Approaching the Collection Limit in Hot Electron Transistors with Ambipolar Hot Carrier Transport

Augmented Extraction Efficiency of a Hot D Exciton in MoS2 via Intervalley Scattering

Prolonging hot carrier cooling, a crucial factor in optoelectronic applications, including hot carrier photovoltaics, presents a significant challenge. High-energy band-nesting excitons within parallel bands offer a promising and underexplored avenue for addressing this issue. Here, we exploit an exceptional D exciton cooling prolongation of 2 to 3 orders of magnitude compared to sub-picosecond in typical transition metal dichalcogenides (TMDs) owing to the complex Coulomb environment and the sequential and mismatch-valley relaxation. Simultaneously, the intervalley scattering upconversion of band-edge excitons with the slow D exciton formation in the metastable Γ valley/hill also reduces the cooling rate. We successfully extract D and C excitons as hot carriers through integrating with various thicknesses of TiOx, achieving the highest efficiency of 98% and 85% at a Ti thickness of 2 nm. Our findings highlight the potential of band-nesting excitons for extending hot carrier cooling time, paving the way for advancements in hot carrier-based optoelectronic devices.

Small Feature-Size Transistors Based on Low-Dimensional Materials: From Structure Design to Nanofabrication Techniques

For several decades after Moore's Law is proposed, there is a continuous effort to reduce the feature-size of transistors. However, as the size of transistors continues to decrease, numerous challenges and obstacles including severe short channel effects (SCEs) are emerging. Recently, low-dimensional materials have provided new opportunities for constructing small feature-size transistors due to their superior electrical properties compared to silicon. Here, state-of-the-art low-dimensional materials-based transistors with small feature-sizes are reviewed. Different from other works that mainly focus on material characteristics of a specific device structure, the discussed topics are utilizing device structure design including vertical structure and nano-gate structure, and nanofabrication techniques to achieve small feature-sizes of transistors. A comprehensive summary of these small feature-size transistors is presented by illustrating their operation mechanism, relevant fabrication processes, and corresponding performance parameters. Besides, the role of small feature-size transistors based on low-dimensional materials in further reducing the small footprint is also clarified and their cutting-edge applications are highlighted. Finally, a comparison and analysis between state-of-art transistors is made, as well as a glimpse into the future research trajectory of low dimensional materials-based small feature-size transistors is briefly outlined.

Integrated Graphene Heterostructures in Optical Sensing

Graphene-an outstanding low-dimensional material-exhibited many physics behaviors that are unknown over the past two decades, e.g., exceptional matter-light interaction, large light absorption band, and high charge carrier mobility, which can be adjusted on arbitrary surfaces. The deposition approaches of graphene on silicon to form the heterostructure Schottky junctions was studied, unveiling new roadmaps to detect the light at wider-ranged absorption spectrums, e.g., far-infrared via excited photoemission. In addition, heterojunction-assisted optical sensing systems enable the active carriers' lifetime and, thereby, accelerate the separation speed and transport, and then they pave new strategies to tune high-performance optoelectronics. In this mini-review, an overview is considered concerning recent advancements in graphene heterostructure devices and their optical sensing ability in multiple applications (ultrafast optical sensing system, plasmonic system, optical waveguide system, optical spectrometer, or optical synaptic system) is discussed, in which the prominent studies for the improvement of performance and stability, based on the integrated graphene heterostructures, have been reported and are also addressed again. Moreover, the pros and cons of graphene heterostructures are revealed along with the syntheses and nanofabrication sequences in optoelectronics. Thereby, this gives a variety of promising solutions beyond the ones presently used. Eventually, the development roadmap of futuristic modern optoelectronic systems is predicted.

GaN/Gr (2D)/Si (3D) Combined High-Performance Hot Electron Transistors

To overcome the problem of minority carrier storage time in bipolar transistors, a hot electron transistor (HET) has been proposed. This device has the advantage of high working speed and some complex logic functions can be completed by using one component. Here, we demonstrate a mixed-dimensional HET composed of GaN/AlN microwires, graphene (Gr), and Si. The electrons between GaN/AlN are injected into graphene by an F-N tunneling mechanism to achieve high speed hot electrons, then cross graphene by ballistic transport, and are collected in a nearly lossless manner through a low-barrier Si. Therefore, the device shows a record DC gain of 16.2, a collection efficiency close to the limit of 99.9% based on the graphene hot electron transistor (GHET), an emitter current density of about 68.7 A/cm², and a high on/off current ratio reaching ∼10⁷. Meanwhile, the current saturation range is wide, beyond those of most GHETs. It has potential applications as a power amplifier.

Hot Carrier Cooling and Trapping in Atomically Thin WS2 Probed by Three-Pulse Femtosecond Spectroscopy

Transition metal dichalcogenides (TMDs) have shown outstanding semiconducting properties which make them promising materials for next-generation optoelectronic and electronic devices. These properties are imparted by fundamental carrier-carrier and carrier-phonon interactions that are foundational to hot carrier cooling. Recent transient absorption studies have reported ultrafast time scales for carrier cooling in TMDs that can be slowed at high excitation densities via a hot-phonon bottleneck (HPB) and discussed these findings in the light of optoelectronic applications. However, quantitative descriptions of the HPB in TMDs, including details of the electron-lattice coupling and how cooling is affected by the redistribution of energy between carriers, are still lacking. Here, we use femtosecond pump-push-probe spectroscopy as a single approach to systematically characterize the scattering of hot carriers with optical phonons, cold carriers, and defects in a benchmark TMD monolayer of polycrystalline WS₂. By controlling the interband pump and intraband push excitations, we observe, in real-time (i) an extremely rapid "intrinsic" cooling rate of ∼18 ± 2.7 eV/ps, which can be slowed with increasing hot carrier density, (ii) the deprecation of this HPB at elevated cold carrier densities, exposing a previously undisclosed role of the carrier-carrier interactions in mediating cooling, and (iii) the interception of high energy hot carriers on the subpicosecond time scale by lattice defects, which may account for the lower photoluminescence yield of TMDs when excited above band gap.

2D Heterostructures for Ubiquitous Electronics and Optoelectronics: Principles, Opportunities, and Challenges

A grand family of two-dimensional (2D) materials and their heterostructures have been discovered through the extensive experimental and theoretical efforts of chemists, material scientists, physicists, and technologists. These pioneering works contribute to realizing the fundamental platforms to explore and analyze new physical/chemical properties and technological phenomena at the micro-nano-pico scales. Engineering 2D van der Waals (vdW) materials and their heterostructures via chemical and physical methods with a suitable choice of stacking order, thickness, and interlayer interactions enable exotic carrier dynamics, showing potential in high-frequency electronics, broadband optoelectronics, low-power neuromorphic computing, and ubiquitous electronics. This comprehensive review addresses recent advances in terms of representative 2D materials, the general fabrication methods, and characterization techniques and the vital role of the physical parameters affecting the quality of 2D heterostructures. The main emphasis is on 2D heterostructures and 3D-bulk (3D) hybrid systems exhibiting intrinsic quantum mechanical responses in the optical, valley, and topological states. Finally, we discuss the universality of 2D heterostructures with representative applications and trends for future electronics and optoelectronics (FEO) under the challenges and opportunities from physical, nanotechnological, and material synthesis perspectives.

2D Cu9 S5 /PtS2 /WSe2 Double Heterojunction Bipolar Transistor with High Current Gain

Bipolar junction transistor (BJT) as one important circuit element is now widely used in high-speed computation and communication for its capability of high-power signal amplification. 2D materials and their heterostructures are promising in building high-amplification and high-frequency BJTs because they can be naturally thin and highly designable in tailoring components properties. However, currently the low emitter injection efficiency results in only moderate current gain achieved in the pioneer researches, severely restraining its future development. Herein, it is shown that an elaborately designed double heterojunction bipolar transistor (DHBT) can greatly promote the injection efficiency, improving the current gain by order of magnitude. In this DHBT high-doping-density wide-bandgap 2D Cu₉ S₅ is used as emitter and narrow-bandgap PtS₂ as base. This heterostructure efficiently suppresses the reverse electron flux from base and increase the injection efficiency. Consequently, the DHBT achieves an excellent current gain (β ≈ 910). This work systematically explores the electrical behavior of 2D materials based DHBT, and provides deep insight of the architecture design for building high gain DHBT, which may promote the applications of 2Dheterojunctions in the fields of integrated circuits.

There is plenty of room at the top: generation of hot charge carriers and their applications in perovskite and other semiconductor-based optoelectronic devices

Hot charge carriers (HC) are photoexcited electrons and holes that exist in nonequilibrium high-energy states of photoactive materials. Prolonged cooling time and rapid extraction are the current challenges for the development of future innovative HC-based optoelectronic devices, such as HC solar cells (HCSCs), hot energy transistors (HETs), HC photocatalytic reactors, and lasing devices. Based on a thorough analysis of the basic mechanisms of HC generation, thermalization, and cooling dynamics, this review outlines the various possible strategies to delay the HC cooling as well as to speed up their extraction. Various materials with slow cooling behavior, including perovskites and other semiconductors, are thoroughly presented. In addition, the opportunities for the generation of plasmon-induced HC through surface plasmon resonance and their technological applications in hybrid nanostructures are discussed in detail. By judiciously designing the plasmonic nanostructures, the light coupling into the photoactive layer and its optical absorption can be greatly enhanced as well as the successful conversion of incident photons to HC with tunable energies can also be realized. Finally, the future outlook of HC in optoelectronics is highlighted which will provide great insight to the research community.

High-Frequency Graphene Base Hot-Electron Transistor

The integration of graphene and other two-dimensional (2D) materials with existing silicon semiconductor technology is highly desirable. This is due to the diverse advantages and potential applications brought about by the consequent miniaturization of the resulting electronic devices. Nevertheless, such devices that can operate at very high frequencies for high-speed applications are eminently preferred. In this work, we demonstrate a vertical graphene base hot-electron transistor that performs in the radio frequency regime. Our device exhibits a relatively high current density (∼200 A/cm²), high common base current gain (α* ∼ 99.2%), and moderate common emitter current gain (β* ∼ 2.7) at room temperature with an intrinsic current gain cutoff frequency of around 65 GHz. Furthermore, cutoff frequency can be tuned from 54 to 65 GHz by varying the collector-base bias. We anticipate that this proposed transistor design, built by the integrated 2D material and silicon semiconductor technology, can be a potential candidate to realize extra fast radio frequency tunneling hot-carrier electronics.

Abnormal Electron Emission in a Vertical Graphene/Hexagonal Boron Nitride van der Waals Heterostructure Driven by a Hot Hole-Induced Auger Process

Understanding the scattering process of field injection hot carriers is important for modulating their behaviors, which is the key for improving the efficiency of charge transfer and energy conversion in hot carrier devices. In this work, a significant electron thermalization induced by Auger scattering between a field injection hot hole and a native cold electron has been observed in a vertical single layer graphene/hexagonal boron nitride/few layer graphene (Gr/hBN/FLG) device by measuring the vacuum electron emission characteristics. For the first time, it is found that vacuum electron emission can be measured under both directions of bias within the device. Furthermore, electrons can be emitted even when the applied bias energy is smaller than the work function of the Gr cathode. Further analysis of the emission electron kinetic energy indicates that the low turn-on bias results from the emission of energetic electrons that are ∼3 eV higher than the Fermi level. A semiquantitative model based on hot hole-induced Auger electron emission is established to reproduce the results. All of these findings not only expand our understanding of the hot carrier scattering process in graphene but also provide insights into the applications of hot carrier devices.