Drain-Induced Multifunctional Ambipolar Electronics Based on Junctionless MoS2.

Applying a drain bias to a strongly gate-coupled semiconductor influences the carrier density of the channel. However, practical applications of this drain-bias-induced effect in the advancement of switching electronics have remained elusive due to the limited capabilities of its current modulation known to date. Here, we show strategies to largely control the current by utilizing drain-bias-induced carrier type switching in an ambipolar molybdenum disulfide (MoS2) field-effect transistor with Pt bottom contacts. Our CMOS-compatible device architecture, incorporating a partially gate-coupled p-n junction, achieves multifunctionality. The ambipolar MoS2 device operates as an ambipolar transistor (on/off ratios exceeding 107 for both NMOS and PMOS), a rectifier (rectification ratio of ∼3 × 106), a reversible negative breakdown diode with an adjustable breakdown voltage (on/off ratio exceeding 109 with a maximum current as high as 10-4 A), and a photodetector. Finally, we demonstrate a complementary inverter (gain of ∼24 at Vdd = 1.5 V), which is highly facile to fabricate without the need for complex heterostructures and doping processes. Our study provides strategies to achieve high-performance ambipolar MoS2 devices and to effectively utilize drain bias for electrical switching.

Molecular Engineering of Rylene Diimides via Sila-Annulation Toward High-Mobility Organic Semiconductors.

The continuous innovation of captivating new organic semiconducting materials remains pivotal in the development of high-performance organic electronic devices. Herein, a molecular engineering by combining sila-annulation with the vertical extension of rylene diimides (RDIs) toward high-mobility organic semiconductors is presented. The unilateral and bilateral sila-annulated quaterrylene diimides (Si-QDI and 2Si-QDI) are designed and synthesized. In particular, the symmetrical bilateral 2Si-QDI exhibits a compact, 1D slipped π-π stacking arrangement through the synergistic combination of a sizable π-conjugated core and intercalating alkyl chains. Combining the appreciable elevated HOMO levels and reduced energy gaps, the single-crystalline organic field-effect transistors (SC-OFETs) based on 2Si-QDI demonstrate exceptional ambipolar transport characteristics with an impressive hole mobility of 3.0 cm2 V-1 s-1 and an electron mobility of 0.03 cm2 V-1 s-1 , representing the best ampibolar SC-OFETs based on RDIs. Detailed theoretical calculations rationalize that the larger transfer integral along the π-π stacking direction is responsible for the achievement of the superior charge transport. This study showcases the remarkable potential of sila-annulation in optimizing carrier transport performances of polycyclic aromatic hydrocarbons (PAHs).

Landauer-QFLPS Model for Mixed Schottky-Ohmic Contact Two-Dimensional Transistors.

Two-dimensional material-based field-effect transistors (2DM-FETs) are playing a revolutionary role in electronic devices. However, before electronic design automation (EDA) for 2DM-FETs can be achieved, it remains necessary to determine how to incorporate contact transports into model. Reported methods compromise between physical intelligibility and model compactness due to the heterojunction nature. To address this, quasi-Fermi-level phase space theory (QFLPS) is generalized to incorporate contact transports using the Landauer formula. It turns out that the Landauer-QFLPS model effectively overcomes the issue of concern. The proposed new formula can describe 2DM-FETs with Schottky or Ohmic contacts with superior accuracy and efficiency over previous methods, especially when describing non-monotonic drain conductance characteristics. A three-bit threshold inverter quantizer (TIQ) circuit is fabricated using ambipolar black phosphorus and it is demonstrated that the model accurately predicts circuit performance. The model could be very effective and valuable in the development of 2DM-FET-based integrated circuits.

Ambipolar Charge Transport in Organic Semiconductors: How Intramolecular Reorganization Energy Is Controlled by Diradical Character.

The charged forms of π-conjugated chromophores are relevant in the field of organic electronics as charge carriers in optoelectronic devices, but also as energy storage substrates in organic batteries. In this context, intramolecular reorganization energy plays an important role in controlling material efficiency. In this work, we investigate how the diradical character influences the reorganization energies of holes and electrons by considering a library of diradicaloid chromophores. We determine the reorganization energies with the four-point adiabatic potential method using quantum-chemical calculations at density functional theory (DFT) level. To assess the role of diradical character, we compare the results obtained, assuming both closed-shell and open-shell representations of the neutral species. The study shows how the diradical character impacts the geometrical and electronic structure of neutral species, which in turn control the magnitude of reorganization energies for both charge carriers. Based on computed geometries of neutral and charged species, we propose a simple scheme to rationalize the small, computed reorganization energies for both n-type and p-type charge transport. The study is supplemented with the calculation of intermolecular electronic couplings governing charge transport for selected diradicals, further supporting the ambipolar character of the investigated diradicals.

Cascaded Logic Gates Based on High-Performance Ambipolar Dual-Gate WSe2 Thin Film Transistors.

Ambipolar dual-gate transistors based on low-dimensional materials, such as graphene, carbon nanotubes, black phosphorus, and certain transition metal dichalcogenides (TMDs), enable reconfigurable logic circuits with a suppressed off-state current. These circuits achieve the same logical output as complementary metal-oxide semiconductor (CMOS) with fewer transistors and offer greater flexibility in design. The primary challenge lies in the cascadability and power consumption of these logic gates with static CMOS-like connections. In this article, high-performance ambipolar dual-gate transistors based on tungsten diselenide (WSe2) are fabricated. A high on-off ratio of 108 and 106, a low off-state current of 100 to 300 fA, a negligible hysteresis, and an ideal subthreshold swing of 62 and 63 mV/dec are measured in the p- and n-type transport, respectively. We demonstrate cascadable and cascaded logic gates using ambipolar TMD transistors with minimal static power consumption, including inverters, XOR, NAND, NOR, and buffers made by cascaded inverters. A thorough study of both the control gate and the polarity gate behavior is conducted. The noise margin of the logic gates is measured and analyzed. The large noise margin enables the implementation of VT-drop circuits, a type of logic with reduced transistor number and simplified circuit design. Finally, the speed performance of the VT-drop and other circuits built by dual-gate devices is qualitatively analyzed. This work makes advancements in the field of ambipolar dual-gate TMD transistors, showing their potential for low-power, high-speed, and more flexible logic circuits.

Digital Keying Enabled by Reconfigurable 2D Modulators.

Energy, area, and bandwidth efficient communication primitives are essential to sustain the rapid increase in connectivity among internet-of-things (IoT) edge devices. While IoT edge-sensing, edge-computing, and edge-storage have witnessed innovation in materials and devices, IoT edge communication is yet to experience such transformation. The aging silicon (Si)-based complementary metal-oxide-semiconductor (CMOS) technology continues to remain the mainstay of communication devices where they are used to implement amplitude, frequency, and phase shift keying (amplitude-shift keying [ASK]/frequency-shift keying [FSK]/phase-shift keying [PSK]). Keying allows digital information to be communicated over a radio channel. While CMOS-based keying devices have evolved over the years, their hardware footprint and energy consumption are major concerns for resource constrained IoT communication. Furthermore, separate circuit designs and hardware elements are needed for each keying scheme and achieving multibit modulation to improve bandwidth efficiency remains a challenge. Here, a reconfigurable modulator is introduced that exploits unique ambipolar transport and programmable Dirac voltage in ultrathin MoTe2 field-effect transistors to achieve ASK, FSK, and PSK modulation. Furthermore, by integrating two programmed MoTe2 field-effect transistors, multibit data modulation is demonstrated, which improves the bandwidth efficiency by 200%. Finally, a frequency quadrupler is also realized exploiting the unique "double-well" transfer characteristic.

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.

Efficient Organic Light-Emitting Transistors Based on High-Quality Ambipolar Single Crystals.

A cyano-substituted styrene derivative is synthesized and successfully prepared to lamellate single crystals through precisely controlling the crystal growth conditions. The lamellate single crystals with regular edge and smooth surface display intrinsically ordered stacking and high quality, all of which are of importance for high optoelectronic performance. The single-component light-emitting transistors based on the lamellate crystals offer striking device performance in terms of record external quantum efficiency of 2.02%, exceeding the benchmark value in this field. Such organic light-emitting single crystals provide a versatile platform for designing and engineering their structures and optoelectronic properties toward light-emitting devices.

Nucleation Control-Triggering Cocrystal Polymorphism of Charge-Transfer Complexes Differing in Physical and Electronic Properties.

Binary charge-transfer complex polymorphs composed of perylene and 4,8-bis(dicyanomethylene)-4,8-dihydrobenzo-[1,2-b:4,5-b']-dithiophene (DTTCNQ) were synthesized separately via a simple artificial nucleation-tailoring method, in both macroscopic and microscopic cocrystal engineering manners. The two polymorphs were testified to be independently thermosalient in the solid state, and the specific self-assembly derived from homogeneous or heterogeneous nucleation by assistance of governable thermodynamic/kinetic drive, leading to a change in the ordered p-n stacking structure. The as-prepared polymorphic microcrystals afforded a significantly varied (opto)electronic property: high n-type transporting and good photoresponsivity for ß-complex, and ambipolar transporting with ignorable photoresponsivity for α-complex, attributing to the different charge-transfer and supramolecular alignment. This work provides us a new route to the exploitation of donor-acceptor complex family, making it possible to develop functional materials and devices based on variable supramolecular binary structures.

2D Molecular Crystal Bilayer p-n Junctions: A General Route toward High-Performance and Well-Balanced Ambipolar Organic Field-Effect Transistors.

Ambipolar organic field-effect transistors (OFETs) are vital for the construction of high-performance all-organic digital circuits. The bilayer p-n junction structure, which is composed of separate layers of p- and n-type organic semiconductors, is considered a promising way to realize well-balanced ambipolar charge transport. However, this approach suffers from severely reduced mobility due to the rough interface between the polycrystalline thin films of p- and n-type organic semiconductors. Herein, 2D molecular crystal (2DMC) bilayer p-n junctions are proposed to construct high-performance and well-balanced ambipolar OFETs. The molecular-scale thickness of the 2DMC ensures high injection efficiency and the atomically flat surface of the 2DMC leads to high-quality p- and n-layer interfaces. Moreover, by controlling the layer numbers of the p- and n-type 2DMCs, the electron and hole mobilities are tuned and well-balanced ambipolar transport is accomplished. The hole and electron mobilities reach up to 0.87 and 0.82 cm2 V-1 s-1 , respectively, which are the highest values among organic single-crystalline double-channel OFETs measured in ambient air. This work provides a general route to construct high-performance and well-balanced ambipolar OFETs based on available unipolar materials.

Anti-Ambipolar Transport with Large Electrical Modulation in 2D Heterostructured Devices.

Van der Waals materials and their heterostructures provide a versatile platform to explore new device architectures and functionalities beyond conventional semiconductors. Of particular interest is anti-ambipolar behavior, which holds potentials for various digital electronic applications. However, most of the previously conducted studies are focused on hetero- or homo- p-n junctions, which suffer from a weak electrical modulation. Here, the anti-ambipolar transport behavior and negative transconductance of MoTe2 transistors are reported using a graphene/h-BN floating-gate structure to dynamically modulate the conduction polarity. Due to the asymmetric electrical field regulating effect on the recombination and diffusion currents, the anti-ambipolar transport and negative transconductance feature can be systematically controlled. Consequently, the device shows an unprecedented peak resistance modulation factor (≈5 × 103 ), and effective photoexcitation modulation with distinct threshold voltage shift and large photo on/off ratio (≈104 ). Utilizing this large modulation effect, the voltage-transfer characteristics of an inverter circuit variant are further studied and its applications in Schmitt triggers and multivalue output are further explored. These properties, in addition to their proven nonvolatile storage, suggest that such 2D heterostructured devices display promising perspectives toward future logic applications.

Ambipolar Conjugated Polymers with Ultrahigh Balanced Hole and Electron Mobility for Printed Organic Complementary Logic via a Two-Step CH Activation Strategy.

High mobility ambipolar conjugated polymers are seriously absent regardless their great potential for flexible and printed plastic devices and circuits. Here, ambipolar polymers with ultrahigh balanced hole and electron mobility are developed via a two-step CH activation strategy. Diketopyrrolopyrrole-benzothiadiazole-diketopyrrolopyrrole (DBD) and its copolymers with thiophene/selenophene units (short as PDBD-T and PDBD-Se) are used as examples. PDBD-Se exhibits highly efficient ambipolar transport with hole and electron mobility up to 8.90 and 7.71 cm2 V-1 s-1 in flexible organic field-effect transistors, presenting a milestone for ambipolar copolymer screening. Based on this performance metrics and good solubility, PDBD-Se is investigated as inkjet-printable semiconductor ink for organic complementary logic circuits. Under ambient processing, maximum hole and electron mobilities reach 6.70 and 4.30 cm2 V-1 s-1 , respectively. Printed complementary inverter and NAND gates with transition voltages near VDD /2 are fabricated, providing an easy-handling, general material for printed electronics and logic.

High Speed Ultraviolet Phototransistors Based on an Ambipolar Fullerene Derivative.

Combining high charge carrier mobility with ambipolar transport in light-absorbing organic semiconductors is highly desirable as it leads to enhanced charge photogeneration, and hence improved performance, in various optoelectronic devices including solar cells and photodetectors. Here we report the development of [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)-based ultraviolet (UV) phototransistors with balanced electron and hole transport characteristics. The latter is achieved by fine-tuning the source-drain electrode work function using a self-assembled monolayer. Opto/electrical characterization of as-prepared ambipolar PC61BM phototransistors reveals promising photoresponse, particularly in the UV-A region (315-400 nm), with a maximum photosensitivity and responsivity of 9 × 103 and 3 × 103 A/W, respectively. Finally, the temporal response of the PC61BM phototransistors is found to be high despite the long channel length (10 s of µm) with typical switching times of <2 ms.

Hole Transport in Exfoliated Monolayer MoS2.

Ideal monolayers of common semiconducting transition-metal dichalcogenides (TMDCs) such as MoS2, WS2, MoSe2, and WSe2 possess many similar electronic properties. As it is the case for all semiconductors, however, the physical response of these systems is strongly determined by defects in a way specific to each individual compound. Here we investigate the ability of exfoliated monolayers of these TMDCs to support high-quality, well-balanced ambipolar conduction, which has been demonstrated for WS2, MoSe2, and WSe2, but not for MoS2. Using ionic-liquid gated transistors, we show that, contrary to WS2, MoSe2, and WSe2, hole transport in exfoliated MoS2 monolayers is systematically anomalous, exhibiting a maximum in conductivity at negative gate voltage ( V G) followed by a suppression of up to 100 times upon further increasing V G. To understand the origin of this difference, we have performed a series of experiments including the comparison of hole transport in MoS2 monolayers and thicker multilayers, in exfoliated and CVD-grown monolayers, as well as gate-dependent optical measurements (Raman and photoluminescence) and scanning tunneling imaging and spectroscopy. In agreement with existing ab initio calculations, the results of all these experiments are consistently explained in terms of defects associated with chalcogen vacancies that only in MoS2 monolayers, but not in thicker MoS2 multilayers nor in monolayers of the other common semiconducting TMDCs, create in-gap states near the top of the valence band that act as strong hole traps. Our results demonstrate the importance of studying systematically how defects determine the properties of 2D semiconducting materials and of developing methods to control them.

Scalable Synthesis of Highly Crystalline MoSe2 and Its Ambipolar Behavior.

Atomically thin, two-dimensional material molybdenum diselenide (MoSe2) has been shown to exhibit significant potential for diverse applications. The intrinsic band gap of MoSe2 allows it to overcome the shortcomings of the zero-band-gap graphene, while its higher electron mobilities when compared to molybdenum disulfide (MoS2) make it more appropriate for practical devices in electronics and optoelectronics. However, its controlled growth has been an ongoing challenge for investigations and practical applications of the material. Here, we present an atmospheric pressure chemical vapor deposition (CVD) method to achieve highly crystalline, single- and few-layered MoSe2 using a SiO2/Si substrate. Our findings suggested that careful optimization of the flow rate can result in the controlled growth of large-area MoSe2 with desired layer numbers due to the adjustment of gaseous MoSe2 partial pressure and nucleation density. The FETs fabricated on such as-synthesized MoSe2 displayed different transport behaviors depending on the layer numbers, which can be attributed to the formation of Se vacancies generated during low flow rates. Monolayer MoSe2 showed n-type characteristics with an Ion/Ioff ratio of ∼106 and a carrier mobility of ∼19 cm2 V-1 s-1, whereas bilayer MoSe2 showed n-type-dominant ambipolar behavior with an Ion/Ioff ratio of ∼105 and a higher mobility of ∼65 cm2 V-1 s-1 for electrons as well as ∼9 cm2 V-1 s-1 for holes. Our results provide a foundation for property-controlled synthesis of MoSe2 and offer insight on the potential applications of our synthesized MoSe2 in electronics and optoelectronics.

Highly Luminescent 2D-Type Slab Crystals Based on a Molecular Charge-Transfer Complex as Promising Organic Light-Emitting Transistor Materials.

A new 2:1 donor (D):acceptor (A) mixed-stacked charge-transfer (CT) cocrystal comprising isometrically structured dicyanodistyrylbenzene-based D and A molecules is designed and synthesized. Uniform 2D-type morphology is manifested by the exquisite interplay of intermolecular interactions. In addition to its appealing structural features, unique optoelectronic properties are unveiled. Exceptionally high photoluminescence quantum yield (ΦF ≈ 60%) is realized by non-negligible oscillator strength of the S1 transition, and rigidified 2D-type structure. Moreover, this luminescent 2D-type CT crystal exhibits balanced ambipolar transport (µh and µe of ≈10-4 cm2 V-1 s-1 ). As a consequence of such unique optoelectronic characteristics, the first CT electroluminescence is demonstrated in a single active-layered organic light-emitting transistor (OLET) device. The external quantum efficiency of this OLET is as high as 1.5% to suggest a promising potential of luminescent mixed-stacked CT cocrystals in OLET applications.

Electrical Performance of a Molecular Organic Semiconductor under Thermal Stress.

The high temperature performance oforganic field-effect transistorsbased on a molecular organic semiconductor with intermediate dimensions, namely X2, is evaluated. Hole mobility is stable, even at 200-250 °C. Changes in device characteristics at high temperature are reversible across multiple cycles of high temperature operation. Measurements at high temperature exhibit larger hysteresis, while at low temperature one observes the emergence of ambipolar transport.

Control of Threshold Voltage for Top-Gated Ambipolar Field-Effect Transistor by Gate Buffer Layer.

The threshold voltage and onset voltage for p-channel and n-channel regimes of solution-processed ambipolar organic transistors with top-gate/bottom-contact (TG/BC) geometry were effectively tuned by gate buffer layers in between the gate electrode and the dielectric. The work function of a pristine Al gate electrode (-4.1 eV) was modified by cesium carbonate and vanadium oxide to -2.1 and -5.1 eV, respectively, which could control the flat-band voltage, leading to a remarkable shift of transfer curves in both negative and positive gate voltage directions without any side effects. One important feature is that the mobility of transistors is not very sensitive to the gate buffer layer. This method is simple but useful for electronic devices where the threshold voltage should be precisely controlled, such as ambipolar circuits, memory devices, and light-emitting device applications.

Ambipolar Transport in Solution-Synthesized Graphene Nanoribbons.

Graphene nanoribbons (GNRs) with robust electronic band gaps are promising candidate materials for nanometer-scale electronic circuits. Realizing their full potential, however, will depend on the ability to access GNRs with prescribed widths and edge structures and an understanding of their fundamental electronic properties. We report field-effect devices exhibiting ambipolar transport in accumulation mode composed of solution-synthesized GNRs with straight armchair edges. Temperature-dependent electrical measurements specify thermally activated charge transport, which we attribute to inter-ribbon hopping. With access to structurally precise materials in practical quantities and by overcoming processing difficulties in making electrical contacts to these materials, we have demonstrated critical steps toward nanoelectric devices based on solution-synthesized GNRs.

Large Enhancement of Carrier Transport in Solution-Processed Field-Effect Transistors by Fluorinated Dielectric Engineering.

The universal role of high-k fluorinated dielectrics in assisting the carrier transport in transistors for a broad range of printable semiconductors is explored. These results present general rules for how to design dielectric materials and achieve devices with a high carrier concentration, low disorder, reliable operation, and robust properties.