Electric-double-layer-gated 2D transistors for bioinspired sensors and neuromorphic devices

ABSTRACT Electric double layer (EDL) gating is a technique in which ions in an electrolyte modulate the charge transport in an electronic material through electrical field effects. A sub-nanogap capacitor is induced at the interface of electrolyte/semiconductor under the external electrical field and the capacitor has an ultrahigh capacitance density (~µF cm−2). Recently, EDL gating technique, as an interfacial gating, is widely used in two-dimensional (2D) crystals for various sophisticated materials characterization and device applications. This review introduces the EDL-gated transistors based on 2D materials and their applications in the field of bioinspired optoelectronic detection, sensing, logic circuits, and neuromorphic computation. GRAPHICAL ABSTRACT


Introduction
Human life has changed a lot since the first transistor was made successfully by William Shockley, John Bardeen and Walter Brattain at Bell Labs in 1947 [1].Transistor is a semiconductor device using as amplifier or electronic switch, which is the basic building block for standardizing the operation of computers, cell phones and other modern electronic circuits [2,3].And transistor is widely used in large-scale and ultra-large-scale integrated circuits by virtue of its high input impedance, low noise, good thermal stability, and simple manufacturing process [4].However, such a high level of integration greatly increases the energy consumption and the heat dissipation.Many approaches have been used to reduce the power consumption of transistors, such as choosing high-κ dielectrics [5,6], electric double layer gating [7,8], improving contact [9,10], and 2D semiconductor materials [11,12].Among these methods, electric double-layer gating shows its promising future in reducing power consumption for its high field.
Electric double layer (EDL) proposed by Helmholtz in 1853 is an important concept in electrochemistry [13].Electric double layer (EDL) gating is a technique using ions in electrolyte to control charge transport in an electronic material by field-effect.Unlike the metal oxide gate dielectric, electrolyte can allow ion conduction, although it is insulator for electrons [14].Under external electric field, cations and anions move rapidly in the electrolyte, producing a strong accumulation of space charge at the interface of electrolyte/semiconductor, forming an electric double layer which has a capacitance as high as 500 µF/cm 2 , and can induce a charge density which is up to > 10 13 /cm 2 [15].Many devices have been studied based on the superior performance of EDL, such as supercapacitors [16,17], printed electronics on paper [18], electrolyte gated thin-film transistors [19], biosensors [20], multifunctional sensory platform [21], etc.As a gate, the electric double layer can effectively modulate the carrier concentration in the semiconductor and reduce the operating voltage of the transistor.EDL-gating is an interfacial technique, which is especially effective on two-dimensional (2D) crystals for its high specific surface area.
In 2D transistors, different 2D materials can be stacked, forming heterostructure transistors, owing to the absence of dangling bonds on their surface, which provide new avenues for the development of various types of devices [32].EDL-gated transistors based on 2D materials combining the advantages of electric double layer and 2D materials pave the way for large integrated circuits with low power consumption.
In this review, we mainly investigate the operation mechanism of EDL-gated transistors based on 2D materials and present the cutting-edge applications.Section 2 first describes the device structure and working mechanism of the EDL-gated transistors with 2D materials.Then, Section 3 gives a discussion on four typical applications of EDL-gated transistors with 2D materials.As shown in Figure 1, the four applications of EDL-gated 2D transistors in photoelectric detection, sensing, logic circuits, and neuromorphic computation have been summarized.The purpose of this review is to provide the latest research works on EDL-gated 2D transistors to researchers entering this field for the first time.Finally, we have also given a summary and perspective for the future development of EDLgated transistors with 2D materials.

EDL-gated transistors
Ion gels are the primary and most common material used to form electric-double-layer transistors.This section focuses on the working principle of ionic gels, thus further introducing EDL gated transistors.

Ionic gels
Ionic gel is a kind of solid mixture with ionic conductivity, which is usually composed of high molecular organic polymer and salt electrolyte materials that can be electrolyzed into ions [33,34].The investigation of ionic liquids has developed vigorously since the fabrication of 1-ethyl-3-methylimidazolium tetrafluoroborate ([EMIM][BF4]) with a good stability in air by Wilkes et al. in1992 [35].Today, ionic gel has received more and more attention due to its excellent flexibility and compatibility, low vapor pressure, high ionic conductivity, good temperature and chemical stability [36][37][38].In ionic gels, it has a wide selection of anions and cations (such as quaternary ammonium salts, phosphates, imidazoles and pyridines, etc.). Figure 2 shows the structure of the cations and anions traditionally used for ionic liquids (ILs).The point to be emphasized here is that the different properties of ILs are due to different combinations of anions and cations.Depending on the type of solvent in the polymer network, ionic gels can be divided into aqueous and non-aqueous ionic gels.Aqueous ionic gels, hydrogels can be achieved by polymerizing vinyl monomers in an electrolyte solution in the presence of a cross-linker and initiator.Non-aqueous ionic gels are normally prepared by solubilizing polymer networks in organic electrolyte solutions [39][40][41][42][43].

Device structures
There are two kinds of EDL-gated 2D transistors.One is planar transistor, and the other is vertical transistor.Figure 3a shows the structure of planar EDL-gated 2D transistor, with a major feature of coplanar source, drain and gate.This kind of structure can be used for liquid or gel electrolytes.It is easy to prepare these kinds of devices.Firstly, the source, drain, and gate are deposited simultaneously on the substrate.Then the electrolyte is deposited on top of the electrodes by spin coating or other methods.
The other kind of EDL-gated 2D transistors has a vertical structure as shown in Figure 3b, in which the gate, source and drain are not in the same plane.The source and drain electrodes are first prepared on the channel materials, and then the electrolyte is deposited on the top of the electrodes by spin coating or other methods.Finally, the gate is fabricated on the electrolyte.

Operation mechanisms
As is known, there are two basic operation mechanisms for electrolyte-gated transistors (EGTs), including electrostatic doping and electrochemical doping, respectively.The electrostatic doping is the main mechanism for EDL transistors, while the electrochemical doping is widely adopted for electrochemical transistors.The biggest difference between the two operation mechanisms is the semiconductor channel material.For EDL transistors, the channel materials are impermeable semiconductors.In contrast, for electrochemical transistors, the channel materials are permeable semiconductor materials.When applying a gate voltage, EDL can be formed at the gateelectrolyte and semiconductor-electrolyte interfaces for the EDL transistors.For electrochemical transistors, there is only EDL formed at the gate/electrolyte interface under applied gate voltage, while the ions on the semiconductor side will diffuse into the semiconductor film and compensate for the induced charge carriers.This review mainly focuses on EDL transistors based on 2D materials and explore their mechanisms and applications [44].
EDL-gated transistors have similar structure and voltage bias mode as the traditional metal -oxide -semiconductor field-effect transistors (MOSFETs).Unlike traditional MOSFETs in which an insulating gate dielectric is used, EDL-gated transistors use an electrolyte, which is electron insulating and ion conductive, as the gate dielectric.The different gate dielectric materials make EDL-gated transistors have different working mechanisms from MOSFETs.MOSFETs work based on gate electric field, which is coupled to semiconductor channel, resulting in electrostatic doping of carriers in the semiconductor channels.This is a pure capacitive charging process and involves only the movement of electrons.In fact, EDL-gated transistors are developed based on the traditional transistors.These new transistors are similar to traditional MOSFETs in function.The channel current of EDL-gated transistors is also controlled by the gate voltage with a difference that the movement of ions in the electrolyte takes the place of a certain gate voltage of MOSFETs.
Figure 3(c-e) explains the detailed working mechanism and energy band bending of the ion-gel transistor.At the beginning (flat-band state), there are no external potential is applied to the transistor through the ion gel dielectrics (V G = 0).The ions of the ionic gel electrolyte disperse freely, the charge carrier density in the channel maintains the pristine value and the conduction band and valence band of the transistor are not bent which keeps in the flat-band state (Figure 3c).When applying a negative potential (V G <0), the transferred negative charge repels the anion to the interface between the ionic gel and the channel, inducing an increase in hole concentration in the channel, as shown in Figure 3d.In this state, the Fermi level of transistors shifts downwards and lead to a downinclination of conduction/valence band.When applying a positive potential (V G > 0), the transferred positive charge repels the cation to the interface between the ionic gel and the channel material, resulting in the accumulation of cations at the interface between the ionic gel and the two-dimensional material, inducing electrons in the channel, as shown in Figure 3e.In this state, the Fermi level of transistors moves upwards and leads to an upinclined conduction/valence band.

The application of EDL-gated transistors
The EDL-gated 2D transistor is a new transistor technology based on the EDL effect with ultrahigh capacitance and the resultant ultra-strong electrostatic field coupled to transistor channel.It has shown broad application prospects in the fields of optical detection, sensing applications, logic devices, and neuromorphic computing.In the field of optical detection, the EDL-gated 2D transistor exhibits high sensitivity and fast response characteristics, enabling efficient detection and conversion of optical signals.It has been intensively used for optical communication, optoelectronics, and optical sensing.For sensing applications, the high specific surface area and charge response characteristics of the EDL-gated 2D transistor make it an excellent sensor capable of highly sensitive detection of external changes.In terms of logic devices, the EDL-gated 2D transistor features low power consumption, high speed, and high voltage gain, making it an important new logic device in the field of microelectronics.In the area of neuromorphic computing, the intrinsic hysteresis properties and high-sensitivity characteristics of the EDL-gated 2D transistor make it an excellent neural device, suitable for constructing artificial neural network and for neuroscientific research.In the following section, we highlight four applications of EDL-gated 2D transistors in photoelectric detection, sensing, logic circuits, and neuromorphic computation in recent years.

Photoelectric devices
Photoelectric device is developed according to the photoelectric effect, which plays an important role in modern electronics [45].They can convert elusive light signals into easily detectable electrical signals.Then, the light signals can be captured, identified and visualized by photoelectric devices, such as photodetectors, which are widely utilized in both civilian and military applications [46][47][48].For example, ultraviolet (UV) photodetectors have been used in environmental monitoring [49,50] and military reconnaissance [51,52]; visible light detectors are used for optical communication [53,54]; wide-range infrared photodetectors are used for night vision [55,56].
In recent years, two-dimensional transition metal dichalcogenides have gained much attention because of their enhanced optical radiation and tunable bandgap for broadband optical detection [57,58].Nevertheless, 2D photoelectric transistors typically operate at high voltages due to the limited electrostatic modulation behavior of traditional gate dielectrics.In this low voltage scenario, 2D photoelectric transistors show very good merits.
Figure 4a shows an ion-gel gate graphene/MoSe 2 heterostructure device fabricated by Gwangtaek et al. [59].The device has a high on/off ratio (3.3 × 10 4 ) and ambipolar behavior tuned by external gate voltage.Compared with other devices, it has a higher external quantum efficiency (66.3%) and responsivity (285.0 mA/W).These advantages of the device making it suitable for highly efficient photocurrent generation and photodetection.
Shen et al. reported a monolayer MoS 2 phototransistor with ion-gel gating.Polyvinylidene fluoride and 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl) imide (PVDF-EMIM-TFSI) ion-gel was used as the electrolyte, showing very good performance (Figure 4b) [60].The ionic gels have very good transparency, about 88-95% in the range of 400 to 1200 nm.The MoS 2 ion-gel transistor has ambipolar transport property due to the high capacitance of electric double layer.And it shows an abnormal negative photoelectrical effect in both the electron branch and hole branch caused by coupling effect of gate voltage and illumination.The photosensitivity of this phototransistor can be extended from visible to short-range infrared region.At 1200 nm illumination, the photoresponsivity of MoS 2 ion-gel phototransistor can even reach 0.90 A/W with a detectivity of 1.88 × 10 11 Jones.The authors declared that this photodetector has a great potential for applications in communications and wearable medicine.

Sensory devices
With the rapid development of Internet of Things (IoTs), sensors and sensing systems have also been developed rapidly [61].Low power consumption and self-powered sensing systems are becoming increasingly important [62,63].Recently, ionic gel electrolyte materials are widely used to transistors for sensing because their large capacitance enables operation at low-voltage [64,65].
Low-power-consuming sensors are important for the applications in the field of Internet of Things.Liu et al. demonstrated an ion gel coated graphene field-effect transistor (GFET) for humidity sensing [66].It is important to note that the ion gel not only works as a gate dielectric layer but also as a humidity sensing layer.The sensitivity of relative humidity (RH), is 0.0014 in the low-sensitivity regime (<71% RH), and 0.0135 in the high-sensitivity regime (>71% RH).The real-time respiratory monitoring is shown in Figure 5a.Exhalation results in a change in the concentration of water molecules surrounding the mouth, which enables ion-gated GFET generating a response signal.The ion-gel gated GEFT humidity sensors can potentially be used for real-time respiratory monitoring in medical diagnostics and emergency assistance.
Triboelectric nanogenerator (TENG) got rapid development since it was first reported in 2012 [67][68][69][70].Combining TENG with transistors forms a new device called tribotronic transistor and combining TENG with ion-gel transistor forms a new device called triboiontronic transistor [71][72][73][74][75][76]. Figure 5b shows a mechanosensation-active matrix based on a direct-contact tribotronic planar graphene transistor array fabricated by Meng et al. [77].The device can recognize the distance of objects approaching, identify different classes of materials, and even distinguish the sound.The tribotronic planar graphene transistor array has a very good sensing performance with a sensitivity of 0.16 mm −1 , a response time about 15 ms, and an excellent durability.The authors believe that such devices have great potential for human-machine interfaces, electronic artificial skin, multifunctional sensors and smart wearable devices.
To better explore the tribotronic graphene transistor, Zhang et al. designed another electric double layer (EDL) gated graphene transistor which is shown in Figure 5c [78].The device is dual-mode field effect transistor.One mode is tribotronic transistor with capacitively coupled ion gel and the other model is ion-gel-gated graphene transistor with a second tribotronic gate.The performance of the device worked in the first mode is better than graphene transistor (on-state current is twice than the original, on/off ratio is four times higher, the field effect mobility increases by ten times).The device can be used as a multiparameter distance sensor (drain current and threshold voltage).When the TENG displacements change by 0.25 mm, the channel current increases sharply by ∼600 μA and the threshold voltage shifts by ∼0.8 V.The novel EDL-gated FETs are believed to be able to inspire more diverse and versatile functional devices to surpass Moore's Law.
With the rapid development of TENG, its applications are becoming more and more widespread.Usman et al. introduced a graphene tribotronic touch sensor which can work with very low operational voltages, as shown in Figure 5d [79].In this system, the potential generated by TENG acts as a gate bias to the ion-gel graphene FET and modulates its current transmission.The sensitivity of the sensor is about 2% kPa −1 , detection limit <1 kPa, response time is around 30 ms.The authors predict that graphene tribotronic devices can work well in electronic skins and touch screens.
PENG also plays an important role in the field of self-powered and multifunctional sensing.Sun et al. described an active-matrix strain sensor array combining piezoelectric nanogenerators (NGs) with coplanar-gate graphene transistors (GTs), as shown in Figure 5e [80].They have a high sensitivity (measurement factor = 389), detection limit (0.008%) and excellent device durability (>3000 bend-release cycles).In addition, transparent and stretchable strain sensors were fabricated on PDMS substrates for monitoring human hand movements.This work plays a significant role in advancing the field of selfpowered sensing artificial skin.

Logic devices
Moore's Law, which has guided the semiconductor industry since 1965, predicts that the concentration of devices on a chip will double every 18 months [81,82].For more than half a century, silicon logic transistors have been used as the construction units of processing and memory in traditional von Neumann systems [83,84].With Moore's Law meeting a bottleneck in size reduction, personalized and multifunctional electronics that integrate 2D materials and self-powered technologies are a new trend in scientific investigation [85].Logic circuits implemented with transistors are the basic units in integrated circuits.They have a broad application in computers, digital control circuits, communication transmission and instrumental devices [72,[86][87][88].
With the development of material science and nanotechnology, EDL gating technique is emerging in the field of logic devices.Choi and coworkers reported an ion-gel-gated low-voltage vertical field-effect transistor (VFET) composed by MoS 2 and graphene or WSe 2 and graphene (Figure 6a) [89].Then they fabricated low-power complementary inverters based on n-type graphene -MoS 2 and p-type graphene -WSe 2 VFETs.When the operating voltage is below 3 V, the devices show good n-type and p-type characteristics with high current density (>3000 A cm −2 ) and on/off current ratio (>10 4 ).The low-power consumption of ion-gel-gated MoS 2 -graphene and graphene -WSe 2 VFET inverter means that these devices can be used as the basis of integrated logic circuits.The ion-gel gated VFETs are promising in transparent, flexible and low-voltage nanoelectronics.
Based on the unique properties of 2D layered ReS 2 , Dathbun et al. investigated the fabrication of large-area ion-gel gated ReS 2 transistors with graphene electrodes, as shown in Figure 6b [88].The ionic gel electrolytes make the transistors present excellent performance, such as high on-current at a voltage below 2 V, high electron mobility, high on/off current ration and good device durability.Finally, the transistors were assembled into logic gate devices such as NOT, AND and NOR gates.Benefited from its architecture, ReS 2 transistor can be fabricated into a wide variety of complex functional digital circuits.
With the rapid development of TENG, triboiontronic transistors have attracted lots of attention from the researchers.Gao et al. reported a triboiontronic MoS 2 FET device (Figure 6c) [74], which is composed of TENG and EDL-gated MoS 2 FET.The potential induced by TENG is used to modulate the concentration of electron and hole in the channel.The triboiontronic MoS 2 transistor shows excellent performance (high current on/off ratio, low threshold value, and steep switching properties).They also fabricated a logic inverter with good gain, low power consumption, and high robustness, based on the triboiontronic transistors.The triboiontronic transistors have great potential for human-computer interaction, electronic skins and smart wearable devices.
Using 2D materials, memories and sensors can be integrated into a single device, which opens a new way to achieve multifunctional neuromorphic computing, especially for artificial perceptual systems.The mechanism of the EDL-gated synaptic transistor is roughly described as follows: protons are driven by presynaptic spikes and move from the electrolytes to the channel surface, then forming an electrostatic gate [32,99].The slow movement of protons through the electrolyte causes specific transistor current to emulate some biological synaptic properties.The first report for neuromorphic EDL transistor based on 2D MoS 2 is proposed by Wan and his coauthors [100].Their team prepares 2D MoS 2 synaptic/neuronal transistors using polyvinyl alcohol as a laterally coupled proton conducting electrolyte.This device successfully simulates basic synaptic functions such as excitatory postsynaptic currents, paired-pulse facilitation, and dynamic filtering of biological synaptic information transmission.Significantly, multiple-input gates and a modulation gate enable synapsedependent logic operations/modulation, multiplicative neural coding and neuronal gain modulation, which are demonstrated experimentally as well.This work is of significant importance in guiding the preparation of 2D materials based artificial synaptic transistors and the study of brain-like computation.
Sharbati et al. fabricated an electrochemical EDL-gated graphene transistor, as shown in Figure 7a [101].The devices have low power consumption (<500 fJ per switching activity), accurate and reversible command of resistance, analog tunability (>250 distinct states), good durability and holding performance, and linear symmetric resistive response.The electrochemical synapses demonstrated basic neuronal functions, such as short-term potentiation/depression, long-term potentiation/depression, and spike timingdependent plasticity.It provides opportunities for hardware implementation of fully functional artificial neural networks for neuromorphic computing.
Photo-synaptic transistors integrate optical detection, memory and processing elements into a single device, which complements the shortcomings in neuromorphic sensing and enriches the functional diversity of neuromorphic information devices.Jiang et al. demonstrated a versatile photoelectronic hybrid-integrated synaptic device which is based on EDL-gated MoS 2 phototransistor (Figure 7b) [102].In this paper, researchers use biopolymer electrolyte (sodium alginate) as the electrolyte and potential synaptic effects are realized in the hybrid devices.The devices can be used as a high-pass electrical filter for sophisticated signal processing in bio-activity.Most importantly, the optoelectronic and spatiotemporal 4D hybrid neuromorphic integration is achieved by simulating non-Hebb rules and Hebb rules using the optoelectronic hybrid integration method.This work has important implications for the realization of multifunctional neural networks in the field of optoelectronic hybrid nanoelectronics and demonstrates the great potential of such devices for complex neuromorphic computing.
Cheng et al. demonstrated a novel neuromorphic-photoelectric device (Figure 7c) [103].The optoelectronic-neuromorphic device was fabricated by 0D-CsPbBr 3 -quantum-dots/2D-MoS 2 heterojunction and ion gel electrolyte.These devices have strong carrier transport capacity due to the superior light absorption capability of calcium titanate quantum dots.Basic neuronal functions were also successfully exhibited by the devices, such as excitatory post-synaptic current (EPSC), paired-pulse facilitation (PPF), long-term memory/short term memory (STM/LTM) and dynamic filtering.The proposed device has the potential to be used in the next generation of brain-like optoelectronic human-computer interaction and cognitive systems.
As Figure 8a shows, a piezotronic ion-gel graphene transistor which has the function of artificial sensory synapse was fabricated by Sun and coworkers [90].One of the highlights of this work is the piezoelectric potential relates spatiotemporal strain information (strain amplitude and duration) to postsynaptic currents.The typical properties of synapses are perfectly simulated in this work.A negative or positive piezoelectricity induced by the deformation of P(VDF-TrFE) film applied to the device to modulate the concentration and type of the transport particles.The authors also demonstrated spatiotemporally correlated strain stimulation by coupling two parallel presynaptic terminals into a gel-gated synaptic transistor and established dynamic modulation relationships.This piezoelectric synaptic device sheds new light on flexible smart sensors for mechanical stimulation, self- powered electronic skin with artificial intelligence, and neuromorphic interfaces for neural robots.
In the field of tribotronics, ion-gel is the most common electrolyte to form EDL. It is interesting to note that Sun and coworkers use proton conductor to induce EDL and fabricate a versatile triboiontronic transistor (Figure 8b) [75], which have a high current on/off ratio over 10 6 , low cutoff current (∼0.04 pA), and steep switching properties (89 μm/dec).The authors used the device to fabricate logic devices and an artificial sensory neuron system.The sensory neuron system successfully demonstrated a mechanical behavior assisted visual imaging system.The new devices can be used for humanmachine interaction, E-skin and smart wearables.
Building on previous work, Sun and coworkers fabricated another device which is a contact-electrification (or triboelectrification)-activated artificial afferent neuron with femtojoule energy (Figure 8c) [111].The device includes TENG and synaptic transistor which can imitate the function of human perception system.When external stimuli are applied to the TENG mechanoreceptor, the triboelectric signal activates the postsynaptic transistors and endows artificial afferents with adaptive capacity.This work represents a promising strategy for the development of future generations of bio-nanotechnology, as well as low-power neuromorphic devices, directly interactive electronic prostheses, and even neuro-robotics.

Summary and Perspective
In summary, we have reviewed the applications of EDL-gated transistors based on 2D materials in the field of photoelectric detection, sensing, logic circuits, neuromorphic computation.EDL-gated transistors have much advantages (low operating voltage, high current modulation capability, fast response speed, low power consumption).The main advantage of EDL-gated transistors is the formation of EDL capacitors that enable EDLgated transistors to be driven at much lower voltages than conventional electronic transistors.This advantage makes it possible to develop flexible devices and other lowpower devices.
But there are a lot of challenges for EDL-gated transistors that need to be solved by researchers' tireless efforts.

Security issue
For sensors used in wearable devices, no harm to the human body is very important.However, the preparation of electrolyte (such as ion gels) requires large amounts of hazardous organic solvents, which are not favored in biomedical applications.This requires the development of some new production processes to reduce the introduction of harmful substances.

Stability and repeatability
Stability and repeatability are critical for practical applications.Some EDL-gated transistors are susceptible to moisture in air that affects performance, it is essential to research on novel packaging of EDL-gated devices in order to improve the durability and stability of the devices.Besides, some metal electrodes may be corroded by electrolytes such as ionic liquids, so it is necessary to find new electrode materials.

Energy consumption issues
The ability to realize an artificial neural system with integrated storage and computation has the potential to break the Moore's Law bottleneck.However, current progress in this field is still limited to a few neural or synaptic devices, and there is no unified standard for the manufacture of these devices.Besides, although the energy consumption of these neuromorphic devices is much lower than that of conventional transistors, it is still higher than the biological level.
In conclusion, EDL-gated transistor is an important advancement in the field of electronics, but these challenges also require the tireless efforts of human beings to make EDL-gated transistor better for human beings.

Disclosure statement
No potential conflict of interest was reported by the author(s).

Figure 2 .
Figure 2. The type of ionic-gel structure.Three different types of ionic-gel composition structure and main ionic-gel anion and cation types.

Figure 3 .
Figure 3. EDL-gating mechanisms on 2D FETs in various configurations and their band diagrams.(a) Co-plane structure and (b) lateral TFT structure.(c) No electric field is applied, the distribution state of ions in the electrolyte and its band diagram.(d) Negative electric field is applied, the distribution state of ions in the electrolyte and its band diagram.(e) Positive electric field is applied, the distribution state of ions in the electrolyte and its band diagram.

Figure 5 .
Figure 5. Applications of sensors formed by EDL-gated transistors.(a) Ion gel coated graphene field effect transistor for humidity sensing applications (b) Mechanosensation-active matrix based on direct-contact tribotronic planar graphene transistor array.(c) Ion gel capacitively coupled tribotronic gating for multiparameter distance sensing.(d) A graphene tribotronic touch sensor that is based on coplanar coupling of a single-electrode-mode triboelectric nanogenerator (S-TENG) and a graphene FET.(e) Active matrix electronic skin strain sensor based on piezopotential-powered graphene transistors.

Figure 6 .
Figure 6.Applications of logical circuits formed by EDL-gated transistors.(a) Low-voltage VFETs and complementary inverters based on graphene -TMDC heterostructures and ion-gel gate dielectrics.(b) Large-area CVD-grown sub-2 V ReS 2 transistors and logic gates.(c) A triboiontronic FET of molybdenum disulfide (MoS 2 ) and an triboiontronic logic inverter.