Systematic Doping Control of CVD Graphene Transistors with Functionalized Aromatic Self‐Assembled Monolayers

Recent reports have shown that self‐assembled monolayers (SAMs) can induce doping effects in graphene transistors. However, a lack of understanding persists surrounding the quantitative relationship between SAM molecular design and its effects on graphene. In order to facilitate the fabrication of next‐generation graphene‐based devices it is important to reliably and predictably control the properties of graphene without negatively impacting its intrinsic high performance. In this study, SAMs with varying dipole magnitudes/directions are utilized and these values are directly correlated to changes in performance seen in graphene transistors. It is found that, by knowing the z‐component of the SAM dipole, one can reliably predict the shift in graphene charge neutrality point after taking into account the influence of the metal electrodes (which also play a role in doping graphene). This relationship is verified through density functional theory and comprehensive device studies utilizing atomic force microscopy, X‐ray photoelectron spectroscopy, Raman spectroscopy, and electrical characterization of graphene transistors. It is shown that properties of graphene transistors can be predictably controlled with SAMs when considering the total doping environment. Additionally, it is found that methylthio‐terminated SAMs strongly interact with graphene allowing for a cleaner graphene transfer and enhanced charge mobility.     

Graphene Solution‐Gated Field‐Effect Transistor Array for Sensing Applications

Graphene, with its unique combination of physical and electronic properties, holds great promise for biosensor and bioelectronic applications. In this respect, the development of graphene solution‐gated field‐effect transistor (SGFET) arrays capable of operation in aqueous environments will establish the real potential of graphene in this rapidly emerging field. Here, we report on a facile route for the scalable fabrication of such graphene transistor arrays and provide a comprehensive characterization of their operation in aqueous electrolytes. An on‐chip structure for Hall‐effect measurements allows the direct determination of charge carrier concentrations and mobilities under electrolyte gate control. The effect of the solution‐gate potential on the electronic properties of graphene is explained using a model that considers the microscopic structure of water at the graphene/electrolyte interface. The graphene SGFETs exhibit a high transconductance and correspondingly high sensitivity, together with an effective gate noise as low as tens of μV. Our study demonstrates that graphene SGFETs, with their facile technology, high transconductance, and low noise promise to far outperform state‐of‐the‐art Si‐based devices for biosensor and bioelectronic applications.     

Contact Resistance in Organic Field‐Effect Transistors: Conquering the Barrier

Organic semiconductors have sparked interest as flexible, solution processable, and chemically tunable electronic materials. Improvements in charge carrier mobility put organic semiconductors in a competitive position for incorporation in a variety of (opto‐)electronic applications. One example is the organic field‐effect transistor (OFET), which is the fundamental building block of many applications based on organic semiconductors. While the semiconductor performance improvements opened up the possibilities for applying organic materials as active components in fast switching electrical devices, the ability to make good electrical contact hinders further development of deployable electronics. Additionally, inefficient contacts represent serious bottlenecks in identifying new electronic materials by inhibiting access to their intrinsic properties or providing misleading information. Recent work focused on the relationships of contact resistance with device architecture, applied voltage, metal and dielectric interfaces, has led to a steady reduction in contact resistance in OFETs. While impressive progress was made, contact resistance is still above the limits necessary to drive devices at the speed required for many active electronic components. Here, the origins of contact resistance and recent improvement in organic transistors are presented, with emphasis on the electric field and geometric considerations of charge injection in OFETs.     

Non‐Volatile Organic Memory Elements Based on Carbon‐Nanotube‐Enabled Vertical Field‐Effect Transistors

High‐performance non‐volatile memory elements based on carbon‐nanotube‐enabled vertical field‐effect transistors (CN‐VFETs) are demonstrated. A thin crosslinking polymer layer, benzocyclobutene (BCB), on top of the gate dielectric acts as the charge storage layer. This results in a large, fully gate sweep programmable, hysteresis in the cyclic transfer curves exhibiting on/off ratios >4 orders of magnitude. The carbon nanotube random network source electrode facilitates charge injection into the charge storage layer, realizing the strong memory effect without sacrificing mobility in the vertical channel. Given their intrinsically simple fabrication and compact size CN‐VFETs could provide a path to cost‐effective, high‐density organic memory devices.     

High Performance Multi‐Level Non‐Volatile Polymer Memory with Solution‐Blended Ferroelectric Polymer/High‐k Insulators for Low Voltage Operation

Polymer ferroelectric‐gate field effect transistors (Fe‐FETs) employing ferroelectric polymer thin films as gate insulators are highly attractive as a next‐generation non‐volatile memory. Furthermore, polymer Fe‐FETs have been recently of interest owing to their capability of storing data in more than 2 states in a single device, that is, they have multi‐level cell (MLC) operation potential for high density data storage. However, among a variety of technological issues of MLC polymer Fe‐FETs, the requirement of high voltage for cell operation is one of the most urgent problems. Here, a low voltage operating MLC polymer Fe‐FET memory with a high dielectric constant (k) ferroelectric polymer insulator is presented. Effective enhancement of capacitance of the ferroelectric gate insulator layer is achieved by a simple binary solution‐blend of a ferroelectric poly(vinylidene fluoride‐co‐trifluoroethylene) (PVDF‐TrFE) (k ≈ 8) with a relaxer high‐k poly(vinylidene‐fluoride–trifluoroethylene–chlorotrifluoroethylene) (PVDF‐TrFE‐CTFE) (k ≈ 18). At optimized conditions, a ferroelectric insulator with a PVDF‐TrFE/PVDF‐TrFE‐CTFE (10/5) blend composition enables the discrete six‐level multi‐state operation of a MLC Fe‐FET at a gate voltage sweep of ±18 V with excellent data retention and endurance of each state of more than 10⁴ s and 120 cycles, respectively.     

High‐Performance Dopamine Sensors Based on Whole‐Graphene Solution‐Gated Transistors

Solution‐gated graphene transistors with graphene as both channel and gate electrodes are fabricated for the first time and used as dopamine sensors with the detection limit down to 1 nM, which is three orders of magnitude better than that of conventional electrochemical measurements. The sensing mechanism is attributed to the change of effective gate voltage applied on the transistors induced by the electro‐oxidation of dopamine at the graphene gate electrodes. The interference from glucose, uric acid, and ascorbic acid on the dopamine sensor is characterized. The selectivity of the dopamine sensor is dramatically improved by modifying the gate electrode with a thin Nafion film by solution process. This work paves the way for developing many other biosensors based on the solution‐gated graphene transistors by specifically functionalizing the gate electrodes. Because the devices are mainly made of graphene, they are potentially low cost and ideal for high‐density integration as multifunctional sensor arrays.     

ReS2‐Based Field‐Effect Transistors and Photodetectors

Atomically thin 2D layered transition metal dichalcogenides (TMDs) have been extensively studied in recent years because of their appealing electrical and optical properties. Here, the fabrication of ReS₂ field‐effect transistors is reported via the encapsulation of ReS₂ nanosheets in a high‐κ Al₂O₃ dielectric environment. Low‐temperature transport measurements allow to observe a direct metal‐to‐insulator transition originating from strong electron–electron interactions. Remarkably, the photodetectors based on ReS₂ exhibit gate‐tunable photoresponsivity up to 16.14 A W^?1 and external quantum efficiency reaching 3168%, showing a competitive device performance to those reported in graphene, MoSe₂, GaS, and GaSe‐based photodetectors. This study unambiguously distinguishes ReS₂ as a new candidate for future applications in electronics and optoelectronics.     

Tuning Contact Resistance in Top‐Contact p‐Type and n‐Type Organic Field Effect Transistors by Self‐Generated Interlayers

Contact resistance significantly limits the performance of organic field‐effect transistors (OFETs). Positioning interlayers at the metal/organic interface can tune the effective work‐function and reduce contact resistance. Myriad techniques offer interlayer processing onto the metal pads in bottom‐contact OFETs. However, most methods are not suitable for deposition on organic films and incompatible with top‐contact OFET architectures. Here, a simple and versatile methodology is demonstrated for interlayer processing in both p‐ and n‐type devices that is also suitable for top‐contact OFETs. In this approach, judiciously selected interlayer molecules are co‐deposited as additives in the semiconducting polymer active layer. During top contact deposition, the additive molecules migrate from within the bulk film to the organic/metal interface due to additive‐metal interactions. Migration continues until a thin continuous interlayer is completed. Formation of the interlayer is confirmed by X‐ray photoelectron spectroscopy (XPS) and cross‐section scanning transmission electron microscopy (STEM), and its effect on contact resistance by device measurements and transfer line method (TLM) analysis. It is shown that self‐generated interlayers that reduce contact resistance in p‐type devices, increase that of n‐type devices, and vice versa, confirming the role of additives as interlayer materials that modulate the effective work‐function of the organic/metal interface.     

A Flexible Reduced Graphene Oxide Field‐Effect Transistor for Ultrasensitive Strain Sensing

A new kind of flexible strain sensor based on a reduced graphene oxide field‐effect transistor (rGO FET) with ultrasensitivity, stability, and repeatability for the detection of tensile and compressive strains is demonstrated. The novelty of the rGO FET strain sensor is the incorporation of a rGO channel as a sensing layer in which the electrical resistance can be greatly modified upon application of an extremely low level of strain resulting in an intrinsically amplified sensing signal. The rGO FET device is ultrasensitive to extremely low strain levels, as low as 0.02%. Due to weak coupling between adjacent nanosheets, therefore, upon applying small levels of strain into the rGO thin film, a modulation of the internanosheet resistance (R_inter) is expected, inducing a large change in the transconductance of the rGO FET. Using a simple printing and self‐assembly process, the facile fabrication of an rGO FET array over a large area is also demonstrated. In addition, the device can detect small and rapid physical movements of the human body.     

High Sensitivity Graphene Field Effect Transistor‐Based Detection of DNA Amplification

Enzymatic DNA amplification‐based approaches involving intercalating DNA‐binding fluorescent dyes and expensive optical detectors are the gold standard for nucleic acid detection. As components of a simplified and miniaturized system, conventional silicon‐based ion sensitive field effect transistors (ISFETs) that measure a decrease in pH due to the generation of pyrophosphates during DNA amplification have been previously reported. In this article, Bst polymerase in a loop‐mediated isothermal amplification (LAMP) reaction combined with target‐specific primers and crumpled graphene field effect transistors (gFETs) to electrically detect amplification by sensing the reduction in primers is used. Graphene is known to adsorb single‐stranded DNA due to noncovalent π–π bonds, but not double‐stranded DNA. This approach does not require any surface functionalization and allows the detection of primer concentrations at the endpoint of reactions. As recently demonstrated, the crumpled gFET over the conventional flat gFET sensors due to their superior sensitivity is chosen. The endpoint of amplification reaction with starting concentrations down to 8 × 10^?21 m in 90 min including the time of amplification and detection is detected. With its high sensitivity and small footprint, this platform will help bring complex lab‐based diagnostic and genotyping amplification assays to the point‐of‐care.     

Solution‐Processed Zinc Oxide as High‐Performance Air‐Stable Electron Injector in Organic Ambipolar Light‐Emitting Field‐Effect Transistors

Electron injection from the source–drain electrodes limits the performance of many n‐type organic field‐effect transistors (OFETs), particularly those based on organic semiconductors with electron affinities less than 3.5 eV. Here, it is shown that modification of gold source–drain electrodes with an overlying solution‐deposited, patterned layer of an n‐type metal oxide such as zinc oxide (ZnO) provides an efficient electron‐injecting contact, which avoids the use of unstable low‐work‐function metals and is compatible with high‐resolution patterning techniques such as photolithography. Ambipolar light‐emitting field‐effect transistors (LEFETs) based on green‐light‐emitting poly(9,9‐dioctylfluorene‐alt‐benzothiadiazole) (F8BT) and blue‐light‐emitting poly(9,9‐dioctylfluorene) (F8) with electron‐injecting gold/ZnO and hole‐injecting gold electrodes show significantly lower electron threshold voltages and several orders of magnitude higher ambipolar currents, and hence light emission intensities, than devices with bare gold electrodes. Moreover, different solution‐deposited metal oxide injection layers are compared. By spin‐coating ZnO from a low‐temperature precursor, processing temperatures could be reduced to 150 °C. Ultraviolet photoemission spectroscopy (UPS) shows that the improvement in transistor performance is due to reduction of the electron injection barrier at the interface between the organic semiconductor and ZnO/Au compared to bare gold electrodes.     

Detailed Characterization of Contact Resistance,Gate‐Bias‐Dependent Field‐Effect Mobility,and Short‐Channel Effects with Microscale Elastomeric Single‐Crystal Field‐Effect Transistors

The organic field‐effect transistor (OFET) has proven itself invaluable as both the fundamental element in organic circuits and the primary tool for the characterization of novel organic electronic materials. Crucial to the success of the OFET in each of these venues is a working understanding of the device physics that manifest themselves in the form of electrical characteristics. As commercial applications shift to smaller device dimensions and structure/property relationships become more refined, the understanding of these phenomena become increasingly critical. Here, we employ high‐performance, elastomeric, photolithographically patterned single‐crystal field‐effect transistors as tools for the characterization of short‐channel effects and bias‐dependent parasitic contact resistance and field‐effect mobility. Redundant characterization of devices at multiple channel lengths under a single crystal allow the morphology‐free analysis of these effects, which is carried out in the context of a device model previously reported. The data show remarkable consistency with our model, yielding fresh insight into each of these phenomena, as well as confirming the utility of our FET design.     

Toward High‐Output Organic Vertical Field Effect Transistors: Key Design Parameters

The performance of C₆₀‐based organic vertical field‐effect transistors (VFETs) is investigated as a function of key geometrical parameters to attain a better understanding of their operation mechanism and eventually to enhance their output current for maximal driving capability. To this end, a 2D device simulation is performed and compared with experimental results. The results reveal that the output current scales mostly with the width of its drain electrode, which is in essence equivalent to the channel width in conventional lateral‐channel transistors, but that of the source electrode and the thickness of C₆₀ layers underneath the source electrode also play subtle but important roles mainly due to the source contact‐limited behavior of the organic VFETs under study. With design strategies acquired from this study, a VFET with an on/off ratio of 5.5 × 10⁵ and on‐current corresponding to a channel length of near 1 μm in a conventional lateral‐channel organic field‐effect transistor (FET) is demonstrated, while the drain width of the VFET and the channel width of the lateral‐channel organic FET are the same.     

Vertical Organic Field‐Effect Transistors

Field‐effect transistors are the fundamental building blocks for electronic circuits and processors. Compared with inorganic transistors, organic field‐effect transistors (OFETs), featuring low cost, low weight, and easy fabrication, are attractive for large‐area flexible electronic devices. At present, OFETs with planar structures are widely investigated device structures in organic electronics and optoelectronics; however, they face enormous challenges in realizing large current density, fast operation speed, and outstanding mechanical flexibility for advancing their potential commercialized applications. In this context, vertical organic field‐effect transistors (VOFETs), composed of vertically stacked source/drain electrodes, could provide an effective approach for solving these questions due to their inherent small channel length and unique working principles. Since the first report of VOFETs in 2004, impressive progress has been witnessed in this field with the improvement of device performance. The aim of this review is to give a systematical summary of VOFETs with a special focus on device structure optimization for improved performance and potential applications demonstrated by VOFETs. An overview of the development of VOFETs along with current challenges and perspectives is also discussed. It is hoped that this review is timely and valuable for the next step in the rapid development of VOFETs and their related research fields.     

Sensing of Alkylating Agents Using Organic Field‐Effect Transistors

Alkylating agents are simple and reactive molecules that are commonly used in many and diverse fields, such as organic synthesis, medicine, and agriculture. Some highly reactive alkylating species are also being used as blister chemical warfare agents. The detection and identification of alkylating agent is not a trivial issue because of their high reactivity and simple structure. Here, a novel polythiophene derivative that is capable of reacting with alkylating agents is reported, along with its application in direct electrical sensing of alkylators using an organic field‐effect transistor, OFET, device. Upon reacting with alkylators, the OFET containing the new polythiophene analogue as its channel becomes conductive, and the gate effect is lost; this is in marked contrast to the response of the OFET to “innocent” vapors, such as alcohols and acetone. By following the drain–source current under gate bias, one can easily follow the processes of absorption of the analyte to the polythiophene channel and their subsequent reaction.     

Fully Integrated Microscale Quasi‐2D Crystalline Molecular Field‐Effect Transistors

Monolithic integration of microscale organic field‐effect transistors (micro‐OFETs) is the only and inevitable path toward low‐cost large‐area electronics and displays. However, to date, such an ultimate technology has not yet evolved due to challenges in positioning and patterning highly crystalline microscale molecular layers as well as in developing micrometer scale integration schemes. In this work, by mastering the local growth of molecular semiconductors on pre‐defined terraces, single‐crystal quasi‐2D molecular layers tens of square micrometers in size are created in dense periodic arrays on a Si substrate. Nondestructive photolithographic processes are developed to pattern micro‐OFETs with mobilities up to 34.6 cm² V^?1 s^?1. This work demonstrates the feasibility to integrate arrays of short‐channel micro‐OFETs into electronic circuitry by highly parallel and size scalable fabrication technologies.     

High‐Density Carrier Accumulation in ZnO Field‐Effect Transistors Gated by Electric Double Layers of Ionic Liquids

Very recently, electric‐field‐induced superconductivity in an insulator was realized by tuning charge carrier to a high density level (1 × 10¹⁴ cm^?2). To increase the maximum attainable carrier density for electrostatic tuning of electronic states in semiconductor field‐effect transistors is a hot issue but a big challenge. Here, ultrahigh density carrier accumulation is reported, in particular at low temperature, in a ZnO field‐effect transistor gated by electric double layers of ionic liquid (IL). This transistor, called an electric double layer transistor (EDLT), is found to exhibit very high transconductance and an ultrahigh carrier density in a fast, reversible, and reproducible manner. The room temperature capacitance of EDLTs is found to be as large as 34 µF cm^?2, deduced from Hall‐effect measurements, and is mainly responsible for the carrier density modulation in a very wide range. Importantly, the IL dielectric, with a supercooling property, is found to have charge‐accumulation capability even at low temperatures, reaching an ultrahigh carrier density of 8×10¹⁴ cm^?2 at 220 K and maintaining a density of 5.5×10¹⁴ cm^?2 at 1.8 K. This high carrier density of EDLTs is of great importance not only in practical device applications but also in fundamental research; for example, in the search for novel electronic phenomena, such as superconductivity, in oxide systems.     

Interlayer Band‐to‐Band Tunneling and Negative Differential Resistance in van der Waals BP/InSe Field‐Effect Transistors

Atomically thin layers of van der Waals (vdW) crystals offer an ideal material platform to realize tunnel field‐effect transistors (TFETs) that exploit the tunneling of charge carriers across the forbidden gap of a vdW heterojunction. This type of device requires a precise energy band alignment of the different layers of the junction to optimize the tunnel current. Among 2D vdW materials, black phosphorus (BP) and indium selenide (InSe) have a Brillouin zone‐centered conduction and valence bands, and a type II band offset, both ideally suited for band‐to‐band tunneling. TFETs based on BP/InSe heterojunctions with diverse electrical transport characteristics are demonstrated: forward rectifying, Zener tunneling, and backward rectifying characteristics are realized in BP/InSe junctions with different thickness of the BP layer or by electrostatic gating of the junction. Electrostatic gating yields a large on/off current ratio of up to 10⁸ and negative differential resistance at low applied voltages (V ≈ 0.2 V). These findings illustrate versatile functionalities of TFETs based on BP and InSe, offering opportunities for applications of these 2D materials beyond the device architectures reported in the current literature.     

Effects of the Ionic Currents in Electrolyte‐gated Organic Field‐Effect Transistors

Polyelectrolytes are promising materials as gate dielectrics in organic field‐effect transistors (OFETs). Upon gate bias, their polarization induces an ionic charging current, which generates a large double layer capacitor (10–500 µF cm^?2) at the semiconductor/electrolyte interface. The resulting transistor operates at low voltages (<1 V) and its conducting channel is formed in ～50 µs. The effect of ionic currents on the performance of the OFETs is investigated by varying the relative humidity of the device ambience. Within defined humidity levels and potential values, the water electrolysis is negligible and the OFETs performances are optimum.     

Dual‐Gate Organic Field‐Effect Transistors as Potentiometric Sensors in Aqueous Solution

Buried electrodes and protection of the semiconductor with a thin passivation layer are used to yield dual‐gate organic transducers. The process technology is scaled up to 150‐mm wafers. The transducers are potentiometric sensors where the detection relies on measuring a shift in the threshold voltage caused by changes in the electrochemical potential at the second gate dielectric. Analytes can only be detected within the Debye screening length. The mechanism is assessed by pH measurements. The threshold voltage shift depends on pH as ΔV_th = (C_top/C_bottom) × 58 mV per pH unit, indicating that the sensitivity can be enhanced with respect to conventional ion‐sensitive field‐effect transistors (ISFETs) by adjusting the ratio of the top and bottom gate capacitances. Remaining challenges and opportunities are discussed.