Diversified regulation of electrons have received much attention to realize a multi-functional transistor, and it is crucial to have a considerable control over the charge carriers in transistors. Ionic gel, as the dielectric material in transistors, facilitates a large capacitance, and high induced-carrier concentrations. This review presents the recent progress in ionic-gated transistors (IGTs) that have good mechanical stability as well as high physical and chemical stability. We first briefly introduce the various applications of IGTs in sensors, neuromorphic transistors, organic transistor circuits, and health detection. Finally, the future perspectives of IGTs are discussed and some possible solutions to the challenges are also proposed.
In the past decade, with the rapid development of the Internet of Things and consumer electronics, human demand for high performance, portability, and wear-ability pushed the upgrade of transistor integration density. However, due to the tunneling effect and other problems, this trend of continuing to use Moore’s Law will inevitably slow down. Therefore, in order to develop more promising integrated circuit applications in the post-Moore era, tremendous efforts are being invested to develop materials and structures aspects to improve the performance of field-effect transistors(FETs)1,2,3. Many inorganic materials with high dielectric constant have been extensively studied, like HfO24 and Al2O35,6. Unfortunately, the high dielectric constant causes the carrier mobility decreased in the transistor because of Fröhlich polarons7.
Meanwhile, a kind of transistor with ionic gate, ionic-gated transistors (IGTs) has attracted widespread attention owing to its large capacitance, high carrier-inducing ability, and low operating voltage8. The function of IGT is similar to that of traditional MOSFET, and the channel current of IGT is also controlled by the gate voltage9. The difference is that IGT replaces the dielectric material with the electrolyte material. When a negative voltage is applied to the gate electrode, cations in the electrolyte accumulate at the gate/electrolyte interface under the electrostatic mechanism, forming a capacitor layer Cge that is opposite to the charge of the gate electrode. At the same time, a capacitor layer Ces is also formed at the channel layer/electrolyte interface. Since the layer spacing of the two significant parallel plate capacitors is ~1 nm, the capacitance effect is significantly greater than that of general insulating layer materials8,10. This type of IGT based on an electric double layer is also named electric double layer transistor (EDLT). In addition, the channel layer material may also allow electrolyte ion implantation, thereby causing electrochemical doping11. Due to the typical EDL effect, the large surface capacitance allows the IGT to operate well at a relatively low operating voltage12 and maintain good switching performance13. Nowadays, the research of ion-gated transistors is booming in various disciplines, and the ion materials include electrolyte solutions14,15,16 and ionic liquids7,17,18,19 with liquid fluidity; as well as ions/proton-conducting polymers20,21,22, and inorganic materials23. As a kind of electrolyte, ionic gel not only has good physical/chemical stability, but also has the characteristics of flexibility, lightweight, and transparency. Therefore, in the field of wearable electronic devices, IGT has demonstrated unquestionable adaptability, portability, and functionality. These characteristics have aroused the research enthusiasm for the application of ionic-gel to IGT. In addition, it is well known that the nervous system controls the activities of the human body through electrical and chemical signals. IGT has the potential advantage of being compatible with biological signals due to its similar electrical characteristics, structure flexible, and low toxic. IGT has also been studied in the context of neuromorphology, memory, and synaptic devices, and is expected to be applied in the field of artificial intelligence24,25,26,27.
In this review, we will briefly list material choices, and introduce the conventional and emerging applications of IGTs in flexible electronics. Recent articles on the sensing function, neuromorphic, and IC function of IGTs will be discussed in detail. The schematic device structure of IGT applications is summarized in Fig. 1. Finally, the future development and problems of IGT sensing and neuromorphic systems are summarized in Section 6.
Materials aspect of ionic-gel-gated transistor
Tremendous efforts have been invested in IGTs, and the dielectric materials consist of polymer electrolytes, polyelectrolytes, ILs, ionic gels, and inorganic nanogranular materials28. Polymer electrolytes and polyelectrolytes have shown an excellent capacitance performance, whereas, they also exhibit a slow ion migration, which limits the application of IGTs in high-speed electronic devices20,29. ILs have good gating functions and faster responses; however, their intrinsic liquid state hinders the use of ILs in practical devices7. Therefore, as an alternative to ILs, ionic gels solve the problem by combining polymer mechanical compatibility and swift switching response. An ionic gel is a semi-solid compound that retains charge owing to the insertion of an IL into the polymer during the drying stage. The colloidal particles in the mixture prevent it from curing completely, but are in the form of a gel. Due to their excellent chemical stability and transmittance, these materials are also widely used as battery electrolyte materials. As a result, they are also named as electrolyte gated transistors (EGT). In the following section, we describe ILs and the polymer grid used in IGTs.
ILs are generally known as room-temperature molten salt. Owing to the low vapor pressure, high ionic conductivity, and good temperature (up to 350 °C), and chemical stabilities, ILs are more suitable for polymer-in-salt systems. In 1992, since Wilkes et al. prepared 1-ethyl-3-methylimidazole tetrafluoroborate ([EMIM][BF4]) with good stability in the air, the research on ionic liquids has flourished30. Ionic gels have a wide selection of anions31,32,33,34, so ionic gels are usually classified according to the types of cations, including quaternary ammonium35,36, phosphonium37, Imidazole38, and pyridine39, etc. Imidazole-based ionic liquids are widely used in emerging electronic devices due to their generally low melting point and many optional substituents40,41. Figure 2 shows the structures of cations and anions conventionally used in IL, and the various properties of IL are determined by the combination of different anions and oxygen ions. Generally, most of the studies on the potential applications of ionic gel have advanced toward electrical and electrochemical devices, including sensors42, super capacitors43, batteries32,39,44,45,46, actuators47,48, electrochromic devices41,49, solar batteries50,51, FET52, and optimization of thermoelectric materials using IGTs22. The manufacturing method, materials, and performance of the reported nanoscale IGTs are compared in Table 1.
ILs, such as [EMIM][TFSI]53,54, [EMIM][TFSA]55,56, and [BMIM][PF6]57, have been reported to be widely used and have good performance. Cho et al. used a chemically stable ionic liquid, [EMIM] [TFSI], and photocurable mixture material to prepare patterned graphene transistors under ultraviolet (UV) radiation58. The presence of ionic liquid not only maintains stability but also exhibits excellent surface capacitance and polarization response speed. Besides being extremely sensitive to external bending and deformation (GF~389), the system can also implement a large-area, highly integrated array on a flexible substrate for motion detection.
However, for in vivo applications, these ILs have not been proven to be safe. Considering these conditions, Tang et al. reported a biocompatible IGT composed of poly(ε-decalactone)-b-poly(DLlactide)-b-poly(ε-decalactone) (DLD) and [P14][TFSI]37. The minimum capacitance of the 20% ionic gel could reach approximately 2 μF cm−2, ensuring that the system could be operated under a low voltage. The prepared IGT using P3HT as the channel layer showed good characteristics for printed electronic applications: 1 cm2 V−1 s−1 mobility, an Ion/Ioff ratio of 105, and low hysteresis. Besides, it showed promising hydrolytic degradation. More recently, Kim et al. reported an ionic gel material made of choline and malic acid as raw materials, and the obtained dielectric layer has good biocompatibility and biodegradability59.
The flexibility of material selection makes it possible to choose different polymers to prepare ionic-gel gate transistors. For most IGTs, polymer monomers are embedded into ionic solutions composed of polymer molecular segments, and the commonly used materials are mainly self-assembled tri-block and di-block co-polymers, such as poly(styrene-block-ethylene oxideblock-styrene) (SOS)53,60, poly(ε-decalactone)-b-poly(DL-lactide)-b-poly(ε-decalactone) (DLD)37, and poly(styrene-b-methylmethacrylate-b-styrene) (SMS)61,62.
The first ionic-liquid-based gel used in an organic film transistor was produced by Lee et al.57 They chose a tri-block co-polymer, (SOS), as the gel network and dissolved it with 1-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF6]) into methylene chloride (Shown in Fig. 3a). Tri-block polymers such as SOS have special structural and chemical properties; the entire structure is divided into two parts: can be dissolved (the middle) and cannot be dissolved (on two sides) in IL. Thus, they are collectively referred to as ABA tri-block polymers. This characteristic makes ABA tri-block co-polymers suitable for the preparation of ionic gels. This easily leads to the design and preparation of an ionic gel that can be utilized to construct various transistors with different requirements. In addition to SOS, other ABA tri-block co-polymers, such as SMS, are also widely used in the preparation of ionic gels61.
According to application scenarios and preparation processes, many chain polymers are also used to prepare IGTs. A transistor that uses P (VDF-HFP) and a gel gate dielectric layer has the characteristics of easy transfer. There is a transistor using P (VDF-HFP) that can easily transfer the gate insulating layer through a “cut and paste” operation55. Except good solvent compatibility, the gel material not only has high tension and structural stability, but also exhibits similar properties to triblock polymers in some specific aspects. For example, PVA, (P(VDF-HFP)) can also be used to form gels with [EMIM][TFSA] ILs. Compared with PS-PMMA-PS gel, their capacitance values decrease with increasing frequency. The results (Fig. 3b) show that the capacitance value of P(VDF-HFP) gel is approximately 1 μF lower than that of SMS gel at a low frequency (10 Hz), however it is slightly higher than that of the SMS gel at a high frequency (100 kHz)56. This proves that the dielectric functions of the two EDL dielectric materials are comparable. To achieve good implantability of flexible electronic devices, biocompatibility must also be ensured for the materials used in co-polymers. Therefore, some researchers use biocompatible materials, such as polysaccharides59, chitosan63,64, and proteins65 instead of cross-linked polymers66. As mentioned before, Kim et al. proposed an ionic gel made of Lewandan polysaccharide and choline-based water-soluble IL cast by a simple solution method (shown in Fig. 3c). This dielectric has the characteristics of good biocompatibility, biodegradability, high transparency, free stranding, and solid state59. Besides, Yang et al. dissolved wheat flour and acetic acid in water and prepared a wheat flour electrolyte membrane by a drop-casting method. The resulting IZO-based thin-film transistor exhibited a switching ratio of 105, and have excellent saturation mobility (15 cm2 V−1 s−1). Accordingly, they realized the simulation of synaptic functions, such as excitatory postsynaptic current (EPSC) and paired-pulse facilitation (PPF)67.
Furthermore, some mixed copolymer has more application compatibility and enables realize photo-crosslinking, specific recognition, acid-base resistance, hydrophilic, and hydrophobic control, etc. by controlling the internal components. A UV-curable ionic gel can easily realize the preparation of ultra-tiny IGTs68. In addition, Liu et al. also prepared a fully stretchable polarized transistor by controlling the PFPE content in the PFPE-DMA gate dielectric layer. They demonstrated a skin-like display that is formed by integrating stretchable TFT array and stretchable organic light-emitting electrochemical cells (OLECs) (Fig. 3d)69. Through the preparation method compatible with traditional MEMS technology, the application of IGTs in fields, like health monitoring and logic operation, will be further expanded.
Sensing applications of ionic-gate transistors (IGTs)
In 1970, Bergveld et al. discovered that after removing the gate of the FET and immersing it in a liquid, the ions in the insulating layer would affect the conductivity of the channel layer. Therefore, the original IGT was used as a sensitive ion sensor and used in physiological detection. Nowadays, IGT has become a hot research topic among multidisciplinary areas including electronics, neuromorphology, biology, and engineering. Various exciting applications of IGT have been developed continuously. For example, due to the excellent light transmittance of ionic gels, IGT also has broad application prospects in the field of optics10. The properties of IGT, such as stable structure, stable chemical properties, and stretchability, have attracted much attention in the field of wearable electronics. Not only can it be used as a mechanical sensor to sense pressure, but it can also be used as a proximity sensor or smart sensor in sensing, thereby improving signal processing efficiency and reducing system power consumption.
In the field of optoelectronics, phototransistors and optical modulators take full advantage of IGTs70. Phototransistors that receive optical signals and convert them into electrical signals can detect light in various bands, therefore, enabling applications of night vision, three-dimensional object recognition, health monitoring, and optical communications71,72. Owing to their three-terminal configurations, phototransistors can effectively reduce noise and amplify electrical signals. Meanwhile, ionic gel electrolytes are introduced due to its high capacitance and transparency to solve the problem of high operating voltages73. Thus, the ionic gel has been used to achieve low operation voltage and high carrier concentration, while the low mobility of the ions and formation of the layered effect can help realize optical modulation.
In order to obtain a strong electric field at the interface between the gate and the channel, EDLT is a promising choice, which can generate high carrier density at low pressure. Many studies have used IGTs to achieve electrical and optical multi-modal control of the channel layer, and further applied to neuromorphological research55. Much more interesting, Ionic-gel-based photonic devices with simple processing and high gate-capacitance, dynamically regulate the graphene chemical potential (Fermi level) (Fig. 4)74. The charge density is electrically tuned at the ionic gel–graphene interface, and the chemical potential of graphene is changed to control its intensity of light absorption. At 1.55 μm wavelength, the transverse-magnetic (TM) mode is increased from −10.8 dB to −8.8 dB, and the transverse electric (TE) mode is reduced from −9.0 dB to −12.1 dB. Depending on these data, the waveguide sample with an ionic-gel gate dielectric can be used as a broadband TM transmission polarizer. In addition, the optical behavior of the ion-gated modulator clearly shows hysteresis due to the slow polarization response time.
Further, a research study has recently demonstrated that photoluminescence (PL) can be reversibly tuned by EDLTs up to 1–2 orders of magnitude at a low gate voltage (Fig. 4c, d)75. The density of electron traps via the interaction of trap with oxygen ion at Vg < 0 is reduced, owing to the transverse gate-electric-field generated by ionic gels, leading to lower probabilities of non-radiative recombination and photogenerated carriers with increased mobility, and accordingly, the PL density is enhanced. The function of reversible control of perovskite luminescence can be advantageous to the potential applications of photons in perovskite materials.
Moreover, optical frequency combs are the basis of modern frequency measurement, astronomical observation, precision spectroscopy, ultrafast optics, and quantum information76,77. Yao et al. pioneered the application of IGTs in graphene-based tunable optical frequency combs (Fig. 4e)78. By coupling the ionic-gel gate-tunable light guide to the silicon nitride photonic microresonator, the second- and higher-order dispersion can be modulated by tuning the Fermi level. They have successfully demonstrated a double-layer IGT that can adjust the Fermi level of graphene to 0.45–0.65 eV, besides, the double-layer IGT is able to retain a cavity mass coefficient of up to 106. Resulting from merging single atomic layer nanoscience with ultrafast optoelectronics, this heterogeneous graphene microcavity can further enhance our understanding of dynamic frequency combs and ultrafast optoelectronics.
The FET-type pressure device is one of the basic component of electronic skin devices that achieve advanced sensing performance, including multi-parameter tracking, high sensitivity, high resolution, and low crosstalk signaling79,80,81,82. In an earlier study, ionic-gel-based FETs have been observed to function by inducing mobile charges in the semi-conductor channel and are utilized to fabricate pressure sensors with low power consumption and high transparency58,83. For example, Khademhosseini et al. demonstrate an IGT-based pressure sensor84. The dielectric layer is fabricated by microstructured ionic hydrogel, shown in Fig. 5a–d. As external pressure is applied to the top of the transistor, the microstructure hydrogel deforms and changes the capacitance of the hydrogel/gate interface, as a result, affecting the number of ions in the channel. Due to the outstanding amplify capability of the organic electrochemistry transistor (OECT), the pressure sensor can detect pressure as low as 20 Pa in this manner. Furthermore, the IGT operates at a lower voltage (0–1 V) due to the high capacitance of the EDL effect, and the entire power consume of sensing device used is only 10–1000 μW. Based on these two advantages, ionic gel-based pressure sensors are also competitive in wearable and implantable sensor applications.
Cho et al. proposed a structure for the fabrication of pressure sensor matrices with a simple process and excellent performance83. The ionic-gel connects the drain and the gate, and the ionic-gel gate capacitance under the double capacitance effect is large (7.9 μF cm−2), which ensures that the graphene FET works at a low gate voltage of less than 2 V. Between the source and drain electrodes, the interdigitated electrode at the bottom is in contact with the graphene backplate under external pressure, resulting in a change in resistance (shown in Fig. 5e). It exhibited high-pressure sensitivity (0.12 kPa−1) and outstanding mechanical durability (more than 2500 cycles). With the help of this structure, the fabrication of common-gate array devices can be easily realized. The pressures on the 4 × 4 GFET matrices can be mapped spatially (Fig. 5f).
However, the use of transistors as direct pressure sensors is limited by structural design difficulties and poor stability, prompting researchers to turn to other methods. The piezoelectric effect is defined as the electric polarization of the piezoelectric material that originates from materials that lack inversion symmetry, caused by the action of external mechanical forces; piezoelectric materials convert random mechanical energy into electric energy (piezopotential power)85,86. The piezotronic transistor, utilizing inner-crystal piezopotential to harvests biochemical and biomechanical energy87,88, while achieving self-powered active digital data processing capacities, such as pressure arrays with high resolution and human-machine interaction89,90.
Ionic-gel electrolyte materials are extensively applied in piezotronic transistors owing to their large capacitance that enables low-voltage transistor operation91,92. The long-range polarization characteristics of ions in the ionic gel allow the placement of the gate electrode and channel to be coplanar. Sun et al.58 described a piezopotential-powered active strain sensor matrix array combining piezoelectric nanogenerators (PENGs) and coplanar-gate graphene transistors based on P(VDF-TrFE), as shown in Fig. 5h. The output pressure sensor signal is maintained at the corresponding value based on the applied strain. The ionic-gel dielectrics can effectively couple the piezoelectric potential to the graphene channel, thereby maintaining the output sensor signal at the corresponding stress value. As a result, excellent sensitivity (GF = 389), ultra-high detection limit (0.008%), and high mechanical durability (> 3000 cycles) are obtained (Fig. 5i). They further developed a piezopotential-programmed non-volatile memory array by integrating ionic-gel-gated FETs and PENGs93. The storage device is programmed by applying external pressure to the PENG, and by grounding the top electrode of the PENG. In contrast, it is easy to flash the stored data by grounding the upper electrode of the PENG (Fig. 5g). Under various external bending strains, multi-level data storage of more than 3 orders of magnitude is achieved with a good memory efficiency, including a high programming/erasing current ratio (> 103), 2-bit multi-level data storage (over 4 levels), efficiency reliability over 100 cycles, and reliable data retention over 3000 s.
With the advent of the Internet of Things (IoTs), non-invasive measurements and real-time health monitoring have become increasingly important in modern medicine, particularly to address the urgent needs of the aging population40. In the process of medical monitoring and diagnosis, equipment with high-quality signals and long-term stability is essential94. In addition, soft devices with good physiological comfort and biological compatibility can effectively minimize the harm to the human body and discomfort of wearing2,95,96,97,98,99. IGTs have the key advantages of low threshold voltages, high amplification, and biocompatibility100,101 that ensure an accurate diagnosis of physical illness. Therefore, IGTs that are intrinsically stretchable, adequately meet the needs of physiological monitoring.
In conventional methods, medical monitoring and diagnosis mainly rely on physiological signals (blood pressure, heart rate, and respiratory rate) and the real-time concentration of biochemical molecules (Na+, K+, glucose). Transistors can achieve an effective amplification of these data and thereby effectively improve the sensitivity of medical sensors. For example, Someya et al.102 fabricated a biocompatible electrode by integrating a gelatinous composite into transistor amplifiers to realize a direct detection of pericardial electrocardiography (Fig. 6a). One-pot fabrication of a conductive gel that is responsive to glucose without enzyme, is integrated into OECTs to detect the patient’s glucose level clinically103. Alcohol dehydrogenase (ADH) and its nicotinamide adenine dinucleotide (NAD+) cofactor are readily immobilized on the OECT to detect ethanol in the range of 0.01 % to 0.2 % blood alcohol content (Fig. 6b)104.
Although gel-based transistors have realized various applications in the medical field, there is still a lot of scope for improvement. The first problem that needs to be solved in this field is the requirement of the high-level accuracy of physiological signals to provide an accurate diagnosis, and this is dependent on the SNR (signal-to-noise ratio)105,106. To obtain a high SNR, a preamplifier may be integrated in close proximity to the sensing electrode to reduce the influence of the wiring noise, thereby realizing active elements with local signal amplifications102,107. The structure of OECT without the gate dielectric layer makes direct electrolytic operation possible, allowing the conversion amplifier to be directly loaded at the location of the target, thereby largely reducing the ratio of noise. Someya et al.108 designed an OECT-based electrophysiological transducer with a high SNR (24 dB). They successfully solved the issue of the inability of transistors to operate for a long time due to electrolyte drying, by integrating an ultra-thin OECT with a nonvolatile gel electrolyte, as shown in Fig. 6c. OECT made of thin gels can adhere well to the skin and effectively monitor the heart signal. It demonstrated stable performance during long-term continuous monitoring (for example, within 3 h) and multiple reuses in tests, lasting more than a week (Fig. 6d).
Another major achievement is the technological advances in flexible and biocompatible materials that not only allow patients to have a better wearing experience, but also facilitates real-time monitoring. Unlike previous studies that have fabricated ultra-thin inorganic semi-conductors or have designed “wavy” structures to achieve flexibility109,110, intrinsically stretchable conductive polymers have become a growing trend. Kim et al.59 synthesized biocompatible, biodegradable, highly transparent, free-standing, and solid-state electrolytes consisting of levan polysaccharide and choline-based water-soluble IL (as shown in Fig. 6e). Levan polysaccharides function as free-standing and solid-state polymeric matrices that are translucent, versatile, and water-soluble, while choline-type water-soluble IL reacts with malic acid, utilizing choline as the cation and malic acid as the anion108. Biodegradable electrolyte-based organic transistors exhibit satisfactory performance, such as low operating voltage (Vds = −1.0 V and Vgs = −2.0 V), good stability, and high on/off ratio (over 103). Organic transistors were incorporated with bio-integrated instruments, such as electrocardiogram (ECG) recordings on human skin and rat heart with a better signal-to-noise ratio (shown in Fig. 6f).
A thread-based transistor (TBT) realized by electrolyte gating of semiconducting carbon nanotube networks on linen threads was proposed by Sameer et al. The thread has an interchangeable module to fit the biological tissue required, can be minimally invasive (for example, sutures), and enables the provision of three-dimensional tissues or organs with a natural interface111. The electrolyte is composed of colloidally scattered nanoparticles of silica and IL gel (a form of an ionic gel) that enables CNT transistors to be electrostatically gated. To achieve a multiplexed diagnostic system based on complete threads, they linked TBTs with thread-based electrochemical sensors (TBEs). As seen in Fig. 6g, completely threaded platforms are thin, extremely durable, and compatible, allowing them to be worn directly on the skin without any silicone substrate or without using a skin needle. As mentioned above, most ionic gels consisting of polymers and ILs are not proven to be biocompatible and biodegradable, and some of them are even toxic to humans. Therefore, it is necessary to apply natural biomaterials such as cellulose or hydrogels with non-toxic synthesis procedures.
Neuromorphology applications of ionic-gate transistors (IGTs)
Another emerging application of IGT is neuromorphic devices, particularly synaptic transistors. In order to simulate the human brain, much efforts have been invested in the ability of parallel processing of data and weight modulation through algorithms and software on the existing computer hardware112,113. However, the human brain performs better than traditional computers while processing real-time sensory data such as images, videos, sounds, and navigation. In addition, compared with the high energy consumption of supercomputers, low energy consumption is also a major advantage of the biological brain114. Synapse transistor is imitated as a synapse, where two neurons are connected functionally, and they are also the key parts of signal transmission, as shown in Fig. 7a. The human brain is a complex network composed of nearly 1011 neurons and 1015 synapses that are highly interconnected, massively parallel, and structurally variable115. Synaptic plasticity, as a functional feature of nerve cell interconnection, gives biology the ability to learn and remember. The appearance of hysteresis of EDLT devices limits its switching speed, and is not suitable as a conventional transistor, but is more suitable as a functional device that mimics synaptic transmission. The bionic simulation of brain nerve structure provides many feasible research scopes. Based on the phenomena of dendrites receiving multi-synaptic signals, Wan et al. controlled the information transmission of neuromorphic transistors through multiple in-plane gates116 (Fig. 7b). Owing to the different distances between the source and drain electrodes and the multiple gates, the spatial position of each gate on the plane is defined by different coordinates. Obviously, the spatial coordinate of the gate closer to the channel layer contributes more significantly to the weight of the transistor. The mapping of distance and EPSC contribution is shown in Fig. 7c. They present EPSC current diagrams for different stimulation directions and the I/O relationship between the rate coding scheme and neurons in different directions. They further verified the influence of stimuli directivity on multi-gate neuromorphic transistors (Fig. 7d).
Real-time control can be achieved through a programmable circuit design. John et al. also utilized P(VDF-HFP) and [EMIM][TFSI] as the dielectric materials to prepare neuromorphic memtransistor elements25. By defining different gate voltages as different harmful inputs, they successfully used IGTs to simulate the modulation of the salient weight of pain receptors. Similar to bio-nociceptors, the satellite threshold adjusting receptors (STARs) can adjust the transfer characteristic curve of the device according to the intensity of the harmful stimulus (Vgs) to make it more sensitive and avoid repeated damage. When Vgs is higher than the threshold voltage and the duration increases, significant transistor drift occurs, as shown in Fig. 8a–f. After a period of time or application of the recovery voltage, the threshold characteristic of IGT can be restored to its original level, as shown in Fig. 8e. The robot can change its sensitivity after receiving harmful stimuli and can realize self-healing and functional recovery after being damaged. The decentralized intelligence they proposed is different from traditional centralized intelligence and is more suitable for the requirements of future robot intelligence in terms of power consumption and application prospects.
Owing to the inherent flexibility of ionic gels, some flexible neuromorphic transistors have also been reported. Shim et al. fabricated rubber-like synaptic transistors using full rubbery elastomeric materials26. Based on the all-rubber materials, this rubber synaptic transistor has realized synaptic behaviors such as pair-pulse facilitation (PPF), synaptic filtering, and current enhancement after excitation. They studied the excitatory postsynaptic current drops from 82.21 µA to 77.15, 44.54, and 41.8 µA without damage, when the rubber synaptic transistor was stretched by 10% and 30% to 50%, as shown in Fig. 8h. With an increase in the pulse width at the frequency of 5 Hz, the synaptic device exhibits the functions of sensory memory, short-term memory, and long-term memory (Fig. 8i). Figure 8j shows that the flexible synaptic transistor is excited at 20 Hz, and the excitatory postsynaptic current response of 0% and 50% strain is significantly different. They defined the gain strength of A20/A1 and used it to measure the effect of strain. Obviously, the strength of the gain is also related to the pulse frequency, pulse width, and pulse peak value. They further used triboelectric nanogenerator as power supplies to build an integrated soft adaptive neural robot (Fig. 8k). This soft neurorobotic system receives external pressure stimuli and produces bending movements, which is suitable for the application of building humanoid robots in the future.
Many inorganic nanomaterials or organic semi-conductor materials have excellent response characteristics to light waves of different wavelengths10,117. Further, inspired by biological visual and nervous systems, Sun et al. demonstrated a flexible, dual-modulation, tunable synaptic FET based on ZnO nanowires as the channel, and sodium alginate as the gate118. IGT is combined with ZnO that has an excellent UV light response, it can also output a higher current than the normal state after multiple UV light stimulations. The optical image, transfer characteristics, and optical responses of the synaptic field-effect transistor (SFET) on a PI substrate are shown in Fig. 9b. The threshold voltage Vth of the SFET moves to the left under the various forces of UV radiation, demonstrating the implementation potential of the system in terms of artificial optical synapses. The light spike stimulates the transistor as the action of a positive gate voltage. A matrix of SFETs has been manufactured and verified to investigate the uses of SFETs in artificial visual systems (As shown in Fig. 9d).
IC applications of ionic-gate transistors (IGTs)
Much effort has been made in applying IGT to IC functions, like inverter, which is the basic function of advanced integrated circuits119. The most commonly used structures of inverters in recent years are two forms: single-component FET and complementary FET120. The single-component FET inverter is consists of one FET and a resistor. Frisbie et al. reported an EDL-FET using SMS ionic-gel as gate dielectric on a poly (ethylenenaphthalate) (PEN) substrate (Fig. 10a). They adopted whole organic material to fabricate this low-cost OFET, instead of Au. The switching Vth is zero, and completed within 0.5 V, shows a high gain at 7121. Furthermore, they used IGT to prepare a CMOS inverter with high response speed and low operating voltage. Frisbie et al used stenciling technology and traditional photolithography to prepare SEAS-N3 ionic-gel gate dielectric layer transistors122 (Fig. 10c). The fabricated transistor is powered by a power supply sub-3 V and the inverter stage delay is close to 50 µs. More importantly, they claim that through further optimization, especially in terms of the ion conductivity of gel, the stage delay can be shortened to the order of 1 µs. There is no doubt that this will greatly expand the potential applications of ionic-gel transistor circuits.
In addition to the functional devices of traditional integrated circuits such as inverters, ionic-gel transistors can also be used as organic memory devices due to their reverse ion characteristics123,124,125. Koo et al. reported nonvolatile transistor memories (NvTMs) devices that depend on the formation of EDL126 (Shown in Fig. 10e). The IGZO film pattern was prepared by spin coating and UV curing. The transfer characteristics of Au nanoparticles device are shown in Fig. 10f. They separately analyzed the induction of the gate on the IGZO surface through the EDL effect under the write voltage Vprog and the erase voltage Vera. Colloidal gold nanoparticles are used at the interface of IGZO, and the organic amine ligand molecules therein serve as the nano-scale tunneling dielectric of the gold nanoparticle core and the IGZO channel, which are not present in the evaporated gold nanoparticles. The tunneling dielectric delays the charge transfer between the Au nanoparticles and IGZO channel, and has a better retention effect on the programming/erasing signal. The memory window obtained when the program/erase voltage is 9 V, which is summarized in Fig. 10g as a function of the colloidal dispersion concentration to form the gold nanoparticle layer. It is verified that an increase in the coverage area of gold nanoparticles on IGZO is beneficial to storing more charges. More importantly, the preservation characteristics of the memory are improved relative to the nanoparticles prepared by thermal evaporation due to the position of the insulating organic ligand inserted between the IGZO channel and the gold nanoparticle sheet. Figure 10h shows the retention characteristics of the programming and erasing signals of the IGZO nonvolatile memory prepared with colloidal gold nanoparticles (5 mg ml−1). Ion-gel gated IGZO NvTM with multiple programming/erase functions showed stable transfer characteristics in a series of cycling tests (Fig. 10i). Obviously, the drive storage device with the participation of electrolyte has a different working mode and control ability from the previous device.
We have reviewed the advances in the realization of the emerging IGTs technology. As mentioned before, the carriers induced by the large EDL-capacitance enable IGTs to be driven at much lower voltages than traditional electronic transistors, which has shown the potential contribution of IGTs in addressing energy security challenges. However, there are un-matched challenges for the processing and preparation of the ionic-gel layers for mass-scale industrialization. Spin coating, printing, and other methods have the problem of contaminating the semi-conductor layer material of the transistor, while the transfer methods generally have low processing accuracy, and the task load is indeed heavy when dealing with high-precision systems. Merging various technologies (R2R combination transfer), maybe more suitable for rapid and low-cost preparation and application of high-precision IGTs, however further research is needed in terms of material adhesion treatment and precision control.
The good compatibility of ionic-gel materials with organic FETs further improves the shortcomings of flexible electronic technology in terms of dielectric materials. The integration of IGTs with portable power can serve a wide range of IoTs applications such as health monitoring and industrial sensing. In addition, integrated neuromorphic transistors can also provide technical support for artificial intelligence and soft robotics. Although the development of neuromorphic devices has attracted much attention, current progress is still limited to a few nerve or synaptic devices, and the functions they realize are all around the basic biological nerve functions. In addition, the energy consumption is much lower than traditional transistors, but still higher than the biological level. Finally, the multi-input neuromorphic transistor provides an alternative model for simulating the multiple connections of biological nerves, the weight modulation between multiple gates is also an important area for further research. To summarize, whether it is the application of the ultra-high electric field facilitated by the EDL characteristics in low-dimensional systems or the realization of biological neural networks through neuromorphic devices, its impact on scientific research and practical applications is subversive.
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This work was supported by the National Science Foundation of China (NSFC, Grant No. 61625404, 61874111, 61888102, 62022079), Young Elite Scientists Sponsorship Program by CAST (2018QNRC001), and the Youth Innovation Promotion Association of Chinese Academy of Sciences under Grant No. 2020115.
The authors declare no competing interests.
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Wang, D., Zhao, S., Yin, R. et al. Recent advanced applications of ion-gel in ionic-gated transistor. npj Flex Electron 5, 13 (2021). https://doi.org/10.1038/s41528-021-00110-2
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