Light-Triggered Ternary Device and Inverter Based on Heterojunction of van der Waals Materials

Light-Triggered Anti-ambipolar Transistor Based on an In-Plane Lateral Homojunction

Currently, the construction of anti-ambipolar transistors (AATs) is primarily based on asymmetric heterostructures, which are challenging to fabricate. AATs used for photodetection are accompanied by dark currents that prove difficult to suppress, resulting in reduced sensitivity. This work presents light-triggered AATs based on an in-plane lateral WSe2 homojunction without van der Waals heterostructures. In this device, the WSe2 channel is partially electrically controlled by the back gate due to the screening effect of the bottom electrode, resulting in a homojunction that is dynamically modulated with gate voltage, exhibiting electrostatically reconfigurable and light-triggered anti-ambipolar behaviors. It exhibits high responsivity (188 A/W) and detectivity (8.94 × 1014 Jones) under 635 nm illumination with a low power density of 0.23 μW/cm2, promising a new approach to low-power, high-performance photodetectors. Moreover, the device demonstrates efficient self-driven photodetection. Furthermore, ternary inverters are realized using monolithic WSe2, simplifying the manufacturing of multivalued logic devices.

Anti-Ambipolar Heterojunctions: Materials, Devices, and Circuits

Anti-ambipolar heterojunctions are vital in constructing high-frequency oscillators, fast switches, and multivalued logic (MVL) devices, which hold promising potential for next-generation integrated circuit chips and telecommunication technologies. Thanks to the strategic material design and device integration, anti-ambipolar heterojunctions have demonstrated unparalleled device and circuit performance that surpasses other semiconducting material systems. This review aims to provide a comprehensive summary of the achievements in the field of anti-ambipolar heterojunctions. First, the fundamental operating mechanisms of anti-ambipolar devices are discussed. After that, potential materials used in anti-ambipolar devices are discussed with particular attention to 2D-based, 1D-based, and organic-based heterojunctions. Next, the primary device applications employing anti-ambipolar heterojunctions, including anti-ambipolar transistors (AATs), photodetectors, frequency doublers, and synaptic devices, are summarized. Furthermore, alongside the advancements in individual devices, the practical integration of these devices at the circuit level, including topics such as MVL circuits, complex logic gates, and spiking neuron circuits, is also discussed. Lastly, the present key challenges and future research directions concerning anti-ambipolar heterojunctions and their applications are also emphasized.

A reconfigurable binary/ternary logic conversion-in-memory based on drain-aligned floating-gate heterojunction transistors

A new type of heterojunction non-volatile memory transistor (H-MTR) has been developed, in which the negative transconductance (NTC) characteristics can be controlled systematically by a drain-aligned floating gate. In the H-MTR, a reliable transition between N-shaped transfer curves with distinct NTC and monolithically current-increasing transfer curves without apparent NTC can be accomplished through programming operation. Based on the H-MTR, a binary/ternary reconfigurable logic inverter (R-inverter) has been successfully implemented, which showed an unprecedentedly high static noise margin of 85% for binary logic operation and 59% for ternary logic operation, as well as long-term stability and outstanding cycle endurance. Furthermore, a ternary/binary dynamic logic conversion-in-memory has been demonstrated using a serially-connected R-inverter chain. The ternary/binary dynamic logic conversion-in-memory could generate three different output logic sequences for the same input signal in three logic levels, which is a new logic computing method that has never been presented before.

Highly Tunable Negative Differential Resistance Device Based on Insulator-to-Metal Phase Transition of Vanadium Dioxide

Negative differential resistance (NDR) based on the band-to-band tunneling (BTBT) mechanism has recently shown great potential in improving the performance of various electronic devices. However, the applicability of conventional BTBT-based NDR devices is restricted by their insufficient performance due to the limitations of the NDR mechanism. In this study, we develop an insulator-to-metal phase transition (IMT)-based NDR device that exploits the abrupt resistive switching of vanadium dioxide (VO₂) to achieve a high peak-to-valley current ratio (PVCR) and peak current density (J_peak) as well as controllable peak and valley voltages (V_peak/valley). When a phase transition is induced in VO₂, the effective voltage bias on the two-dimensional channel is decreased by the reduction in the VO₂ resistance. Accordingly, the effective voltage adjustment induced by the IMT results in an abrupt NDR. This NDR mechanism based on the abrupt IMT results in a maximum PVCR of 71.1 through its gate voltage and VO₂ threshold voltage tunability characteristics. Moreover, V_peak/valley is easily modulated by controlling the length of VO₂. In addition, a maximum J_peak of 1.6 × 10⁶ A/m² is achieved through light-tunable characteristics. The proposed IMT-based NDR device is expected to contribute to the development of various NDR devices for next-generation electronics.

Diverse field-effect characteristics and negative differential transconductance in a graphene/WS2/Au phototransistor with a Ge back gate

We propose an infrared-sensitive negative differential transconductance (NDT) phototransistor based on a graphene/WS₂/Au double junction with a SiO₂/Ge gate. By changing the drain bias, diverse field-effect characteristics can be achieved. Typical p-type and n-type behavior is obtained under negative and positive drain bias, respectively. And NDT behavior is observed in the transfer curves under positive drain bias. It is believed to originate from competition between the top and bottom channel currents in stepped layers of WS₂ at different gate voltages. Moreover, this phototransistor shows a gate-modulated rectification ratio of 0.03 to 88.3. In optoelectronic experiments, the phototransistor exhibits a responsivity of 2.76 A/W under visible light at 532 nm. By contrast, an interesting negative responsivity of -29.5 µA/W is obtained and the NDT vanishes under illumination by infrared light at 1550 nm. A complementary inverter based on two proposed devices of the same structure is constructed. The maximum voltage gain of the complementary inverter reaches 0.79 at a supply voltage of 1.5 V. These results demonstrate a new method of realizing next-generation two- and three-dimensional electronic and optoelectronic multifunctional devices.

Looking Beyond 0 and 1: Principles and Technology of Multi-Valued Logic Devices

Ever since the invention of solid-state transistors, binary devices have dominated the electronics industry. Although the binary technology links the natural property of devices to be in the ON or OFF state with two logic levels, it provides the least possible information content per interconnect. Multi-valued logic (MVL) has long been considered as a means of improving the computation efficiency and reducing the power consumption of modern chips. In view of the power density limits of the conventional complementary metal-oxide-semiconductor technology, MVL technologies have recently gained even more attention, and various MVL unit devices based on conventional and emerging materials have been proposed. Herein, the recent achievements toward the development of compact MVL unit devices are reviewed. First, basic principles of MVL technologies are introduced by describing methods of obtaining multiple logic states and discussing radix-related aspects of MVL computation. Next, MVL unit devices are classified and overviewed with emphasis on principles of operation, technologies, and applications. Finally, a comparative discussion of strengths and weaknesses is provided for each class of MVL devices, and the review concludes with the outlook for the MVL field.

Multivalued Logic for Optical Computing with Photonically Enabled Chiral Bio-organic Structures

Photonic bio-organic multiphase structures are suggested here for integrated thin-film electronic nets with multilevel logic elements for multilevel computing via a reconfigurable photonic bandgap of chiral biomaterials. Herein, inspired by an artificial intelligence system with efficient information integration and computing capability, the photonically active dielectric layer of chiral nematic cellulose nanocrystals is combined with printed-in p- and n-type organic semiconductors as a bifunctional logical element. These adaptive logic elements are capable of triggering tailored quantized electrical output signals under light with different photon energy and at the different photonic bandgaps of the active dielectric layer. The bifunctional structures enable complex memory behavior upon repetitive changes of photonic bandgap (controlled by expansion/contraction of chiral nematic pitch) and photon energy (controlled by light absorption wavelength of complementary organic semiconductor layers), exhibiting effectively a reconfigurable ternary logic response. This proof-of-concept bio-assisted multivalued logic structure facilitates an optical computing system for low-power optical information processing integrated with human-machine interfaces.

Recent progress in 2D hybrid heterostructures from transition metal dichalcogenides and organic layers: properties and applications in energy and optoelectronics fields

Atomically thin transition metal dichalcogenides (TMDs) present extraordinary optoelectronic, electrochemical, and mechanical properties that have not been accessible in bulk semiconducting materials. Recently, a new research field, 2D hybrid heteromaterials, has emerged upon integrating TMDs with molecular systems, including organic molecules, polymers, metal-organic frameworks, and carbonaceous materials, that can tailor the TMD properties and exploit synergetic effects. TMD-based hybrid heterostructures can meet the demands of future optoelectronics, including supporting flexible, transparent, and ultrathin devices, and energy-based applications, offering high energy and power densities with long cycle lives. To realize such applications, it is necessary to understand the interactions between the hybrid components and to develop strategies for exploiting the distinct benefits of each component. Here, we provide an overview of the current understanding of the new phenomena and mechanisms involved in TMD/organic hybrids and potential applications harnessing such valuable materials in an insightful way. We highlight recent discoveries relating to multicomponent hybrid materials. Finally, we conclude this review by discussing challenges related to hybrid heteromaterials and presenting future directions and opportunities in this research field.

Mixed-Dimensional Anti-ambipolar Phototransistors Based on 1D GaAsSb/2D MoS2 Heterojunctions

The incapability of modulating the photoresponse of assembled heterostructure devices has remained a challenge for the development of optoelectronics with multifunctionality. Here, a gate-tunable and anti-ambipolar phototransistor is reported based on 1D GaAsSb nanowire/2D MoS₂ nanoflake mixed-dimensional van der Waals heterojunctions. The resulting heterojunction shows apparently asymmetric control over the anti-ambipolar transfer characteristics, possessing potential to implement electronic functions in logic circuits. Meanwhile, such an anti-ambipolar device allows the synchronous adjustment of band slope and depletion regions by gating in both components, thereby giving rise to the gate-tunability of the photoresponse. Coupled with the synergistic effect of the materials in different dimensionality, the hybrid heterojunction can be readily modulated by the external gate to achieve a high-performance photodetector exhibiting a large on/off current ratio of 4 × 10⁴, fast response of 50 μs, and high detectivity of 1.64 × 10¹¹ Jones. Due to the formation of type-II band alignment and strong interfacial coupling, a prominent photovoltaic response is explored in the heterojunction as well. Finally, a visible image sensor based on this hybrid device is demonstrated with good imaging capability, suggesting the promising application prospect in future optoelectronic systems.

Energy-Band Engineering by Remote Doping of Self-Assembled Monolayers Leads to High-Performance IGZO/p-Si Heterostructure Photodetectors

Metal oxide semiconductors are of great interest for enabling advanced photodetectors. However, operational instability and the absence of an appropriate doping technique hinder practical development and commercialization. Here, a strategy is proposed to dramatically increase the conventional photodetection performance, having superior stability in operational and environmental atmospheres. By performing energy-band engineering through an octadecylphosphonic acid (ODPA) self-assembled-monolayer-based doping treatment, the proposed indium-gallium-zinc oxide (IGZO)/p-Si heterointerface devices exhibit greatly enhance the photoresponsive characteristics, including a photoswitching current ratio with a 100-fold increase, and photoresponsivity and detectivity with a 15-fold increase each. The observed ODPA doping effects are investigated through comprehensive analysis with X-ray photoelectron spectroscopy (XPS), atomic force microscopy (AFM), and Kelvin probe force microscopy (KPFM). Furthermore, the proposed photodetectors, fabricated at a 4 in. wafer scale, demonstrate its excellent operation robustness with consistent performance over 237 days and 20 000 testing cycles.

Dielectric engineering enable to lateral anti-ambipolar MoTe2heterojunction

Atomically two-dimensional (2D) materials have generated widespread interest for novel electronics and optoelectronics. Specially, owing to atomically thin 2D structure, the electronic bandgap of 2D semiconductors can be engineered by manipulating the surrounding dielectric environment. In this work, we develop an effective and controllable approach to manipulate dielectric properties of h-BN through gallium ions (Ga⁺) implantation for the first time. And the maximum surface potential difference between the intrinsic h-BN (h-BN) and the Ga⁺implanted h-BN (Ga⁺-h-BN) is up to 1.3 V, which is characterized by Kelvin probe force microscopy. More importantly, the MoTe₂transistor stacked on Ga⁺-h-BN exhibits p-type dominated transfer characteristic, while the MoTe₂transistor stacked on the intrinsic h-BN behaves as n-type, which enable to construct MoTe₂heterojunction through dielectric engineering of h-BN. The dielectric engineering also provides good spatial selectivity and allows to build MoTe₂heterojunction based on a single MoTe₂flake. The developed MoTe₂heterojunction shows stable anti-ambipolar behaviour. Furthermore, we preliminarily implemented a ternary inverter based on anti-ambipolar MoTe₂heterojunction. Ga⁺implantation assisted dielectric engineering provides an effective and generic approach to modulate electric bandgap for a wide variety of 2D materials. And the implementation of ternary inverter based on anti-ambipolar transistor could lead to new energy-efficient logical circuit and system designs in semiconductors.

Systematic Control of Negative Transconductance in Organic Heterojunction Transistor for High-Performance, Low-Power Flexible Ternary Logic Circuits

Organic multi-valued logic (MVL) circuits can substantially improve the data processing efficiency in highly advanced wearable electronics. Organic ternary logic circuits can be implemented by utilizing the negative transconductance (NTC) of heterojunction transistors (H-TRs). To achieve high-performance organic ternary logic circuits, the range of NTC in H-TRs must be optimized in advance to ensure the well-defined intermediate logic state in ternary logic inverters (T-inverters). Herein, a simple and efficient strategy, which enables the systematic control of the range and position of NTC in H-TRs is presented. Each thickness of p-/n-type semiconductor in H-TRs is adjusted to control the channel conductivity. Furthermore, asymmetric source/drain (S/D) electrode structure is newly developed for H-TRs, which can adjust the amount of hole and electron injection, independently. Based on the semiconductor thickness variation and asymmetric S/D electrodes, the T-inverter exhibits full-swing operation with three distinguishable logic states, resulting in unprecedentedly high static noise margin (≈48% of the ideal value). Moreover, a flexible T-inverter with an ultrathin polymer dielectric is demonstrated, whose operating voltage is less than 8 V. The proposed strategy is fully compatible with the conventional integrated circuit design, which is highly desirable for broad applicability and scalability for various types of T-inverter production.

Conception of a Smart Artificial Retina Based on a Dual-Mode Organic Sensing Inverter

The human visual system enables perceiving, learning, remembering, and recognizing elementary visual information (light, colors, and images), which has inspired the development of biomimicry visual system-based electronic devices. Photosensing and synaptic devices are integrated into these systems to realize elementary information storage and recognition to imitate image processing. However, the severe restrictions of the monotonic light response and complicated circuitry design remain challenges for the development of artificial visual devices. Here, the concept of a smart artificial retina based on an organic optical sensing inverter device that can be operated as a multiwavelength photodetector and recorder is reported first. The device exhibits a light-triggered broadband (red/green/blue) response, a low energy consumption as low as ±5 V, and an ultrafast response speed (<300 ms). Moreover, the multifunctional component is also combined within a single cell for health monitoring of the artificial retina during light surveillance to avoid retinopathy. Proof-of-concept devices, by simplifying the circuitry and providing dual-mode functions, can contribute significantly to the development of bionics design and broaden the horizon for smart artificial retinas in the human visual system.

A Bi-Anti-Ambipolar Field Effect Transistor

Multistate logic is recognized as a promising approach to increase the device density of microelectronics, but current approaches are offset by limited performance and large circuit complexity. We here demonstrate a route toward increased integration density that is enabled by a mechanically tunable device concept. Bi-anti-ambipolar transistors (bi-AATs) exhibit two distinct peaks in their transconductance and can be realized by a single 2D-material heterojunction-based solid-state device. Dynamic deformation of the device reveals the co-occurrence of two conduction pathways to be the origin of this previously unobserved behavior. Initially, carrier conduction proceeds through the junction edge, but illumination and application of strain can increase the recombination rate in the junction sufficiently to support an alternative carrier conduction path through the junction area. Optical characterization reveals a tunable emission pattern and increased optoelectronic responsivity that corroborates our model. Strain control permits the optimization of the conduction efficiency through both pathways and can be employed in quaternary inverters for future multilogic applications.

Bias-controlled multi-functional transport properties of InSe/BP van der Waals heterostructures

Van der Waals (vdW) heterostructures, consisting of a variety of low-dimensional materials, have great potential use in the design of a wide range of functional devices thanks to their atomically thin body and strong electrostatic tunability. Here, we demonstrate multi-functional indium selenide (InSe)/black phosphorous (BP) heterostructures encapsulated by hexagonal boron nitride. At a positive drain bias (V_D), applied on the BP while the InSe is grounded, our heterostructures show an intermediate gate voltage (V_BG) regime where the current hardly changes, working as a ternary transistor. By contrast, at a negative V_D, the device shows strong negative differential transconductance characteristics; the peak current increases up to ~5 μA and the peak-to-valley current ratio reaches 1600 at V_D = −2 V. Four-terminal measurements were performed on each layer, allowing us to separate the contributions of contact resistances and channel resistance. Moreover, multiple devices with different device structures and contacts were investigated, providing insight into the operation principle and performance optimization. We systematically investigated the influence of contact resistances, heterojunction resistance, channel resistance, and the thickness of BP on the detailed operational characteristics at different V_D and V_BG regimes.

Complementary Driving between 2D Heterostructures and Surface Functionalization for Surpassing Binary Logic Devices

Recently, for overcoming the fundamental limits of conventional silicon technology, multivalued logic (MVL) circuits based on two-dimensional (2D) materials have received significant attention for reducing the power consumption and the complexity of integrated circuits. Compared with the conventional silicon complementary metal oxide semiconductor technology, new functional heterostructures comprising 2D materials can be readily implemented, owing to their unique inherent electrical properties. Furthermore, their process integration does not pose issues of lattice mismatch at junction interfaces. This facilitates the realization of new functional logic gate circuit configurations. However, the reported three-valued NOT gates (ternary inverters) based on 2D materials require stringent operating conditions and complex fabrication processes to obtain three distinct logic states. Herein, a general structure of MVL devices based on a simple series connection of 2D materials with partial surface functionalization is demonstrated. By arranging three 2D materials exhibiting p-type, ambipolar, and n-type conductivities, ternary inverter circuits can be established based on the complementary driving between 2D heterotransistors. This ternary inverter circuit can be further improved for quaternary inverter circuits by controlling the charge neutral point of partial ambipolar 2D materials using surface functionalization, which is an effective and nondestructive doping method for 2D materials.

MoS2/pentacene hybrid complementary inverter based photodetector with amplified voltage-output

A sensitive photodetection based on a novel hybrid CMOS inverter has been demonstrated. Unlike common photo-current type photodetectors, which convert optical signals to current, the CMOS inverter realizes voltage-output, overcoming the difficulty to monitor current signal in the range of nA. The hybrid CMOS logic inverter employs n-channel MoS₂ nanosheet/perovskite heterojunction FET and p-channel organic pentacene FET in a planar architecture. In order to obtain high performance, we adopt the interdigital electrodes for the pentacene FET to enhance the current density of the p-channel, and stack perovskite on the MoS₂ channel to modify the threshold voltage of the n-channel. As a result, a CMOS inverter with a voltage gain of more than ten is obtained. When V_IN is around the transition voltage (-38 V), the inverter can obtain stable optical detection signal, the V_OUT changes from 6 V in dark to 1 V under 633 nm light exposure. This finding indicates the potential to fabricate visible light detecting devices with voltage-output based on the inverter and may be further applicable for a photo-logic circuit.

Negative differential transconductance device with a stepped gate dielectric for multi-valued logic circuits

Multi-valued logic (MVL) technology is a promising approach for improving the data-handling capabilities and decreasing the power consumption of integrated circuits. This is especially attractive as conventional complementary metal-oxide-semiconductor technology is approaching its scaling and power density limits. Here, an ambipolar WSe₂ field-effect transistor with two or more negative-differential-transconductance (NDT) regions in its transfer characteristic (NDTFET) is proposed for MVL applications of various radices. The operation and charge carrier transport mechanism of the NDTFET are studied first by Kelvin probe force microscopy, electrical, and capacitance-voltage measurements. Next, strategies for increasing the number of NDT regions and engineering the NDTFET transfer characteristic are discussed. Finally, the extensibility and tunability of our concept are demonstrated by adapting NDTFETs as core devices for ternary, quaternary, and quinary MVL inverters through simulations, where only WSe₂ is employed as a channel material for all devices comprising the inverters. The MVL inverter operation principle and the mechanism of the multiple logic state formation are analyzed in detail. The proposed concept is practically verified by the fabrication of a ternary inverter.

Distinctive Field-Effect Transistors and Ternary Inverters Using Cross-Type WSe2/MoS2 Heterojunctions Treated with Polymer Acid

The electrical and optical characteristics of two-dimensional (2D) transition-metal dichalcogenides (TMDCs) can be improved by surface modification. In this study, distinctive field-effect transistors (FETs) were realized by forming cross-type 2D WSe₂/MoS₂ p-n heterojunctions through surface treatment using poly(methyl methacrylate-co-methacrylic acid) (PMMA-co-PMAA). The FETs were applied to new ternary inverters as multivalued logic circuits (MVLCs). Laser confocal microscope photoluminescence spectroscopy indicated the generation of trions in the WSe₂ and MoS₂ layers, and the intensity decreased after PMMA-co-PMAA treatment. For the cross-type WSe₂/MoS₂ p-n heterojunction FETs subjected to PMMA-co-PMAA treatment, the channel current and the region of anti-ambipolar transistor characteristics increased considerably, and ternary inverter characteristics with three stable logic states, "1", "1/2", and "0", were realized. Interestingly, the intermediate logic state 1/2, which results from the negative differential transconductance characteristics, was realized by the turn-on of all component FETs, as the current of the FETs increased after PMMA-co-PMAA treatment. The electron-rich carboxyl acid moieties in PMMA-co-PMAA can undergo coordination with the metal Mo or W atoms present in the Se or S vacancies, respectively, resulting in the modulation of charge density. These features yielded distinctive FETs and ternary inverters for MVLCs using cross-type WSe₂/MoS₂ heterojunctions.

A multiple negative differential resistance heterojunction device and its circuit application to ternary static random access memory

For increasing the restricted bit-density in the conventional binary logic system, extensive research efforts have been directed toward implementing single devices with a two threshold voltage (VTH) characteristic via the single negative differential resistance (NDR) phenomenon. In particular, recent advances in forming van der Waals (vdW) heterostructures with two-dimensional crystals have opened up new possibilities for realizing such NDR-based tunneling devices. However, it has been challenging to exhibit three VTH through the multiple-NDR (m-NDR) phenomenon in a single device even by using vdW heterostructures. Here, we show the m-NDR device formed on a BP/(ReS2 + HfS2) type-III double-heterostructure. This m-NDR device is then integrated with a vdW transistor to demonstrate a ternary vdW latch circuit capable of storing three logic states. Finally, the ternary latch is extended toward ternary SRAM, and its high-speed write and read operations are theoretically verified.

Design and Implementation of Ternary Logic Integrated Circuits by Using Novel Two-Dimensional Materials

With the approaching end of Moore’s Law (that the number of transistors in a dense integrated circuit doubles every two years), the logic data density in modern binary digital integrated circuits can hardly be further improved due to the physical limitation. In this aspect, ternary logic (0, 1, 2) is a promising substitute to binary (0, 1) because of its higher number of logic states. In this work, we carry out a systematical study on the emerging two-dimensional (2D) materials (MoS2 and Black Phosphorus)-based ternary logic from individual ternary logic devices to large scale ternary integrated circuits. Various ternary logic devices, including the standard ternary inverter (STI), negative ternary inverter (NTI), positive ternary inverter (PTI) and especially the ternary decrement cycling inverter (DCI), have been successfully implemented using the 2D materials. Then, by taking advantage of the optimized ternary adder algorithm and the novel ternary cycling inverter, we design a novel ternary ripple-carry adder with great circuitry simplicity. Our design shows about a 50% reduction in the required number of transistors compared to the existing ternary technology. This work paves a new way for the ternary integrated circuits design, and shows potential to fulfill higher logic data density and a smaller chip area in the future.

2D Materials Based Optoelectronic Memory: Convergence of Electronic Memory and Optical Sensor

The continuous development of electron devices towards the trend of "More than Moore" requires functional diversification that can collect data (sensors) and store (memories) and process (computing units) information. Considering the large occupation proportion of image data in both data center and edge devices, a device integration with optical sensing and data storage and processing is highly demanded for future energy-efficient and miniaturized electronic system. Two-dimensional (2D) materials and their heterostructures have exhibited broadband photoresponse and high photoresponsivity in the configuration of optical sensors and showed fast switching speed, multi-bit data storage, and large ON/OFF ratio in memory devices. In addition, its ultrathin body thickness and transfer process at low temperature allow 2D materials to be heterogeneously integrated with other existing materials system. In this paper, we overview the state-of-the-art optoelectronic random-access memories (ORAMs) based on 2D materials, as well as ORAM synaptic devices and their applications in neural network and image processing. The ORAM devices potentially enable direct storage/processing of sensory data from external environment. We also provide perspectives on possible directions of other neuromorphic sensor design (e.g., auditory and olfactory) based on 2D materials towards the future smart electronic systems for artificial intelligence.

Negative Transconductance Heterojunction Organic Transistors and their Application to Full-Swing Ternary Circuits

Multivalued logic (MVL) computing could provide bit density beyond that of Boolean logic. Unlike conventional transistors, heterojunction transistors (H-TRs) exhibit negative transconductance (NTC) regions. Using the NTC characteristics of H-TRs, ternary inverters have recently been demonstrated. However, they have shown incomplete inverter characteristics; the output voltage (V_OUT ) does not fully swing from V_DD to G_ND . A new H-TR device structure that consists of a dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) layer stacked on a PTCDI-C13 layer is presented. Due to the continuous DNTT layer from source to drain, the proposed device exhibits novel switching behavior: p-type off/p-type subthreshold region /NTC/ p-type on. As a result, it has a very high on/off current ratio (≈10⁵ ) and exhibits NTC behavior. It is also demonstrated that an array of 36 of these H-TRs have 100% yield, a uniform on/off current ratio, and uniform NTC characteristics. Furthermore, the proposed ternary inverter exhibits full V_DD -to-G_ND swing of V_OUT with three distinct logic states. The proposed transistors and inverters exhibit hysteresis-free operation due to the use of a hydrophobic gate dielectric and encapsulating layers. Based on this, the transient operation of a ternary inverter circuit is demonstrated for the first time.

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.

Photoinduced Doping To Enable Tunable and High-Performance Anti-Ambipolar MoTe2/MoS2 Heterotransistors

van der Waals (vdW) p-n heterojunctions formed by two-dimensional nanomaterials exhibit many physical properties and deliver functionalities to enable future electronic and optoelectronic devices. In this report, we demonstrate a tunable and high-performance anti-ambipolar transistor based on MoTe₂/MoS₂ heterojunction through in situ photoinduced doping. The device demonstrates a high on/off ratio of 10⁵ with a large on-state current of several micro-amps. The peak position of the drain-source current in the transfer curve can be adjusted through the doping level across a large dynamic range. In addition, we have fabricated a tunable multivalue inverter based on the heterojunction that demonstrates precise control over its output logic states and window of midlogic through source-drain bias adjustment. The heterojunction also exhibits excellent photodetection and photovoltaic performances. Dynamic and precise modulation of the anti-ambipolar transport properties may inspire functional devices and applications of two-dimensional nanomaterials and their heterostructures of various kinds.

Multi-Valued Logic Circuits Based on Organic Anti-ambipolar Transistors

Multivalued logic circuits, which can handle more information than conventional binary logic circuits, have attracted much attention as a promising way to improve the data-processing capabilities of integrated circuits. In this study, we developed a ternary inverter based on organic field-effect transistors (OFET) as a potential component of high-performance and flexible integrated circuits. Key elements are anti-ambipolar and n-type OFETs connected in series. First, we demonstrate an organic ternary inverter that exhibits three distinct logic states. Second, the operating voltage was greatly reduced by taking advantage of an Al₂O₃ gate dielectric. Finally, the operating voltage was finely tuned by the designing of the device geometry. These results are achievable owing to the flexible controllability of the device configuration, suggesting that the organic ternary inverter plays an important role with regard to high-performance organic integrated circuits.

Carrier Transport and Photoresponse in GeSe/MoS2 Heterojunction p-n Diodes

Simple stacking of thin van der Waals 2D materials with different physical properties enables one to create heterojunctions (HJs) with novel functionalities and new potential applications. Here, a 2D material p-n HJ of GeSe/MoS₂ is fabricated and its vertical and horizontal carrier transport and photoresponse properties are studied. Substantial rectification with a very high contrast (>10⁴ ) through the potential barrier in the vertical-direction tunneling of HJs is observed. The negative differential transconductance with high peak-to-valley ratio (>10⁵ ) due to the series resistance change of GeSe, MoS₂ , and HJs at different gate voltages is observed. Moreover, strong and broad-band photoresponse via the photoconductive effect are also demonstrated. The explored multifunctional properties of the GeSe/MoS₂ HJs are expected to be important for understanding the carrier transport and photoresponse of 2D-material HJs for achieving their use in various new applications in the electronics and optoelectronics fields.

Interface Engineering for Controlling Device Properties of Organic Antiambipolar Transistors

The main purpose of this study is to establish a guideline for controlling the device properties of organic antiambipolar transistors. Our key strategy is to use interface engineering to promote carrier injection at channel/electrode interfaces and carrier accumulation at a channel/dielectric interface. The effective use of carrier injection interlayers and an insulator layer with a high dielectric constant (high-k) enabled the fine tuning of device parameters and, in particular, the onset (V_on) and offset (V_off) voltages. A well-matched combination of the interlayers and a high-k dielectric layer achieved a low peak voltage (0.25 V) and a narrow on-state bias range (2.2 V), indicating that organic antiambipolar transistors have high potential as negative differential resistance devices for multivalued logic circuits.