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Bilayer graphene has attracted interest for its unique properties, including interesting electrical behavior when one layer is slightly rotated relative to the other. However, the quality of large-area bilayer graphene is often limited by the layer-plus-island growth mode in which islands of thicker graphene present as unavoidable impurities. Here, we report the observation of the layer-by-layer, Frank-van der Merwe (FM) growth mode in bilayer graphene where multilayer impurities are suppressed. Instead of the conventional surface adhesive energy, it is found that interface adhesive energy is possible to be tuned with an oxidative pretreatment. The FM-grown bilayer graphene is of AB-stacking or with small-twisting-angle (θ = 0-5°), which is more mechanically robust compared to monolayer graphene, facilitating a free-standing wet transfer technology.

Vertical gallium nitride (GaN) power devices are enabling next-generation power electronic devices and systems with higher energy efficiency, higher power density, faster switching, and smaller form factor. In Part I of this review, we have reviewed the basic design principles and physics of building blocks of vertical GaN power devices, i.e., Schottky barrier diodes and p-n diodes. Key topics such as materials engineering, device engineering, avalanche breakdown, and leakage mechanisms are discussed. In Part II of this review, several more advanced power rectifiers are discussed, including junction barrier Schottky (JBS) rectifiers, merged p-n/Schottky (MPS) rectifiers, and trench metal–insulator–semiconductor barrier Schottky (TMBS) rectifiers. Normally- OFF GaN power transistors have been realized in various advanced device structures, including current aperture vertical electron transistors (CAVETs), junction field-effect transistors (JFETs), metal–oxide–semiconductor field-effect transistors (MOSFETs), and fin field-effect transistors (FinFETs). A detailed analysis on their performance metrics is provided, with special emphasis on the impacts of key fabrication processes such as etching, ion implantation, and surface treatment. Lastly, exciting progress has been made on selective area doping and regrowth, a critical process for the fabrication of vertical GaN power devices. Various materials characterization techniques and surface treatments have proven to be beneficial in aiding this rapid development. This timely and comprehensive review summarizes the current progress, understanding, and challenges in vertical GaN power devices, which can serve as not only a gateway for those interested in the field but also a critical reference for researchers in the wide bandgap semiconductor and power electronics community.

The strength of interlayer coupling critically affects the physical properties of 2D materials such as black phosphorus (BP), where the electronic structure depends sensitively on layer thickness. Rigid-layer vibrations reflect directly the interlayer coupling strength in 2D van der Waals solids, but measurement of these characteristic frequencies is made difficult by sample instability and small Raman scattering cross sections in atomically thin elemental crystals. Here, we overcome these challenges in BP by performing resonance-enhanced low-frequency Raman scattering under an argon-protective environment. Interlayer breathing modes for atomically thin BP were previously unobservable under conventional (nonresonant) excitation but became strongly enhanced when the excitation energy matched the sub-band electronic transitions of few-layer BP, down to bilayer thicknesses. The measured out-of-plane interlayer force constant was found to be 10.1 × 1019 N/m3 in BP, which is comparable to graphene. Accurate characterization of the interlayer coupling strength lays the foundation for future exploration of BP twisted structures and heterostructures.

Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices1,3. However, owing to metal-induced gap states (MIGS)4–7, energy barriers at the metal–semiconductor interface—which fundamentally lead to high contact resistance and poor current-delivery capability—have constrained the improvement of two-dimensional semiconductor transistors so far2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS2; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS2, WS2 and WSe2. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore’s law. Electric contacts of semimetallic bismuth on monolayer semiconductors are shown to suppress metal-induced gap states and thus have very low contact resistance and a zero Schottky barrier height.

The authors acknowledge financial support from the Air Force Office of Scientific Research under the MURI-FATE programs (Grant Nos. FA9550-15-1-0514 and FA9550-16-1-0031) as well as partial support by the ARO project W911NF-19-10372 (award title: Optical Communication with Synthetic Cells). V.S. was supported by the KAUST sensor project CRF2015-SENSORS-2719. This work was carried out in part using MIT’s Microsystems Technology Laboratories and MIT.nano

Recording, modelling and understanding tactile interactions is important in the study of human behaviour and in the development of applications in healthcare and robotics. However, such studies remain challenging because existing wearable sensory interfaces are limited in terms of performance, flexibility, scalability and cost. Here, we report a textile-based tactile learning platform that can be used to record, monitor and learn human–environment interactions. The tactile textiles are created via digital machine knitting of inexpensive piezoresistive fibres, and can conform to arbitrary three-dimensional geometries. To ensure that our system is robust against variations in individual sensors, we use machine learning techniques for sensing correction and calibration. Using the platform, we capture diverse human–environment interactions (more than a million tactile frames) and show that the artificial-intelligence-powered sensing textiles can classify humans’ sitting poses, motions and other interactions with the environment. We also show that the platform can recover dynamic whole-body poses, reveal environmental spatial information and discover biomechanical signatures. Large-scale sensing textiles that can conform to arbitrary three-dimensional geometries and are created through digital machine knitting of piezoresistive fibres can be used to record, monitor and learn human–environment interactions.

Field emitters are attracting much attention recently because of their potential for high-frequency and harsh-environment applications. Although silicon is the most studied semiconductor for field emitters, III-Nitrides are also very promising thanks to their tunable electron affinities. Two main challenges exist for low turn-on-voltage III-Nitrides field emitters: lack of a self-aligned gate and tip-sharpening technologies. In this work, we demonstrate self-aligned-gated GaN field emitter arrays whose tips are sharpened by a wet-based digital etching technology. This technology allows to reduce the tip size from 40 nm down to 20 nm. Gated GaN field emitters with a turn-on voltage ( $\text{V}_{GE,ON}$ ) of 20 V and current density of 150 mA/cm2 at $\text{V}_{GE} =50$ V have been demonstrated.

Despite several theoretically proposed two-dimensional (2D) diamond structures, experimental efforts to obtain such structures are in initial stage. Recent high-pressure experiments provided significant advancements in the field, however, expected properties of a 2D-like diamond such as sp3 content, transparency and hardness, have not been observed together in a compressed graphene system. Here, we compress few-layer graphene samples on SiO2/Si substrate in water and provide experimental evidence for the formation of a quenchable hard, transparent, sp3-containing 2D phase. Our Raman spectroscopy data indicates phase transition and a surprisingly similar critical pressure for two-, five-layer graphene and graphite in the 4–6 GPa range, as evidenced by changes in several Raman features, combined with a lack of evidence of significant pressure gradients or local non-hydrostatic stress components of the pressure medium up to ≈ 8 GPa. The new phase is transparent and hard, as evidenced from indentation marks on the SiO2 substrate, a material considerably harder than graphene systems. Furthermore, we report the lowest critical pressure ( ≈ 4 GPa) in graphite, which we attribute to the role of water in facilitating the phase transition. Theoretical calculations and experimental data indicate a novel, surface-to-bulk phase transition mechanism that gives hint of diamondene formation.

FinFET is the backbone device technology for CMOS electronics at deeply scaled technology nodes per Moore’s law. Recently, the FinFET concept has been leveraged to develop a new generation of vertical power transistors based on wide-bandgap (WBG) and ultrawide-bandgap (UWBG) semiconductors for kilovolts and high-power applications. The sidewall gate-stack in a vertical power FinFET can rely on either a metal–oxide–semiconductor (MOS) structure or a p-n junction, rendering a Fin-MOSFET or a fin-based junction field-effect transistor (Fin-JFET), respectively. Although the device technologies are still at the early stage of development, 1.2-kV-class WBG gallium nitride (GaN) power Fin-MOSFETs have demonstrated one of the highest static and switching performances in all similarly rated power transistors; UWBG gallium oxide power Fin-MOSFETs have shown high performance up to a breakdown voltage over 2.6 kV. Early UWBG diamond lateral power Fin-MOSFETs have also been demonstrated. Meanwhile, GaN power Fin-JFETs are currently under active development. This article provides a comprehensive tutorial and review of the background and recent advances in WBG and UWBG vertical power FinFETs. It covers fundamental device physics, device and process development, as well as the static and switching performance of various power Fin-MOSFETs and Fin-JFETs. This article is concluded by identifying the current challenges and exciting research opportunities in this very dynamic research field.

Power and RF electronics applications have spurred massive investment into a range of wide and ultrawide bandgap semiconductor devices which can switch large currents and voltages rapidly with low losses. However, the end systems using these devices are often limited by the parasitics of integrating and driving these chips from the silicon complementary metal–oxide-semiconductor-based design (CMOS) circuitry necessary for complex control logic. For that reason, implementation of CMOS logic directly in the wide bandgap platform has become a way for each maturing material to compete. This review examines potential CMOS monolithic and hybrid approaches in a variety of wide bandgap materials.

Dr. Xu Zhang is an assistant professor in the Department of Electrical and Computer Engineering at Carnegie Mellon University. He obtained his BS in physics from University of Science and Technology of China and his PhD in electrical engineering from Massachusetts Institute of Technology. He was an Enrico Fermi Fellow at Argonne National Laboratory from 2018 to 2019. His research interests include nanoelectronics, nanophotonics, and emerging materials for energy harvesting, sensing, and communication applications. Dr. Jesus Grajal received his PhD in electrical engineering from Universidad Politecnica de Madrid (UPM) in 1998. Since 2017, he has been a Full Professor with the Signals, Systems, and Radiocommunications Department at UPM. His current research interests include semiconductor device modeling and high-frequency circuit and system design. Dr. Marisa Lopez-Vallejo received a PhD in electronic engineering from Universidad Politecnica de Madrid (UPM) in 1999. She was with Bell Laboratories, Lucent Technologies, Murray Hill, NJ, USA as a Member of the Technical Staff. Since 2016, she has been a Full Professor with the Electronic Engineering Department at UPM. Her major interests are in the area of low power, PVT-aware design, computer-aided diagnostic methods and tools, emerging memories, and application-specific high-performance programmable architectures. Elaine McVay is pursuing a PhD in the Department of Electrical Engineering and Computer Science at MIT. Her research interests include 2D material/thin-film photovoltaics and infrared detectors. Dr. Tomas Palacios is a Professor in the Department of Electrical Engineering and Computer Science at MIT. He received his PhD from the University of California - Santa Barbara in 2006 and his undergraduate degree in Telecommunication Engineering from the Universidad Politecnica de Madrid. His current research focuses on demonstrating new electronic devices and applications for novel semiconductor materials such as graphene and gallium nitride. His work has been recognized with multiple awards, including the Presidential Early Career Award for Scientists and Engineers, the IEEE George Smith Award, and the NSF, ONR, and DARPA Young Faculty Awards, among many others.

Trichalcogenide semiconductors could provide a platform for quasi-one-dimensional electronic devices, on condition that their tunable doping capability is established. This work presents a synthetic strategy towards degenerately doped titanium trisulfide nanostructures that feature ultralow carrier activation barriers and exceptional ambient stability. The anomalous electrical conductivity of the synthesized nanostructures is revealed both experimentally and computationally to be associated with high-density S22- vacancies, which are generated spontaneously during the chemical vapor deposition process and act effectively as the electron dopants. These results hence portend the tantalizing possibility of constructing nanoscale electronics with defect-engineered trichalcogenide semiconductors.

Technology advancements in history have often been propelled by material innovations. In recent years, two-dimensional (2D) materials have attracted substantial interest as an ideal platform to construct atomic-level material architectures. In this work, we design a reaction pathway steered in a very different energy landscape, in contrast to typical thermal chemical vapor deposition method in high temperature, to enable room-temperature atomic-layer substitution (RT-ALS). First-principle calculations elucidate how the RT-ALS process is overall exothermic in energy and only has a small reaction barrier, facilitating the reaction to occur at room temperature. As a result, a variety of Janus monolayer transition metal dichalcogenides with vertical dipole could be universally realized. In particular, the RT-ALS strategy can be combined with lithography and flip-transfer to enable programmable in-plane multiheterostructures with different out-of-plane crystal symmetry and electric polarization. Various characterizations have confirmed the fidelity of the precise single atomic layer conversion. Our approach for designing an artificial 2D landscape at selective locations of a single layer of atoms can lead to unique electronic, photonic, and mechanical properties previously not found in nature. This opens a new paradigm for future material design, enabling structures and properties for unexplored territories.

The performance of GaN CMOS technology was systematically evaluated through experimentally calibrated simulations. Logic inverters based on self-aligned p-FETs and n-FinFETs with W p /W n =5 exhibit 1.4 V of logic-low noise margin (NML) and 2.8 V of high noise margin (NMH) for V DD =5 V. Simulations also confirmed that GaN CMOS logic building blocks are operational with negligible performance degradation at high temperature (300 °C).

GaN vertical finFETs are capable of high breakdown voltages and large current density without the need for regrowth or p-type doping. These attributes make them a promising candidate for next-generation high-frequency power applications. Here, we demonstrate the first high frequency characterization of vertical GaN finFETs. These devices utilize a highly insulating sapphire substrate and a quasi-vertical design that avoids the expensive bulk-GaN substrates typically used in vertical power applications. Moreover, contacts remain on the top of the wafer, thereby enabling high-frequency characterization and potentially easing future system integration.

Daily human activities, e.g., locomotion, exercises, and resting, are heavily guided by the tactile interactions between the human and the ground. In this work, leveraging such tactile interactions, we propose a 3D human pose estimation approach using the pressure maps recorded by a tactile carpet as input. We build a low-cost, high-density, large-scale intelligent carpet, which enables the real-time recordings of human-floor tactile interactions in a seamless manner. We collect a synchronized tactile and visual dataset on various human activities. Employing a state-of-the-art camera-based pose estimation model as supervision, we design and implement a deep neural network model to infer 3D human poses using only the tactile information. Our pipeline can be further scaled up to multi-person pose estimation. We evaluate our system and demonstrate its potential applications in diverse fields.

This paper presents the first analytical model for the ON-resistance ( ${R}_{ \mathrm{\scriptscriptstyle ON}}$ ) in vertical power FinFETs. The model allows to extract the channel mobility and series resistance and to separate the current conduction through the bulk fin channel and the accumulation-mode metal–oxide–semiconductor (MOS) channel. The model was validated by experiments and simulations. The extracted series resistance was verified by measuring a diode fabricated in the same wafer with the FinFETs. At the same time, simulations using the extracted channel mobility and series resistance agreed well with the experiments. The model was then used to analyze a 1200 V GaN vertical power FinFET. The main ${R}_{ \mathrm{\scriptscriptstyle ON}}$ component was identified to be from the drift layer and the substrate, while the gate-modulated channel resistance only accounts for ~13% of the total device ${R}_{ \mathrm{\scriptscriptstyle ON}}$ . Our model enables parameter extraction from the dc characteristics of a single device, and therefore, provides a fast and easy way to understand, analyze, and design vertical power FinFETs. Our model can also be adjusted to allow for fast and accurate parameter extraction in other power transistors with a vertical gate-modulated channel, such as trench MOSFETs.

This letter reports the demonstration of N-polar Al0.8Ga0.2N/AlN continuously-graded-channel polarization-doped field-effect transistors (PolFETs) on SiC. A PolFET with a source to drain distance of $12~\mu \text{m}$ exhibited a maximum drain current of 62.8 mA/mm and an ON/OFF current ratio of $1.1\times 10^{{4}}$ . The maximum drain current was stable between 20 °C and 250 °C operating temperatures. With the addition of 30-nm-thick Al2O3 gate insulator the maximum drain current increased to 126 mA/mm.

In this letter, we demonstrate a recessed-gate p-channel MISFET using a p-GaN (70 nm)/Al 0.2Ga0.8N (15 nm)/GaN heterostructure grown by metalorganic chemical vapor deposition (MOCVD) on a Si substrate. A major benefit of such epi-structure is that it contains both a 2-dimensional electron gas (2-DEG) and a 2-dimensional hole gas (2-DHG), hence making it suitable for the GaN-based complementary circuit implementation. In addition, the 2-DEG at the interface of the AlGaN/GaN heterostructure is depleted due to the p-GaN layer on top giving rise to normally- OFF operation for the n-channel transistors. A contact resistivity of $\textsf {4.9}\times \textsf {10}^{-\textsf {6}}\Omega \cdot \textsf {cm}^{\textsf {2}}$ is obtained for the ohmic contact to the hole channel for a p-GaN Mg concentration of $\textsf {2}\times \textsf {10}^{\textsf {20}}\, \textsf {cm}^{-\textsf {3}}$ . Long channel ( ${L} _{\textsf {g}}=\textsf {3}\,\,{\boldsymbol \mu } \text{m}$ and ${L} _{\textsf {sd}}=\textsf {9}\,\,{\boldsymbol \mu } \text{m}$ ) p-type transistors show an ON–OFF current ratio of ~105 and ON resistance of $\sim 2.3~\text{k}{\Omega } \cdot \textsf {mm}$ at ${V} _{\textsf {GS}}=-\textsf {11}$ V.

We investigate the performance of 5-nm gate length GaN nMOS nanowire field effect transistor (GaN-NW-nFET) of various geometrical shapes, around the limits of cross-sectional scalability, using atomistic quantum transport simulations. For square cross-sections, the benchmarking results with the simulated Si-NW-nFET reveal over 30% enhancement in GaN drive current in both low standby power (LP − $I_{ \mathrm{\scriptscriptstyle OFF}}=1$ nA/ $\mu \text{m}$ ) and high performance (HP − $I_{ \mathrm{\scriptscriptstyle OFF}}=100$ nA/ $\mu \text{m}$ ) applications. Further performance enhancement is observed with the use of non-square geometries that are akin to GaN’s wurtzite crystal structure. Particularly, for ${T}_{\text {nw}}=2.4 $ mn, triangular cross-section GaN-NW-nFETs exhibit the smallest subthreshold swing, down to 62 mv/decade, excellent drive current, ${I}_{\text {DSAT}} > 2\times 10^{6}\,\, \mu \text {A}/ \mu \text {m}^{2}$ and superior energy-delay product compared to simulated Si-NW-nFET.

The mechanical and electronic properties of two-dimensional materials make them promising for use in flexible electronics1–3. Their atomic thickness and large-scale synthesis capability could enable the development of ‘smart skin’1,3–5, which could transform ordinary objects into an intelligent distributed sensor network6. However, although many important components of such a distributed electronic system have already been demonstrated (for example, transistors, sensors and memory devices based on two-dimensional materials1,2,4,7), an efficient, flexible and always-on energy-harvesting solution, which is indispensable for self-powered systems, is still missing. Electromagnetic radiation from Wi-Fi systems operating at 2.4 and 5.9 gigahertz8 is becoming increasingly ubiquitous and would be ideal to harvest for powering future distributed electronics. However, the high frequencies used for Wi-Fi communications have remained elusive to radiofrequency harvesters (that is, rectennas) made of flexible semiconductors owing to their limited transport properties9–12. Here we demonstrate an atomically thin and flexible rectenna based on a MoS2 semiconducting–metallic-phase heterojunction with a cutoff frequency of 10 gigahertz, which represents an improvement in speed of roughly one order of magnitude compared with current state-of-the-art flexible rectifiers9–12. This flexible MoS2-based rectifier operates up to the X-band8 (8 to 12 gigahertz) and covers most of the unlicensed industrial, scientific and medical radio band, including the Wi-Fi channels. By integrating the ultrafast MoS2 rectifier with a flexible Wi-Fi-band antenna, we fabricate a fully flexible and integrated rectenna that achieves wireless energy harvesting of electromagnetic radiation in the Wi-Fi band with zero external bias (battery-free). Moreover, our MoS2 rectifier acts as a flexible mixer, realizing frequency conversion beyond 10 gigahertz. This work provides a universal energy-harvesting building block that can be integrated with various flexible electronic systems. Integration of an ultrafast flexible rectifier made from a two-dimensional material with a flexible antenna achieves wireless energy harvesting of Wi-Fi radiation, which could power future flexible electronic systems.

With the recent interest in wearable electronics and smart garments, digital fabrication of sensing and interactive textiles is in increasing demand. Recently, advances in digital machine knitting offer opportunities for the programmable, rapid fabrication of soft, breathable textiles. In this paper, we present KnitUI, a novel, accessible machine-knitted user interface based on resistive pressure sensing. Employing conductive yarns and various machine knitting techniques, we computationally design and automatically fabricate the double-layered resistive sensing structures as well as the coupled conductive connection traces with minimal manual post-processing. We present an interactive design interface for users to customize KnitUI’s colors, sizes, positions, and shapes. After investigating design parameters for the optimized sensing and interactive performance, we demonstrate KnitUI as a portable, deformable, washable, and customizable interactive and sensing platform. It obtains diverse applications, including wearable user interfaces, tactile sensing wearables, and artificial robot skin.

Transition metal oxides (TMOs) are promising electrochromic (EC) materials for applications such as smart windows and displays, yet the challenge still exists to achieve good flexibility, high coloration efficiency and fast response simultaneously. MXenes (e.g. Ti3C2Tx) and their derived TMOs (e.g. 2D TiO2) are good candidates for high-performance and flexible EC devices because of their 2D nature and the possibility of assembling them into loosely networked structures. Here we demonstrate flexible, fast, and high-coloration-efficiency EC devices based on self-assembled 2D TiO2/Ti3C2Tx heterostructures, with the Ti3C2Tx layer as the transparent electrode, and the 2D TiO2 layer as the EC layer. Benefiting from the well-balanced porosity and connectivity of these assembled nanometer-thick heterostructures, they present fast and efficient ion and electron transport, as well as superior mechanical and electrochemical stability. We further demonstrate large-area flexible devices which could potentially be integrated onto curved and flexible surfaces for future ubiquitous electronics., Though two-dimensional (2D) nanocomposite materials have been attractive for flexible electrochromic (EC) devices, achieving both high performance and durability remains a challenge. Here, the authors report high-performance, environmentally stable 2D MXene/transition metal oxide-based EC devices.

Atomically thin transition metal dichalcogenides (TMDs), together with their polymorphism, provide promising alternatives for next generation electronic devices and a platform to explore exotic quantum phenomena. However, a large-scale synthesis method that can reliably produce high-quality two-dimensional (2D) TMDs with controlled phase is still lacking. Instead, TMDs with high concentration of defects and defect-stabilized metastable crystalline phases are often obtained via conventional chemical vapor deposition. Here we developed a liquid-vapor (LV) technique to exploit liquid precursors to significantly suppress the equilibrium shift to the decomposition direction and successfully synthesized high-quality TMDs. We highlight the importance of exploiting the synergism of equilibrium and kinetics to facilitate the synthesis reaction (forward) and to impede decomposition (reverse). A high concentration of reactants in the liquid phase also maximizes the kinetic rate of defect repairing. We demonstrated the advantages of LV method by synthesizing diverse high-quality 2D metal tellurides with controllable polymorphs, which would be challenging, if not impossible, to realize by using conventional methods due to weak metal-tellurium bonds, thermal instability and the co-existence of mixed crystalline phases. In particular, we successfully synthesized high-quality monolayer 2H MoTe2, which is only possible when Te defect level is substantially suppressed. Our approach provides a new paradigm in high-quality and large-scale materials synthesis and can be readily extended to a variety of quantum materials, potentially accelerating both research and industrial initiatives.

This work demonstrates a p-MOSFET with a GaNM.l 0.2 Ga 0.8 N (20 nm)/GaN heterostructure grown by metal-organic-chemical vapor deposition (MOCVD) on 6-inch Si substrate with optimized self-aligned process. Devices with source-to-drain distance, L SD , varying from 1 µm down to 200 nm were fabricated. Significant field-induced acceptor ionization was found in these devices at high drain voltages. The device with L SD =200 nm shows a record combination of I ON of ~45 mA/mm and ON-OFF ratio of ~104 when compared with other p-channel transistor demonstrations. The device also exhibits Enhancement-mode operation with threshold voltage of -0.5V. The best device shows an ON-current of 100 mA/mm but at the expense of lower on-off ratio of ~102.

We demonstrate for the first time a vacuum transistor fabricated based upon Si operating in excess of 38 kV applied bias, a voltage normally reserved for high bandgap materials such as SiC and GaN. This is a proof of a concept demonstration of a high-performance vacuum transistor with the potential of having a semiconductor-like footprint. The device consists of a gated field emission array or an electron source, a vacuum drift region and a metal anode. Electrons are emitted from the gated field emission array into vacuum through tunneling, travel through vacuum and are collected at the anode. In this case, the vacuum determines the properties of transport and the high voltage isolation. Important figures of merit: the Baliga, Johnson and power vs. frequency figures of merit (FOMs) are theoretically derived and show the benefits of using a vacuum drift channel, especially for applications which require both high frequency and power. The origin of the improved performance comes from the high critical field of vacuum and most importantly the unbounded nature of the electron velocity due to a lack of scattering.

Some recent developments in 2D and 3D GaN devices and their improved performance parameters such as efficiency, fT, linearity, power density and switching speed are briefly outlined. Most of the cases briefed are for applications in RF with one example for power electronics and another for GaN integrated circuit.

In this paper we present a fully CMOS-compatible fabrication process for GaN-on-Si monolithic microwave integrated circuits (MMICs) on 200-mm-diameter wafers. This process also enables wafer-level 3D integration of GaN MMICs with Si CMOS circuits to enhance performance and functionality while reducing the size, weight, power, as well as the cost. We demonstrate our progress towards full 3D integration, including a discussion on GaN HEMT fabrication and bonding with a Si CMOS wafer. With the use of an InA1N barrier layer we show a GaN-on-Si HEMT output power of 4.5 W/mm and PAE of 53% at 10 GHz.

Transition metal dichalcogenide (TMD) materials have emerged as promising candidates for thin-film solar cells due to their wide bandgap range across the visible wavelengths, high absorption coefficient, and ease of integration with both arbitrary substrates and conventional semiconductor technologies. However, reported TMD-based solar cells suffer from relatively low external quantum efficiencies (EQE) and low open circuit voltage due to unoptimized design and device fabrication. This paper studies Pt/WSe₂ vertical Schottky junction solar cells with various WSe₂ thicknesses in order to find the optimum absorber thickness. Also, we show that the devices' photovoltaic performance can be improved via Al₂O₃ passivation, which increases the EQE up to 29.5% at 410 nm wavelength incident light. The overall resulting short circuit current improves through antireflection coating, surface doping, and surface trap passivation effects. Thanks to the Al₂O₃ coating, this work demonstrates a device with an open circuit voltage (VOC) of 380 mV and a short circuit current density (JSC) of 10.7 mA/cm². Finally, the impact of Schottky barrier height inhomogeneity at the Pt/WSe2 contact is investigated as a source of open circuit voltage lowering in these devices., NSF (Grant DMR-1231319), AFOSR (Grant FA9550-15-1-0514)

GaN vertical power FinFETs are promising high voltage switches for the next generation of high-frequency power electronics applications. Thanks to a vertical fin channel, the device offers excellent electrostatic and threshold voltage control, eliminating the need for epitaxial regrowth 1 or p-type doping 2 unlike other vertical GaN power transistors. Vertical GaN FinFETs with 1200 V breakdown voltage (BV), 5 A current rating and excellent switching figure of merit have been demonstrated recently on free-standing GaN substrates 3 . Despite this promising performance, the commercialization of these devices has been limited by the high cost ($50-$100/cm 2 ) and small diameter (~ 2 inch) of free-standing GaN substrates. The use of GaN-on-Si wafers could reduce the substrate cost by ×1000, however the growth of the thick (~10 μm or thicker) drift layers required for kV class applications is extremely challenging on Si. Alternatively, GaN grown on engineered substrates (QST ® ) with a matched thermal expansion coefficient could enable low-cost vertical GaN FinFETs with thick (>10 μm) drift layers and large wafer diameters (8-12 inch). In this work, we demonstrate GaN power FinFETs on engineered substrates for the first time.

By 2030, about 80% of all US electricity is expected to flow through power electronics. This will require power electronic devices and circuits with much higher efficiency and smaller form-factor than today's silicon-based systems. III-Nitride semiconductors and other ultra-wide bandgap materials are ideal platforms for the new generation of power electronics thanks to the combination of excellent transport properties and the high critical electric field enabled by their wide bandgap [1]. This talk will discuss recent progress in our group in developing high voltage power transistors and diodes based on wide bandgap materials.

A vacuum channel transistor is the ultimate wide band-gap structure with potential for high Johnson figure of merit (~10 14 V/s) due to no electron scattering and no impact ionization/breakdown [1] , [2] . Hence, nanoscale vacuum channel transistors (NVCTs) can possibly outperform solid-state transistors in terms of speed, breakdown voltage and reliability in harsh environments [1] . Carriers are injected into the channel by electron tunneling across a barrier narrowed by an electric field. Such electron sources can be realized using nanoscale gated Si field emitter arrays (FEAs) with high packing densities (≥10 8 /cm 2 ) and self-aligned apertures which have low turn-on voltage (8.5 V), low operating voltage, high current density (150 A/cm 2 ) and long lifetime (>300 hours) [3] . The barrier height is nonetheless sensitive to adsorption/desorption of gas molecules, resulting in large current variations in poor vacuum, which can also generate energetic ions that erode the emitter. Hence FEAs require costly and bulky ultra-high vacuum (UHV) systems for reliability [4] . Using multi-layers of graphene (Gr) that withstand high pressure gradients and, are transparent to electrons, but impervious to gas molecules, can enable operation of these FEAs in poor vacuum [5] , [6] . In this paper, Gr layers are used to encapsulate such FEAs with two self-aligned gates ( Fig. 1 ); this structure allows an independent control of the bias applied to the Gr layer, and significantly reduces the volume to be encapsulated.

Electron devices based on field emitters (FE) are promising for harsh-environments and high-frequency electronics thanks to their radiation hardness and scattering-free electron transport. Si field emitters with a sub-10 nm tip radius and self-aligned gates have demonstrated sub-20 V turn-on operation [1] , [2] . However, stability and operating voltage still need further improvement to enable circuit applications. III-Nitrides are excellent candidates to overcome these issues because of their strong bonding energies [3] and tunable electron affinities [4] . So far, there are few demonstrations of III-Nitride field emitters with self-aligned gates, which are critical to reduce the gate-emitter voltage (V GE ). In this work, a novel GaN nanowire (NW) field emitter based on self-aligned gates is demonstrated to reduce the gate-emitter turn-on voltage (V GE, ON ) below 30 V. To the best of our knowledge, this represents the lowest control voltage in any GaN field emitter device, opening an opportunity for using III-N in integrated field emitters.

© 2020 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim Advanced microscopy and/or spectroscopy tools play indispensable roles in nanoscience and nanotechnology research, as they provide rich information about material processes and properties. However, the interpretation of imaging data heavily relies on the “intuition” of experienced researchers. As a result, many of the deep graphical features obtained through these tools are often unused because of difficulties in processing the data and finding the correlations. Such challenges can be well addressed by deep learning. In this work, the optical characterization of 2D materials is used as a case study, and a neural-network-based algorithm is demonstrated for the material and thickness identification of 2D materials with high prediction accuracy and real-time processing capability. Further analysis shows that the trained network can extract deep graphical features such as contrast, color, edges, shapes, flake sizes, and their distributions, based on which an ensemble approach is developed to predict the most relevant physical properties of 2D materials. Finally, a transfer learning technique is applied to adapt the pretrained network to other optical identification applications. This artificial-intelligence-based material characterization approach is a powerful tool that would speed up the preparation, initial characterization of 2D materials and other nanomaterials, and potentially accelerate new material discoveries.

Two-dimensional (2D) transition metal nitrides (TMNs) are new members in the 2D materials family with a wide range of applications. Particularly, highly crystalline and large area thin films of TMNs are potentially promising for applications in electronic and optoelectronic devices; however, the synthesis of such TMNs has not yet been achieved. Here, we report the synthesis of few-nanometer thin Mo5N6 crystals with large area and high quality via in situ chemical conversion of layered MoS2 crystals. The structure and quality of the ultrathin Mo5N6 crystal are confirmed using transmission electron microscopy, Raman spectroscopy and X-ray photoelectron spectroscopy. The large lateral dimensions of Mo5N6 crystals are inherited from the MoS2 crystals that are used for the conversion. Atomic force microscopy characterization reveals the thickness of Mo5N6 crystals is reduced to about 1/3 of the MoS2 crystal. Electrical measurements show the obtained Mo5N6 samples are metallic with high electrical conductivity (~ 100 {\Omega} sq-1), which is comparable to graphene. The versatility of this general approach is demonstrated by expanding the method to synthesize W5N6 and TiN. Our strategy offers a new direction for preparing 2D TMNs with desirable characteristics, opening a door for studying fundamental physics and facilitating the development of next generation electronics., Comment: 41 pages, 6 Figures, Accepted by Science Advances