Ambipolar Field-effect Transistors Research Articles

High-Performance One-Dimensional Sub-5 nm Transistors Based on Poly(p-phenylene ethynylene) Molecular Wires.

Poly(p-phenylene ethynylene) (PPE) molecular wires are one-dimensional materials with distinctive properties and can be applied in electronic devices. Here, the approach called first-principles quantum transport is utilized to investigate the PPE molecular wire field-effect transistor (FET) efficiency limit through the geometry of the gate-all-around (GAA) instrument. It is observed that the n-type GAA PPE molecular wire FETs with a suitable gate length (Lg = 5 nm) and underlap (UL = 1, 2, 3 nm) can gratify the on-state current (Ion), power dissipation (PDP), and delay period (τ) concerning the conditions in 2028 to achieve the higher performance (HP) request of the International Roadmap for Device and Systems (IRDS, 2022 version). In contrast, the p-type GAA PPE molecular wire FETs with Lg = 5, 3 nm, and UL of 1, 2, 3 nm could gratify the Ion, PDP, and τ concerning the 2028 needs to achieve the HP request of the IRDS in 2022, while Lg = 5 and UL = 3 nm could meet the Ion and τ concerning the 2028 needs to achieve the LP request of the IRDS in 2022. More importantly, this is the first one-dimensional carbon-based ambipolar FET. Therefore, the GAA PPE molecular wire FETs could be a latent choice to downscale Moore's law to 3 nm.

Illustrating the pertinacious interlayer charge compression effect in van der Waals heterointerfaces

Heterojunctions of two-dimensional (2D) van der Waals (vdW) nanomaterials often exhibit unusual, “non-textbook” bonding mechanism that involves distinct orbital coupling within the compressed vdW gap. To unravel this atypical bonding mechanism for 2D heterointerfaces, using density-functional theory calculations, we examine the compressed charge redistribution and interface dipoles for the heterostructures of ReSe2/graphene, ReSe2/h-BN, and a triply-stacked ReSe2/h-BN/graphene heterointerface. Here, we report the optimized atomic structures, electronic density-of-states, (integrated) planar-averaged electron density differences, and tip-inclusive scanning tunneling microscopy simulations for these heterostructures, while focusing on the explicit contributions of the conductive graphene and insulating h-BN substrates to the interlayer confined charges and dipoles. Using other 2D heterosystems, we also demonstrate that this charge compression effect within the vdW gap is ubiquitous and general regardless of the nature of the substrates and supports. Upon applying an external electric field, we clearly demonstrate a tunable control (in both magnitude and sign of these interface dipoles) in 2D vdW heterostructures and advance the precise engineering of pertinacious interlayer compressed charges/interface dipoles for the next-generation ambipolar field-effect (nano)transistor (FET) applications.

Porphyrin-fused graphene nanoribbons

Graphene nanoribbons (GNRs), nanometre-wide strips of graphene, are promising materials for fabricating electronic devices. Many GNRs have been reported, yet no scalable strategies are known for synthesizing GNRs with metal atoms and heteroaromatic units at precisely defined positions in the conjugated backbone, which would be valuable for tuning their optical, electronic and magnetic properties. Here we report the solution-phase synthesis of a porphyrin-fused graphene nanoribbon (PGNR). This PGNR has metalloporphyrins fused into a twisted fjord-edged GNR backbone; it consists of long chains (>100 nm), with a narrow optical bandgap (~1.0 eV) and high local charge mobility (>400 cm2 V–1 s–1 by terahertz spectroscopy). We use this PGNR to fabricate ambipolar field-effect transistors with appealing switching behaviour, and single-electron transistors displaying multiple Coulomb diamonds. These results open an avenue to π-extended nanostructures with engineerable electrical and magnetic properties by transposing the coordination chemistry of porphyrins into graphene nanoribbons.

Influence of the Bound Electron-Hole Pairs Dissociation Probability Field Dependence Form on the Photocurrent and Spatial Resolution of Organic Field Phototransistors

In this work, numerical simulations are used to study ambipolar organic field-effect phototransistors, in which a spatially localized photoelectric effect can take place. This effect consists in the fact that there is a small spatially localized photosensitive region in the transistor channel, the position of which can be controlled by changing the gate voltage. The purpose of this work is to analyze the relationship between the form of the field dependence of the bound electron-hole pairs (𝑒/ℎ pairs) dissociation probability and characteristics of the studied ambipolar phototransistors such as normalized photocurrent, spatial resolution, and response time. It is shown that the optimal form of the field dependence of 𝑒/ℎ pairs dissociation probability is stepwise-like form, which can provide a high spatial resolution at high values of the normalized photocurrent without degrading the response time of the phototransistor. This shape can be achieved when the organic semiconductor has an extremely narrow distribution of 𝑒/ℎ pairs by size, described by the delta function. Also, on the example of several distributions of various shapes, it is shown that a decrease in the width of the distribution leads to an increase in the spatial resolution. Approaches to the selection and modification of organic semiconductor materials that would provide the most pronounced spatially localized photoelectric effect in ambipolar field-effect transistors are discussed.

Carbon nanotube neurotransistors with ambipolar memory and learning functions

In recent years, neuromorphic computing has gained attention as a promising approach to enhance computing efficiency. Among existing approaches, neurotransistors have emerged as a particularly promising option as they accurately represent neuron structure, integrating the plasticity of synapses along with that of the neuronal membrane. An ambipolar character could offer designers more flexibility in customizing the charge flow to construct circuits of higher complexity. We propose a novel design for an ambipolar neuromorphic transistor, utilizing carbon nanotubes as the semiconducting channel and an ion-doped sol–gel as the polarizable gate dielectric. Due to its tunability and high dielectric constant, the sol–gel effectively modulates the conductivity of nanotubes, leading to efficient and controllable short-term potentiation and depression. Experimental results indicate that the proposed design achieves reliable and tunable synaptic responses with low power consumption. Our findings suggest that the method can potentially provide an efficient solution for realizing more adaptable cognitive computing systems.Impact statementThe huge amount of data generated by the current society makes it necessary to explore new computing methods with higher efficiency to overcome the bottleneck formed between data storage and processing tasks. Neuromorphic computing aims at emulating the functioning of our brain, which performs both tasks utilizing the same hardware. Here, we propose ambipolar field-effect transistors based on carbon nanotubes with a polarizable gate dielectric, capable of providing memory functions reminiscent of neuronal synapses, at both polarities of the device. The ambipolar characteristic doubles the possibilities of previously demonstrated neurotransistors. The short-term and ambipolar behavior of the device can find its place in novel applications in the future. Machine learning-enabled gas sensing is an excellent example, where real-time processing of large amounts of data is beneficial. In addition, interaction with oxidative and reductive gases will result in dual responses due to the ambipolarity of the transistor, along with the possibility of storing the sensing data.Graphical abstract

Self-Assembled Monolayer Doping for MoTe2 Field-Effect Transistors: Overcoming PN Doping Challenges in Transition Metal Dichalcogenides.

Transition metal dichalcogenides (TMDs) have gained significant attention as next-generation semiconductor materials that could potentially overcome the integration limits of silicon-based electronic devices. However, a challenge in utilizing TMDs as semiconductors is the lack of an established PN doping method to effectively control their electrical properties, unlike those of silicon-based semiconductors. Conventional PN doping methods, such as ion implantation, can induce lattice damage in TMDs. Thus, chemical doping methods that can control the Schottky barrier while minimizing lattice damage are desirable. Here, we focus on the molybdenum ditelluride (2H-MoTe2), which has a hexagonal phase and exhibits ambipolar field-effect transistor (FET) properties due to its direct band gap of 1.1 eV, enabling concurrent transport of electrons and holes. We demonstrate the fabrication of p- or n-type unipolar FETs in ambipolar MoTe2 FETs using self-assembled monolayers (SAMs) as chemical dopants. Specifically, we employ 1H,1H,2H,2H perfluorooctyltriethoxysilane and (3-aminopropyl)triethoxysilane as SAMs for chemical doping. The selective SAMs effectively increase the hole and electron charge transport capabilities in MoTe2 FETs by 18.4- and 4.6-fold, respectively, due to the dipole effect of the SAMs. Furthermore, the Raman shift of MoTe2 by SAM coating confirms the successful p- and n-type doping. Finally, we demonstrate the fabrication of complementary inverters using SAMs-doped MoTe2 FETs, which exhibit clear full-swing capability compared to undoped complementary inverters.

Full two-dimensional ambipolar CFET-like architecture for switchable logic circuits

As the scaling of integrated circuits based on silicon semiconductors becomes increasingly challenging due to the minimum feature size being close to the physical limit, the urgent demand for alternative strategies has fuelled the rapid growth of techniques and material innovations. Here, we report on the fabrication of vertically stacked ambipolar complementary field-effect transistor that is fully composed of two-dimensional materials of WSe2/h-BN/graphene/h-BN/WSe2 heterostructures. The ambipolar feature of the top and bottom WSe2 FET enables a switchable inverter behavior with a favorable voltage gain of up to 75, which can work in both the first and third quadrants. Based on the switchable characteristics, a large voltage swing circuit for single photon avalanche detectors is proposed without any bulky negative-voltage components. This work could open a new pathway for future two-dimensional electronics and ultimate monolithic 3D high-density integration circuits.

Two-dimensional material-based complementary ambipolar field-effect transistors with ohmic-like contacts

Ambipolar field-effect transistors (FETs) possessing both electron and hole carriers enable implementation of novel reconfigurable transistors, artificial synaptic transistors, and output polarity controllable (OPC) amplifiers. Here, we fabricated a two-dimensional (2D) material-based complementary ambipolar FET and investigated its electrical characteristics. Properties of ohmic-like contacts at source/drain sides were verified from output characteristics and temperature-dependent measurements. The symmetry of electron and hole currents can be easily achieved by optimization of the MoS2 or WSe2 channels, different from the conventional ambipolar FET with fundamental issues related to Schottky barriers. In addition, we demonstrated successful operation of a complementary inverter and OPC amplifier, using the fabricated complementary ambipolar FET based on 2D materials.

Control of Carrier Polarity and Schottky Barriers in Layered PdSe2 Field-Effect Transistors

Palladium diselenide (PdSe2) has been discovered as an intriguing two-dimensional (2D) semiconductor for its unique pentagonal crystalline structure, electrical and optical anisotropy, thickness-modulated band gap, robust air stability, and high carrier mobility, demonstrating great potential in field-effect transistors (FETs), photodetectors, and thermoelectric devices. Controlling the carrier polarity and electrical contact Schottky barriers is of great significance for realizing high-performance PdSe2 optoelectronic devices. Here, by combining work-function measurement and electrical transport, we observe a thickness-modulated carrier polarity transition in layered PdSe2 FETs from n-type unipolar to intrinsic ambipolar and p-type unipolar behavior by simply varying the number of layers of PdSe2. Due to the weak Fermi-level pinning in few-layer PdSe2, n-type and ambipolar FETs are achieved by contacting PdSe2 to Sc, Ti, and Pd electrodes, respectively. The Schottky barrier heights are determined by temperature-dependent transport, showing a 15 meV barrier for the Sc contact at the electron side and 62 and 93 meV barriers for Ti and Pd contacts, both at the hole side, respectively. We further demonstrate ozone-treatment-induced hole doping in PdSe2, which effectively converts the PdSe2 FETs from n-type to p-type. The doping is robust in the ambient environment over 3 months. In-plane homojunction on a PdSe2 flake is realized by selective ozone doping, demonstrating typical diode rectifying behavior. The ability to control the carrier polarity and Schottky barriers in layered PdSe2 makes this 2D semiconductor highly feasible for complementary metal-oxide semiconductor device integration and high-performance photodetectors based on atomically thin p–n junctions.

Comprehensive Polarity Regulation of WSe2 Field‐Effect Transistors Enabled by Combining Contact Engineering and Plasma Doping

Two‐dimensional semiconductors are considered as promising candidates in next‐generation nanoelectronics. The polarity regulation, however, has been a great obstacle to their applications. Herein, a strategy to comprehensively modulate the polarity of WSe2 field‐effect transistors (FETs) by combining contact engineering and plasma doping is demonstrated. N‐type and ambipolar WSe2 FETs are obtained by indium (In) and chromium (Cr) contact, respectively. Meanwhile Cr contact and mild oxygen plasma doping are employed simultaneously to realize p‐type WSe2 FET. High on/off ratio of ≈107 has been achieved for both n‐type and p‐type WSe2 FETs. Subsequently, they are connected in series to construct a homogeneous complementary logic inverter and a lateral p–n diode. Anti‐ambipolar transfer characteristics, therefore, are accessed from the inverter. And the forward to backward rectifying ratio reaches 106 for the p–n diode. The proposed strategy paves the way for practical applications of WSe2 FETs in logic circuits and optoelectronics.

The fluorination effect: the importance of backbone planarity in achieving high performance ambipolar field effect transistors

Understanding the strategy of fluorination to control π-conjugation and intra and interchain interactions for the rational design of improved semiconducting polymers.

Comparison between modulations of contact and channel potential in nitrogen dioxide gas response of ambipolar carbon nanotube field-effect transistors

Carbon nanotubes (CNTs) are promising materials for gas sensing because of their large specific area and high sensitivity to charge differentiation. In CNT-based field-effect transistors (FETs) for gas sensing, both CNT potential modulation in the channels and Schottky barrier height modulation at the CNT/metal electrode contact influence the current properties. However, researchers have not used Schottky barrier height modulation for gas detection. To investigate and compare the effects of Schottky barrier height modulation and CNT channel potential modulation on NO2 gas exposure, we fabricated ambipolar CNT FETs by the dielectrophoretic assembly. We exposed CNT FET gas sensors to N2 gas containing 100-ppb NO2 and observed two different responses in the electric properties: a steady current shift in the positive direction in the hole-conduction region because of the channel potential modulation, and an abrupt decrease in transconductance in the electron-conduction region because of the Schottky barrier modulation. The CNT channels and CNT/metal contact both contributed to the sensor response, and the modulation rate of the Schottky barrier was higher than that of the CNT potential shift in the channel.

Absorption and Emission of Chemically and Electrochemically Doped Graphene Nanoribbons

Graphene nanoribbons (GNRs) are narrow stripes of graphene that exhibit unique optical and electronic properties depending on their width and edge type. Recent advances in the bottom-up synthesis of GNRs have enabled precise control over their structure and thus their bandgap. However, the systematic optical and electrical characterization of GNRs obtained from solution-mediated reactions has so far been prevented by the poor stability and processability of GNR dispersions. Here we employ liquid cascade centrifugation (LCC) to obtain size-selected and stable dispersions of bottom-up synthesized 9-aGNRs in toluene and tetrahydrofuran and investigate their intrinsic optical properties. Dispersions and films of these 9-aGNRs show well-defined absorption and photoluminescence bands between 800 and 1000 nm, with relative intensities depending on the LCC-fraction. The best GNR dispersions show photoluminescence quantum yields of up to 70 %. Trions are charged excitons with red-shifted emission that can be created in doped low-dimensional semiconductors, such as carbon nanotubes. Theoretical studies predict large trion binding energies (> 300 meV) in graphene nanoribbons (depending on their width). Hence their optical properties upon doping and possible new features are of interest. Chemical doping of our 9-aGNR dispersions with the strong molecular p-dopant F4-TCNQ and electrochemical doping of GNR thin films in ambipolar electrolyte-gated field-effect transistors results in the expected bleaching of the main absorption bands but also the emergence of new redshifted, charge-induced absorption peaks that may indicate the existence of trions in GNRs.

Channel-Length-Modulated Avalanche Multiplication in Ambipolar WSe2 Field-Effect Transistors.

Recently there has been growing interest in avalanche multiplication in two-dimensional (2D) materials and device applications such as avalanche photodetectors and transistors. Previous studies have mainly utilized unipolar semiconductors as the active material and focused on developing high-performance devices. However, fundamental analysis of the multiplication process, particularly in ambipolar materials, is required to establish high-performance electronic devices and emerging architectures. Although ambipolar 2D materials have the advantage of facile carrier-type tuning through electrostatic gating, simultaneously allowing both carrier types in a single channel poses an inherent difficulty in analyzing their individual contributions to avalanche multiplication. In ambipolar field-effect transistors (FETs), two phenomena of ambipolar transport and avalanche multiplication can occur, and both exhibit secondary rise of output current at high lateral voltage. We distinguished these two competing phenomena using the method of channel length modulation and successfully analyzed the properties of electron- and hole-initiated multiplication in ambipolar WSe2 FETs. Our study provides a simple and robust method to examine carrier multiplication in ambipolar materials and will foster the development of high-performance atomically thin electronic devices utilizing avalanche multiplication.

High-Performance Bidirectional Chemical Sensor Platform Using Double-Gate Ion-Sensitive Field-Effect Transistor with Microwave-Assisted Ni-Silicide Schottky-Barrier Source/Drain

This study proposes a bidirectional chemical sensor platform using ambipolar double-gate ion-sensitive field-effect transistors (ISFET) with microwave-assisted Ni-silicide Schottky-barrier (SB) source and drain (S/D) on a fully depleted silicon-on-insulator (FDSOI) substrate. The microwave-assisted Ni-silicide SB S/D offer bidirectional turn-on characteristics for both p- and n-type channel operations. The p- and n-type operations are characterized by high noise resistance as well as improved mobility and excellent drift performance, respectively. These features enable sensing regardless of the gate voltage polarity, thus contributing to the use of detection channels based on various target substances, such as cells, antigen-antibodies, DNA, and RNA. Additionally, the capacitive coupling effect existing between the top and bottom gates help achieve self-amplified pH sensitivity exceeding the Nernst limit of 59.14 mV/pH without any additional amplification circuitry. The ambipolar FET sensor performance was evaluated for bidirectional electrical characteristics, pH detection in the single-gate and double-gate modes, and reliability in continuous and repetitive operations. Considering the excellent characteristics confirmed through evaluation, the proposed ambipolar chemical sensor platform is expected to be applicable to various fields including biosensors. And through linkage with subsequent studies, various medical applications and precision detector operations for specific markers will be possible.

Control of Charge Transport Properties in Molybdenum Diselenide Field-Effect Transistors for Enhanced Noise-Margin and Inverter Characteristics

Ambipolar field-effect transistor (FET) capable of transporting both holes and electrons has the advantage of operating as both p-type and n-type with a single device depending on the voltage bias condition. Among transition metal dichalcogenides (TMDs), molybdenum diselenide (MoSe₂), which has ambipolar properties, exhibits n-type dominant electrical characteristics. As a result, a digital inverter implemented only using pristine MoSe₂ FETs exhibited asymmetric and limited output swing characteristics because of the difficulty in pull-up operation. Therefore, a method for improving the p-type characteristics of MoSe₂ is needed to implement a complementary metal–oxide–semiconductor (CMOS)-like digital inverter using only MoSe₂. In this study, an n-type dominant MoSe₂ FET was adjusted to have p-type dominant characteristics through annealing process. Raman spectroscopy was used to compare the transistor with previous studies. Statistical analysis showed that the charge transport behavior was reproduced in multiple devices. The p-type enhanced MoSe₂ FET was applied as a pull-up transistor of a digital inverter to present the practicality of the annealing process to improve the p-type characteristics of the MoSe₂ FET. Eventually, a more symmetrical output swing and improved noise margin were achieved compared to a digital inverter composed of only n-type dominant MoSe₂ FETs.

Ambipolar Field Effect Transistor Based on ZnO/Anthracene Nanocomposite As an Active Single Layer for Balanced Hole and Electron Mobility

Ambipolar Field Effect Transistor Based on ZnO/Anthracene Nanocomposite As an Active Single Layer for Balanced Hole and Electron Mobility

Synthesis and characterization of naphthalene derivatives for two-component heterojunction-based ambipolar field-effect transistors complemented with copper hexadecafluorophthalocyanine (F16CuPc).

In order to develop organic semiconductor materials with good performance, herein, a series of naphthalene derivatives were designed and synthesized by a “building-blocks approach” connected through α-bond, double bond, and triple bond, respectively. Thin-film transistors were fabricated in single-component and two-component modes based on these naphthalene derivatives by combining the F16CuPc as the n-type material. The ambipolar performance was investigated by adjusting the device preparation procedure with the hole and electron mobility of up to 10−2 cm2 V−1 s−1. Furthermore, the electrical performance was also improved to 0.73 cm2 V−1 s−1 using the two-component bilayer configuration.

Wafer scale synthesis of organic semiconductor nanosheets for van der Waals heterojunction devices

Organic semiconductors (OSC) are widely used for consumer electronic products owing to their attractive properties such as flexibility and low production cost. Atomically thin transition metal dichalcogenides (TMDs) are another class of emerging materials with superior electronic and optical properties. Integrating them into van der Waals (vdW) heterostructures provides an opportunity to harness the advantages of both material systems. However, building such heterojunctions by conventional physical vapor deposition (PVD) of OSCs is challenging, since the growth is disrupted due to limited diffusion of the molecules on the TMD surface. Here we report wafer-scale (3-inch) fabrication of transferable OSC nanosheets with thickness down to 15 nm, which enable the realization of heterojunction devices. By controlled dissolution of a poly(acrylic acid) film, on which the OSC films were grown by PVD, they can be released and transferred onto arbitrary substrates. OSC crystal quality and optical anisotropy are preserved during the transfer process. By transferring OSC nanosheets (p-type) onto prefabricated electrodes and TMD monolayers (n-type), we fabricate and characterize various electronic devices including unipolar, ambipolar and antiambipolar field-effect transistors. Such vdW p-n heterojunction devices open up a wide range of possible applications ranging from ultrafast photodetectors to conformal electronics.

Azaacenes Based Electroactive Materials: Preparation, Structure, Electrochemistry, Spectroscopy and Applications-A Critical Review.

This short critical review is devoted to the synthesis and functionalization of various types of azaacenes, organic semiconducting compounds which can be considered as promising materials for the fabrication of n-channel or ambipolar field effect transistors (FETs), components of active layers in light emitting diodes (LEDs), components of organic memory devices and others. Emphasis is put on the diversity of azaacenes preparation methods and the possibility of tuning their redox and spectroscopic properties by changing the C/N ratio, modifying the nitrogen atoms distribution mode, functionalization with electroaccepting or electrodonating groups and changing their molecular shape. Processability, structural features and degradation pathways of these compounds are also discussed. A unique feature of this review concerns the listed redox potentials of all discussed compounds which were normalized vs. Fc/Fc+. This required, in frequent cases, recalculation of the originally reported data in which these potentials were determined against different types of reference electrodes. The same applied to all reported electron affinities (EAs). EA values calculated using different methods were recalculated by applying the method of Sworakowski and co-workers (Org. Electron. 2016, 33, 300–310) to yield, for the first time, a set of normalized data, which could be directly compared.