Energy-Efficient Tunneling Field-Effect Transistors for Low-Power Device Applications: Challenges and Opportunities

Electric Field-Driven Conformational Changes in Molecular Memristor and Synaptic Behavior

This paper demonstrates the use of molecular artificial synapses in neuromorphic computing systems designed for low energy consumption. A molecular junction, based on self-assembled monolayers (SAMs) of alkanethiolates terminated with 2,2'-bipyridine complexed with cobalt chloride, exhibits synaptic behaviors with an energy consumption of 8.0 pJ µm-2. Conductance can be modulated simply by applying pulses in the incoherent charge transport (CT) regime. Charge injection in this regime allows molecules to overcome the low energy barrier for C─C bond rotations, resulting in conformational changes in the SAMs. The reversible potentiation/depression process of conductance achieves 90% accuracy in recognizing patterns from the Modified National Institute of Standards and Technology (MNIST) handwritten digit database. The molecular junction further exhibits both rectifying and conductance hysteresis behaviors, showing potential for use in selector-free synaptic arrays that efficiently suppress sneak currents.

Mixed-Valence Coordination Strategy Creating Ordered Ternary Ultrasmall Homo-/Hetero-structures Driven by Lattice Match for Advanced Photochromism and Encryption Applications

Overcoming the challenges of integrating disparate components in nanoarchitectures, this study introduces a straightforward strategy based on a mixed-valence coordination approach, creating an ordered ternary heterostructure integrated with ultrasmall homojunction. This singular ordered homojunction-heterostructure unites ultrathin 1D rutile TiO2 nanowires (NWs) and ultrathin anatase TiO2 NWs with 0D Prussian Blue Analogs (PBAs) nanoparticles (NPs), all exhibiting crystallographic oriented alignment with each other, forming a ternary mesocrystals. Experimental and theoretical insights disclose that the complex interplay between these dissimilar components is governed by a spontaneous lattice match effect, which not only optimizes but also directs the charge transfer, thereby enhancing both efficiency and stability. It also allows for tailoring the valence states of Fe within the PBA, fine-tuning of the composite's photochromic properties, and introducing abundant defect structures that foster strong interaction with oxygen molecules, enabling controllable color-switching dynamics. Consequently, the FeII 1-xFeIII x-PBA/TiO2 exhibits an optimized ternary structure of R-TiO2/A-TiO2/PBA, demonstrating exceptional photoelectronic properties, significantly enhancing photochromism and secure encryption capabilities. These insights establish a solid foundation for engineering sophisticated complex-ordered nanoarchitectures, advancing sustainable energy and environmental technologies.

Sub-1K Cold-Electron Quantum Well Switching at Room Temperature

Quantum states can provide means to systematically manipulate the transport of electrons. Here we present electron transport across quasi-bound states of two heterogeneous quantum wells (QWs), where the transport of thermally excited electrons is blocked or enabled depending on the relative positions of the two quasi-bound states, with an abrupt current onset occurring when the two QW states align. The QW switch comprises a source (Cr), QW1 (Cr2O3), QW2 (SnOx, x < 2), a tunneling barrier (SiO2), and a drain (Si), where the effective electron mass of QW1 (m*QW1) is selected to be larger than QW2 (m*QW2). The current-voltage (I-V) measurements of the fabricated devices show abrupt current onsets, with the current transition occurring within 0.25 mV, corresponding to an effective electron temperature of 0.8 K at room temperature. Since transistor power consumption is fundamentally tied to effective electron temperature, this sub-1K cold-electron QW switching holds promise for highly energy-efficient computing.

WSe2 Negative Capacitance Field-Effect Transistor for Biosensing Applications

Field-effect transistor (FET) biosensors based on two-dimensional (2D) materials are highly sought after for their high sensitivity, label-free detection, fast response, and ease of on-chip integration. However, the subthreshold swing (SS) of FETs is constrained by the Boltzmann limit and cannot fall below 60 mV/dec, hindering sensor sensitivity enhancement. Additionally, the gate-leakage current of 2D material biosensors in liquid environments significantly increases, adversely affecting the detection accuracy and stability. Based on the principle of negative capacitance, this paper presents for the first time a two-dimensional material WSe2 negative capacitance field-effect transistor (NCFET) with a minimum subthreshold swing of 56 mV/dec in aqueous solution. The NCFET shows a significantly improved biosensor function. The pH detection sensitivity of the NCFET biosensor reaches 994 pH-1, nearly an order of magnitude higher than that of the traditional two-dimensional WSe2 FET biosensor. The Al2O3/HfZrO (HZO) bilayer dielectric in the NCFET not only contributes to negative capacitance characteristics in solution but also significantly reduces the leakage in solution. Utilizing an enzyme catalysis method, the WSe2 NCFET biosensor demonstrates a specific detection of glucose molecules, achieving a high sensitivity of 4800 A/A in a 5 mM glucose solution and a low detection limit (10-9 M). Further experiments also exhibit the ability of the biosensor to detect glucose in sweat.

Quantum Graphene Asymmetric Devices for Harvesting Electromagnetic Energy

We present here the fabrication at the wafer level and the electrical performance of two types of graphene diodes: ballistic trapezoidal-shaped graphene diodes and lateral tunneling graphene diodes. In the case of the ballistic trapezoidal-shaped graphene diode, we observe a large DC current of 200 µA at a DC bias voltage of ±2 V and a large voltage responsivity of 2000 v/w, while in the case of the lateral tunneling graphene diodes, we obtain a DC current of 1.5 mA at a DC bias voltage of ±2 V, with a voltage responsivity of 3000 v/w. An extended analysis of the defects produced during the fabrication process and their influences on the graphene diode performance is also presented.

Reduced subthreshold swing in a vertical tunnel FET using a low-work-function live metal strip and a low-k material at the drain

In this research paper, a vertical tunnel field-effect transistor (TFET) structure containing a live metal strip and a material with low dielectric constant is designed, and its performance metrics are analyzed in detail. Low-k SiO2 is incorporated in the channel-drain region. A live molybdenum metal strip with low work function is placed in a high-k HfO2 layer in the source-channel region. The device is examined by the parameters I off, subthreshold swing, threshold voltage, and I on/I off ratio. The introduction of a live metal strip in the dielectric layer closer to the source-channel interface results in a minimum subthreshold slope and a good I on/I off ratio. The low-k material at the drain reduces the gate-to-drain capacitance. Both the SiO2 layer and the live metal strip show excellent leakage current reduction to 1.4 × 10-17 A/μm. The design provides a subthreshold swing of 5 mV/decade, which is an excellent improvement in TFETs, an on-current of 1.00 × 10-5 A/μm, an I on/I off ratio of 7.14 × 1011, and a threshold voltage of 0.28 V.

Two-Dimensional Tunneling Memtransistor with Thin-Film Heterostructure for Low-Power Logic-in-Memory Complementary Metal-Oxide Semiconductor

With the demand for high-performance and miniaturized semiconductor devices continuously rising, the development of innovative tunneling transistors via efficient stacking methods using two-dimensional (2D) building blocks has paramount importance in the electronic industry. Hence, 2D semiconductors with atomically thin geometries hold significant promise for advancements in electronics. In this study, we introduced tunneling memtransistors with a thin-film heterostructure composed of 2D semiconducting MoS2 and WSe2. Devices with the dual function of tuning and memory operation were realized by the gate-regulated modulation of the barrier height at the heterojunction and manipulation of intrinsic defects within the exfoliated nanoflakes using solution processes. Further, our investigation revealed extensive edge defects and four distinct defect types, namely monoselenium vacancies, diselenium vacancies, tungsten vacancies, and tungsten adatoms, in the interior of electrochemically exfoliated WSe2 nanoflakes. Additionally, we constructed complementary metal-oxide semiconductor-based logic-in-memory devices with a small static power in the range of picowatts using the developed tunneling memtransistors, demonstrating a promising approach for next-generation low-power nanoelectronics.

Overcoming the Unfavorable Effects of "Boltzmann Tyranny:" Ultra-Low Subthreshold Swing in Organic Phototransistors via One-Transistor-One-Memristor Architecture

Organic phototransistors (OPTs), as photosensitive organic field-effect transistors (OFETs), have gained significant attention due to their pivotal roles in imaging, optical communication, and night vision. However, their performance is fundamentally limited by the Boltzmann distribution of charge carriers, which constrains the average subthreshold swing (SSave ) to a minimum of 60 mV/decade at room temperature. In this study, an innovative one-transistor-one-memristor (1T1R) architecture is proposed to overcome the Boltzmann limit in conventional OFETs. By replacing the source electrode in an OFET with a memristor, the 1T1R device exploits the memristor's sharp resistance state transitions to achieve an ultra-low SSave of 18 mV/decade. Consequently, the 1T1R devices demonstrate remarkable sensitivity to photo illumination, with a high specific detectivity of 3.9 × 109  cm W-1 Hz1/2 , outperforming conventional OPTs (4.9 × 104  cm W-1 Hz1/2 ) by more than four orders of magnitude. The 1T1R architecture presents a potentially universal solution for overcoming the detrimental effects of "Boltzmann tyranny," setting the stage for the development of ultra-low SSave devices in various optoelectronic applications.

The Roadmap of 2D Materials and Devices Toward Chips

Due to the constraints imposed by physical effects and performance degradation, silicon-based chip technology is facing certain limitations in sustaining the advancement of Moore's law. Two-dimensional (2D) materials have emerged as highly promising candidates for the post-Moore era, offering significant potential in domains such as integrated circuits and next-generation computing. Here, in this review, the progress of 2D semiconductors in process engineering and various electronic applications are summarized. A careful introduction of material synthesis, transistor engineering focused on device configuration, dielectric engineering, contact engineering, and material integration are given first. Then 2D transistors for certain electronic applications including digital and analog circuits, heterogeneous integration chips, and sensing circuits are discussed. Moreover, several promising applications (artificial intelligence chips and quantum chips) based on specific mechanism devices are introduced. Finally, the challenges for 2D materials encountered in achieving circuit-level or system-level applications are analyzed, and potential development pathways or roadmaps are further speculated and outlooked.

Low temperature passivation of silicon surfaces for enhanced performance of Schottky-barrier MOSFET

By using a simple device architecture along with a simple process design and a low thermal-budget of a maximum of 100 °C for passivating metal/semiconductor interfaces, a Schottky barrier MOSFET device with a low subthreshold slope of 70 mV dec-1could be developed. This device is enabled after passivation of the metal/silicon interface (found at the source/drain regions) with ultra-thin SiOxfilms, followed by the e-beam evaporation of high- quality aluminum and by using atomic-layer deposition for HfO2as a gate oxide. All of these fabrication steps were designed in a sequential process so that a gate-last recipe could minimize the defect density at the aluminum/silicon and HfO2/silicon interfaces, thus preserving the Schottky barrier height and ultimately, the outstanding performance of the transistor. This device is fully integrated into silicon after standard CMOS-compatible processing, so that it could be easily adopted into front-end-of-line or even in back-end-of-line stages of an integrated circuit, where low thermal budget is required and where its functionality could be increased by developing additional and fast logic.

Interface engineering strategy for multisource spintronic devices via TMPS4 modulation of black-phosphorus

Interface engineering is an effective strategy for significantly providing performance improvements in logic operations and information storage techniques, given the quantum effects characteristic of pristine two-dimensional ferromagnetic (FM) materials. Furthermore, the van der Waals (vdW) heterostructures enable a more efficient design of logic components. Herein, two novel designs of van der Waals heterostructures are proposed, black-phosphorus (BP)/TMPS4(TM = Cr, Fe-Mn), where the CrPS4 relies on odd and even layers for FM state - antiferromagnetic (AFM) state alternation, which is used to modulate BP with high hole mobility but no magnetism. Based on the first principles, the simulation results show that the two heterojunctions exhibit high stability, carrier mobility, and thermoelectric effects, of which the BP/CrPS4 heterojunction is specifically modulated to a type II electronic bandstructure. These two structures are applied in multi-source logic devices. The device performances show that the devices have spin Sebeck effect (SSE), perfect spin filtering effect (SFE), high extinction ratio (1347), and high thermal magnetoresistivity (1011). The above results suggest BP/TMPS4 bilayers as promising candidates for spin-based vdW devices and facilitate the future development of atomically thin magnetic information storage.

Two-Dimensional Transition Metal Dichalcogenide Tunnel Field-Effect Transistors for Biosensing Applications

Field-effect transistor (FET) biosensors based on two-dimensional (2D) materials have drawn significant attention due to their outstanding sensitivity. However, the Boltzmann distribution of electrons imposes a physical limit on the subthreshold swing (SS), and a 2D-material biosensor with sub-60 mV/dec SS has not been realized, which hinders further increase of the sensitivity of 2D-material FET biosensors. Here, we report tunnel FETs (TFETs) based on a SnSe₂/WSe₂ heterostructure and observe the tunneling effect of a 2D material in aqueous solution for the first time with an ultralow SS of 29 mV/dec. A bilayer dielectric (Al₂O₃/HfO₂) and graphene contacts, which significantly reduce the leakage current in solution and contact resistance, respectively, are crucial to the realization of the tunneling effect in solution. Then, we propose a novel biosensing method by using tunneling current as the sensing signal. The TFETs show an extremely high pH sensitivity of 895/pH due to ultralow SS, surpassing the sensitivity of FET biosensors based on a single 2D material (WSe₂) by 8-fold. Specific detection of glucose is realized, and the biosensors show a superb sensitivity (3158 A/A for 5 mM), wide sensing range (from 10^-9 to 10^-3 M), low detection limit (10^-9 M), and rapid response rate (11 s). The sensors also exhibit the ability of monitoring glucose in complex biofluid (sweat). This work provides a platform for ultrasensitive biosensing. The discovery of the tunneling effect of 2D materials in aqueous solution may stimulate further fundamental research and potential applications.

Electrical Nanobiosensors for Nucleic Acid Based Diagnostics

Recent advances in nanotechnologies have promoted the iterative updating of nucleic acid sensors. Among various sensing technologies, the electrical nanobiosensor is regarded as one of the most promising prospects to achieve rapid, precise, and point-of-care nucleic acid based diagnostics. In this Perspective, we introduce recent progresses in electrical nanobiosensors for nucleic acid detection. First, the strategies for improving detection performance are summarized, including chemical amplification and electrical amplification. Then, the detection mechanism of electrical nanobiosensors, such as electrochemical biosensors, field-effect transistors, and photoelectric enhanced biosensors, is illustrated. At the same time, their applications in cancer screening, pathogen detection, gene sequencing, and genetic disease diagnosis are introduced. Finally, challenges and future prospects in clinical application are discussed.

Performance tunability of field-effect transistors using MoS2(1-x)Se2xalloys

Ultra-thin channel materials with excellent tunability of their electronic properties are necessary for the scaling of electronic devices. Two-dimensional materials such as transition metal dichalcogenides (TMDs) are ideal candidates for this due to their layered nature and great electrostatic control. Ternary alloys of these TMDs show composition-dependent electronic structure, promising excellent tunability of their properties. Here, we systematically compare molybdenum sulphoselenide (MoS_2(1-x)Se_2x) alloys, MoS₁Se₁and MoS_0.4Se_1.6. We observe variations in strain and carrier concentration with their composition. Using them, we demonstrate n-channel field-effect transistors (FETs) with SiO₂and high-kHfO₂as gate dielectrics, and show tunability in threshold voltage, subthreshold slope (SS), drain current, and mobility. MoS₁Se₁shows better promise for low-power FETs with a minimum SS of 70 mV dec^-1, whereas MoS_0.4Se_1.6, with its higher mobility, is suitable for faster operations. Using HfO₂as gate dielectric, there is an order of magnitude reduction in interface traps and 2× improvement in mobility and drain current, compared to SiO₂. In contrast to MoS₂, the FETs on HfO₂also display enhancement-mode operation, making them better suited for CMOS applications.

Design and synthesis of a 3D flexible film electrode based on a sodium carboxymethyl cellulose–polypyrrole@reduced graphene oxide composite for supercapacitors

A novel, environmentally friendly and freestanding 3D flexible film electrode (CMC–PPy@RGO) was prepared by a simple vacuum filtration method.