It exhibited an ultra-low voltage (1.5 V) and a good performance with a high current on/off ratio (Ion/off) of 1 × 105, a large electron mobility (µ) of 47.5 cm2/V s, and a small subthreshold swing (S) of 0.26 V/dec, respectively. The inverter can be realized by using such a single MoS2 EDL FET with a gain of ∼4 at the operation voltage of only ∼1 V. Most importantly, the neuronal AND logic computing can be also demonstrated by using such a double-lateral-gate single MoS2 EDL transistor. These results show an effective step for future applications of 2D MoS2 FETs for integrated electronic engineering and low-energy environment-friendly green electronics.

Enhancing photoresponse by synergy of electric double layer gate and illumination in single CuTCNQ NW field effect transistors

<p class="BodyText1"><span lang="EN-IN">We report that photoresponse of </span><span lang="EN-US">a single metal-organic charge transfer complex Cu:TCNQ nanowire (NW)</span><span lang="EN-IN"> can be enhanced simultaneously by illumination as well as applying a gate bias in an Electric Double Layer Field Effect Transistor (EDL-FET) configuration fabricated on </span><span lang="EN-US">Cu:TCNQ </span><span lang="EN-IN">as a channel.</span><span lang="EN-IN">It is observed that applying a bias using an EDL gate to a n-channel Cu:TCNQ single NW FET, one can enhance the photoresponse of the Cu:TCNQ substantially over that which arise from the photoconductive response alone. </span><span lang="EN-US">Electron-hole pairs that generate in the NW under illuminated of wavelength 400nm gives rise photo current. Also, electric double layer induce negative charges in the NW channel which effectively increases the carrier concentration, contributing to better response in conduction. </span><span lang="EN-IN">The effect reported here has a generic nature that gives rise to a class of gated photodetectors of different photoresponsive materials.</span></p>

イオン液体/ルブレン単結晶界面の周波数変調AFMによる構造解析

The structural properties of ionic liquid/rubrene single-crystal interfaces were investigated using frequency modulation atomic force microscopy. The spontaneous dissolution of rubrene molecules into the ionic liquid was triggered by surface defects such as an oxidized rubrene or a vacancy, leading to the formation of a clean interface irrespective of the initial conditions. Force curve measurements revealed that a few solvation layers of ionic liquid molecules formed at the interface. We have also measured electric characteristics of electric double layer field-effect transistor based on the ionic liquid/rubrene single crystal interfaces. In contrast to usual devices, the mobility of field induced career was found to gradually increase with time for a day, consistent with the time scale of the spontaneous dissolution. These specific properties are discussed with respect to the microscopic understanding of electric double-layer transistors.

Carrier mobility and scattering lifetime in electric double-layer gated few-layer graphene

We fabricate electric double-layer field-effect transistor (EDL-FET) devices on mechanically exfoliated few-layer graphene. We exploit the large capacitance of a polymeric electrolyte to study the transport properties of three, four and five-layer samples under a large induced surface charge density both above and below the glass transition temperature of the polymer. We find that the carrier mobility shows a strong asymmetry between the hole and electron doping regime. We then employ ab initio density functional theory (DFT) calculations to determine the average scattering lifetime from the experimental data. We explain its peculiar dependence on the carrier density in terms of the specific properties of the electrolyte we used in our experiments.

Gradual improvements of charge carrier mobility at ionic liquid/rubrene single crystal interfaces

We report evolution of electric characteristics of an electric double layer field-effect transistor based on the ionic liquid/rubrene single crystal interfaces. In contrast to usual devices, the field effect mobility was found to gradually increase with time for a day, followed by minor long-term fluctuations. Although the details of the evolution were somewhat device dependent, the final values of the mobility turned out to be 3–4 times larger irrespective of the initial values. These observations are explained by the evolution of the flat interface by defect-induced spontaneous dissolution of rubrene molecules at the ionic liquid/rubrene single crystal interfaces, revealed by frequency modulation atomic force microscopy.

Field Effect Control of <inline-formula> <tex-math notation="TeX">$T_{\rm c}$</tex-math></inline-formula> in Y-Ba-Cu-O Electric Double Layer Transistors

Electric-double-layer field-effect-transistors (EDLFETs) were fabricated using 10.5-12.9 nm thick YBa 2 Cu 3 O 7 (YBCO) films and N, N-Diethyl-Nmethyl-N-(2-methoxyethyl) ammonium bis (trifluoromethanesulfonyl) imide (DEME-TFSI), and critical temperature (T c ) and vortex behavior were evaluated in the devices. Decrease in sheet resistance and increase in Tc were observed when negative gate voltage (V G ) was applied, and opposite variation was observed in the case of positive gate voltage. At V G = 0 V, T c onset was ~70 K, and T c0 was ~47 K. V G = -5 V resulted in T c onset of 83.3 K and T c0 of 61.8 K, and those in V G = +2 V were 65.0 K and 40.0 K. To discuss carrier doping mechanism, activation energy for vortex motion and irreversibility temperature were measured at V G = 0 V and -3 V. Activation energy and irreversibility temperature were improved by negative gate voltage. This shows that carrier doping was performed throughout thickness of film since vortices moved with length scale larger than coherence length. This suggests that electrochemical reaction such as oxygen ion injection strongly affected carrier density in the present devices.

Electric-double-layer transistors with thin crystals of FeSe1−xTex (x = 0.9 and 1.0)

Field-effect transistor (FET) devices using thin crystals of FeSe1−xTex (x = 0.9 and 1.0) have been fabricated with an electric-double-layer (EDL) capacitor. Despite the presence of substantial quantities of electron and hole carriers in the bulk due to the semimetallic electronic structure of FeSe1−xTex, we have observed p-channel depletion-type FET characteristics, in contrast to the n-channel normally on FET characteristics of a Bi2Se3 EDL FET. In FeSe1−xTex, the mobile carriers, holes, are depleted in the channel region by accumulating electrons, resulting in a decrease in conductivity. This result is consistent with the experimentally observed positive Hall coefficient at room temperature.

Creating Novel Transport Properties in Electric Double Layer Field Effect Transistors Based on Layered Materials

ABSTRACTWe present a study on the liquid/solid interface, which can be electrostatically doped to a high carrier density (n~1014 cm-2) by electric-double-layer gating. Using micro-cleavage technique on the layered materials: ZrNCl and graphene, atomically flat channel surfaces can be easily prepared. Intrinsic high carrier density transport regime is accessed at the channel interface of electric double-layer field effect transistor, where novel transport properties are unveiled as the field-induced superconductivity on the ZrNCl with high transition temperature at 15 K, and accessing a high carrier density up to 2×1014 cm-2 in graphene and its multi-layers.

Electric Double Layer Gate Field-Effect Transistors Based on Si

Electric double layer field-effect transistors (EDL-FETs) were fabricated using single crystal Si wafer as the active semiconductor and various characteristics were studied including dynamic response against step-function gate bias. The static FET mobility was more than 100 cm2 V-1 s-1. The response time of the drain current was 20 µs for ionic liquid and 3 ms for poly(ethylene glycol) (PEG) solution of LiBF4. Unexpected fast response was observed at a certain “speed up bias” condition. This effect will be useful to switching circuits using EDL-FETs.

Electric Double Layer Transistor of Organic Semiconductor Crystals in a Four-Probe Configuration

Electric double layer field-effect transistors (EDL-FETs) of rubrene single crystals using a liquid polymer electrolyte were studied in a four-probe configuration. The channel sheet resistance and the contact resistance normalized by the contact width were 0.24 MΩ and 0.83 kΩ cm in the ON-state, respectively. These values are smaller than those of previously reported conventional rubrene crystal FETs, corroborating the high carrier density accumulation in the EDL-FET. In addition, peculiar peak behaviour in the transfer characteristics did not appear in the gate voltage dependence of four-probe conductivity. This observation leads us to conclude that the peak is ascribed to the increase of the contact resistance in the high gate voltage region.