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Time-dependent nonequilibrium Green's functions are used to study electron transport properties in a device consisting of two linear chain leads and a time-dependent interlead coupling that is switched on non-adiabatically. We derive a numerically exact expression for the particle current and examine its characteristics as it evolves in time from the transient regime to the long-time steady-state regime. We find that just after switch-on, the current initially overshoots the expected long-time steady-state value, oscillates and decays as a power law, and eventually settles to a steady-state value consistent with the value calculated using the Landauer formula. The power-law parameters depend on the values of the applied bias voltage, the strength of the couplings, and the speed of the switch-on. In particular, the oscillating transient current decays away longer for lower bias voltages. Furthermore, the power-law decay nature of the current suggests an equivalent series resistor-inductor-capacitor circuit wherein all of the components have time-dependent properties. Such dynamical resistive, inductive, and capacitive influences are generic in nano-circuits where dynamical switches are incorporated. We also examine the characteristics of the dynamical current in a nano-oscillator modeled by introducing a sinusoidally modulated interlead coupling between the two leads. We find that the current does not strictly follow the sinusoidal form of the coupling. In particular, the maximum current does not occur during times when the leads are exactly aligned. Instead, the times when the maximum current occurs depend on the values of the bias potential, nearest-neighbor coupling, and the interlead coupling. [ABSTRACT FROM AUTHOR]

The thermoelectric properties of silicon nanowires with different shapes, sizes, and orientations are theoretically investigated using sp3d5s* tight-binding model coupled with ballistic transport approach. We found that the thermoelectric properties significantly depend on nanowire geometry. Compared to [111] and [100] nanowires, n-doped and p-doped [110] nanowires show the worst performance in terms of power factor per cross-section area and figure of merit (ZT). As nanowire size decreases, thermoelectric properties of nanowires can be enhanced. As a result, triangular nanowires with side length of 1 nm have the best results of ZT and it can be enhanced to 1.5 and 0.85 for an n-type nanowire along [111] orientation and a p-type nanowire along [100] orientation, respectively. For extremely narrow nanowires, thermoelectric properties are only dependent on the number of the transmission modes instead of material properties such as carrier effective mass. Moreover, cross-section shape and thermal conductance contributed by electrons play important roles in ZT while their influence can be ignored for large size nanowires. Even though smaller size nanowires have better performance with the consideration of the single nanowire thermoelectric properties, they might be less efficient than larger diameter nanowires, as packing space is not very dense. [ABSTRACT FROM AUTHOR]

A real-space quantum transport simulator for graphene nanoribbon (GNR) metal-oxide-semiconductor field-effect transistors (MOSFETs) has been developed and used to examine the ballistic performance of GNR MOSFETs. This study focuses on the impact of quantum effects on these devices and on the effect of different type of contacts. We found that two-dimensional (2D) semi-infinite graphene contacts produce metal-induced-gap states (MIGS) in the GNR channel. These states enhance quantum tunneling, particularly in short channel devices, they cause Fermi level pinning and degrade the device performance in both the ON-state and OFF-state. Devices with infinitely long contacts having the same width as the channel do not indicate MIGS. Even without MIGS quantum tunneling effects such as band-to-band tunneling still play an important role in the device characteristics and dominate the OFF-state current. This is accurately captured in our nonequilibrium Greens’ function quantum simulations. We show that both narrow (1.4 nm width) and wider (1.8 nm width) GNRs with 12.5 nm channel length have the potential to outperform ultrascaled Si devices in terms of drive current capabilities and electrostatic control. Although their subthreshold swings under forward bias are better than in Si transistors, tunneling currents are important and prevent the achievement of the theoretical limit of 60 mV/dec. [ABSTRACT FROM AUTHOR]

In the presence of structural disorders, carrier conduction via localized hopping sites emerges in two-dimensional systems and results in a unique thermopower characteristic with T1/3 dependence. The disorders induced potential differences of hopping sites leading to energy variations along current-carrying paths. A systematic thermoelectric study is presently required in comprehending the statistical effects. Therefore, we proposed a statistical model of the Seebeck coefficient on the basis of percolation theory and hopping mechanisms. With this model, the carrier density and temperature dependences can be practically predicted. Key parameters can be extracted by calibration to molybdenum disulfide and black phosphorus experiments, providing a deeper insight into device physics. Moreover, a Mott-like analytical model is developed to investigate the parametric dependence. The thermopower deviations from the noninteracting Mott picture at high and low temperatures are analyzed. Finally, the temperature dependence on the thermoelectric figure of merit is evaluated in a variable range hopping regime. Our model is essential for a reliable prediction of the disorder induced statistical effects on thermoelectric behaviors, which guides both device optimization and material engineering. [ABSTRACT FROM AUTHOR]

A theoretical model based on the Landau-Lifshitz-Bloch equation is developed to study the effect of damping-like spin-orbit torque in ferrimagnets, which can capture many experimental findings. For example, the sample changes from Gd to FeCo dominate by increasing temperature, the damping-like spin-orbit torque has a peak at the magnetization compensation temperature, and angular-momentum compensation temperature increases as a function of Gd concentration. In contrast to the ferromagnet system, the switching trajectory in ferrimagnets is found to be precession free. The two sublattices are not always collinear, which produce a large exchange field affecting the magnetization dynamics. The study of material composition shows the existence of an oscillation region at intermediate current density, induced by the nondeterministic switching. Compared to the Landau-Lifshitz-Gilbert model, our developed model based on the Landau-Lifshitz-Bloch equation enables the systematic study of the spin-torque effect and the evaluation of ferrimagnet-based devices. [ABSTRACT FROM AUTHOR]

Using non-equilibrium Green's function (NEGF), we investigate spin transport properties of a U-shaped, multi-terminal device based on two-dimensional topological insulator (2DTI), such as HgTe quantum well (HgTe QW). The results show that this 2DTI multi-terminal device can be implemented as a perfect spin filter operating in the absence of ferromagnetic electrodes and external magnetic field. The electrically tunable spin polarization can approach 100%. Moreover, the device can redistribute electrons with opposite spins from an unpolarized source to different leads, resulting in a bifunctional device, which can simultaneously operate as a spin filter and a spin separator. These interesting transport properties can be attributed to the unique helical edge states in a quantum spin Hall system. The performance of the U-shaped device is robust against the weak disorder and the variation in widths and bias voltages, but deteriorates in the presence of the strong disorder and vacancies. The capability of generating highly polarized and controllable spin currents in 2DTI indicates the application of topologically protected and low-powered spintronics. [ABSTRACT FROM AUTHOR]

Ferrimagnets perform versatile properties, attributed to their antiferromagnetic sublattice coupling and finite net magnetization. Despite extensive research, the inhomogeneous dynamics in ferrimagnets, including domain walls and magnons, remain not fully understood. Therefore, we adopted a multi-spin model by considering the effect of the spin torques and explored the localized phase-dependent and inhomogeneous THz-oscillation dynamics in a ferrimagnetic spin-chain. Our results demonstrate that the exchange oscillation mode, induced by spin transfer torque, exhibits three typical phases, and the oscillation frequency is dominated by a joint effective field derived in the spin-chain. We also found that the localized spin configurations can be used to tune the bandwidth and sensitivity of the frequency response. Furthermore, we propose an anti-parallel exchange length to reveal the inhomogeneity in the ferrimagnetic spin-chain, which could serve as a valuable tool for characterizing the spin dynamics of these systems. Our findings offer understandings beyond uniform spin-dynamics in ferrimagnets. Ferrimagnets present advantages over their anitferromagnetic and ferromagnetic counterparts for applications in ultrafast spintronics but the dynamics of the non-uniform spin textures are not as well understood. Here, the authors investigate the inhomogeneous oscillation dynamics in the ferrimagnetic system via a multi-spin model. [ABSTRACT FROM AUTHOR]

The effect of phase-transition from the quantum-spin-hall to the band-insulator phase on the transport through a three-terminal U-shape spin-separator has been computationally investigated via non-equilibrium green function formalism. Two-dimensional group-IV elements have been comprehensively appraised as the device material. The device separates the unpolarized current injected at the source-terminal into nearly 100% spin-polarized currents of the opposite polarities at the two drain terminals. The phase-transition activated by the electric-field orthogonal to the device is shown to extensively influence the current magnitude and its spin-polarization, and the effect is stronger for materials with smaller intrinsic spin-orbit coupling. Moreover, the device length and the area under field are shown to critically affect the device characteristics on phase change. It is shown that the same device can be operated as a spin-filter by inducing phase-transition selectively in the channel. The results are important for designing spin-devices from Group-IV monolayers. [ABSTRACT FROM AUTHOR]

The proposition of integrating the 2-D materials and ferroelectric negative-capacitance MOSFETs (NCFETs) was expected to have excellent steep slope I – V characteristics due to the combined advantages of both ultrathin body channel material and negative-capacitance effect. Recent experiments demonstrated the excellent steep slope features of 2-D material NCFETs. Therefore, it was important to understand the device physics of this characteristic, its performance projection, and the benchmark with NCFETs based on conventional Group IV materials. In this article, we conducted a computational study of 2-D material NCFETs based on MoS2 under the nonequilibrium Green’s function (NEGF)-Poisson self-consistent scheme with free-Gibbs energy calculation to judge the operating path in polarization–electric field (P–E) relation. Monolayer MoS2 (1L-MoS2) was chosen as channel material while various thickness of ultrathin body silicon (UTB-Si) for benchmark purpose. The results showed an excellent average subthreshold slope (SS) of ~25 mV/dec in 1L-MoS2 NCFETs compared to ~32 mV/dec of the UTB-Si counterparts. It was found that the steep-slope features in 1L-MoS2 NCFETs could be attributed to the more optimized capacitance matching in 1L-MoS2 NCFETs and tunneling currents at the subthreshold region. [ABSTRACT FROM AUTHOR]

Using ab-initio method and ballistic transport model, we study electron and phonon energy dispersion relations of monolayer transition-metal dichalcogenides: MoS2, MoSe2, WS2, and WSe2. Their electron and heat transports as well as their thermoelectric properties are also studied under linear response regime with different doping types, crystal orientations, and temperatures. Our results show that electron and phonon transports are not very sensitive to crystal orientations because the differences between group velocity and transmission of these carriers along different transport directions are not significant. Furthermore, as temperature increases, first peak values of thermoelectric figure of merit (ZT1st peak) increase linearly except for monolayer n-type WSe2/MoSe2 and p-type WS2, which have higher increasing rates when temperature is high due to the electron transport contribution from an additional valley. Among these various conditions, the results show that all monolayers have similar ZT1st peak at low temperatures below 100 K, and p-type monolayer MoS2 has the largest ZT1st peak at room temperature while n-type WSe2 has the largest ZT1st peak at high temperatures. [ABSTRACT FROM AUTHOR]

Carrier transport through a graphene zigzag nanoribbon (ZNR) is possible to be blocked by a p-n profile implemented along its transport direction. However, we found that in cases of analogous materials with significant intrinsic spin-orbit coupling (SOC), i.e. silicene and germanene, such a profile on ZNR of these materials allows transmission mostly through spin-orbit coupled energy window due to the loss of the parity of wave functions at different energies caused by SOC. Next, a p-i-n scheme on germanene ZNR is proposed to simultaneously permit edge transmission and decimate bulk transmission. The transmission spectrum is shown to mitigate the effect of thermal broadening on germanene and silicene ZNR based spin-separators by improving spin polarization yield by 400% and 785%, respectively, at 300 K. The importance of proper gate voltage and position for such performance is further elucidated. Finally, the modulation the current output of the proposed U-shape p-i-n device while maintaining its spin polarization is discussed. [ABSTRACT FROM AUTHOR]

In this paper, we investigate the relationship between size, shape, and the performance of the voltage-controlled magnetic anisotropy (VCMA)-based magnetic tunnel junctions (MTJs) suitable for gigabit scale MRAMs with 10 years of retention time. A Fokker–Planck simulation framework is developed to model the magnetization dynamics in the presence of thermal noise. Here, we numerically show that the optimization of the MTJ geometry can significantly improve the performance of the VCMA-based MTJs. Using an elliptical MTJ with aspect ratio (AR) of 3 reduces the required supply voltage and energy consumption by 80.7% and 92%, respectively, compared to a circular MTJ when the minimum feature size is 50 nm. The design requirements on the VCMA coefficient, ${\xi }$ , are also reduced by 67%. However, the influence of AR is observed to diminish with reduction in the minimum feature size of the MTJ. For instance, when the minimum feature size is 20 nm, the required supply voltage and energy consumption reduce only by 60.29% and 65.30%, respectively. We, then, perform a comprehensive scaling analysis on the VCMA-based MTJs by varying size, geometry, and ${\xi }$ of the MTJs. The predicted scaling trends are then compared with those of the STT MTJs. [ABSTRACT FROM AUTHOR]

Ferroelectric (FE) negative capacitance (NC) MOSFETs is investigated in depth with a full real-space atomic simulator. For the first time, Gibbs-free energy of the whole atomic system is employed to determine the appropriate operation region in polarization-electric-field (${P}$ – ${E}$) path of the FE layer for FE MOSFETs. With different FE thicknesses, the operation path of the FE layer in MOS structures can be hysteresis or hysteresis-free. For MOSFET structures, the operation path of FE layer will vary with different FE materials. With proper device parameters, the NC effect in FE layer can be obtained with minimum Gibbs-free energy of the MOS and MOSFET structures. The body factor, ${dV}_{G}/{d} \Psi _{S}$ , is obtained with the real-space structure to reflect the influence of the FE layer and optimize the device performance. With the FE layer of the same thickness, the NC-FET with longer channel has a larger body factor which causes the tradeoff between low tunneling current and small body factor. This tradeoff makes it difficult to improve the subthreshold swing by changing the gate length. In addition, the effect of floating metal layer between FE and SiO2 layers in NC-FETs is studied in detail with atomic level real-space analysis for different gate lengths. The metal layer has a stronger influence on the electric characteristics of NC-FETs with longer channel. [ABSTRACT FROM AUTHOR]

A comprehensive understanding of the disorder-induced transport characteristics in resistive random-access memory (RRAM) is critical for its thermal stability analysis and analog switching for the coming neuromorphic computing application. Superior to the previous transport mechanisms which are only valid within their respective ranges of temperatures, we propose a unified physics-based model that can accurately predict the transport dependence on all temperature ranges up to 300 K. By utilizing percolation theory and the Fermi Golden Rule, the probability distributions for both the tunnel junction energy barrier and gap distance based statistical resistance model are described. It is found that different programming cycles and resistance states contribute to transition behavior between various low-temperature transport mechanisms. Moreover, the model can also investigate the dependence of electrical characteristics on defect generation like radiation damage. Therefore, it quantitatively relates the thermal stability and percolation effects to the structural disorders in RRAM. The good agreement between the simulation and experimental results indicates that our physics-based model can provide an accurate prediction of temperature and disorder dependent effects in RRAMs. [ABSTRACT FROM AUTHOR]

Since it is undesirable to require an external magnetic field for on-chip memory applications, we investigate the use of a Rashba effective field alternatively for assisting the electric-field-induced switching operation of a three terminal perpendicular magnetic tunnel junction (pMTJ). By conducting macro-spin simulations, we show that a pMTJ with a thermal stability of 61 can be switched in 0.5 ns, consuming a switching energy of 6 fJ, and the voltage operation margin can be improved to 0.8 ns. Furthermore, the results also demonstrate that a heavy metal system that can provide a large field-like torque rather than the damping-like torque is favored for the switching. [ABSTRACT FROM AUTHOR]

The conventional computing method based on the von Neumann architecture is limited by a series of problems such as high energy consumption, finite data exchange bandwidth between processors and storage media, etc., and it is difficult to achieve higher computing efficiency. A more efficient unconventional computing architecture is urgently needed to overcome these problems. Neuromorphic computing and stochastic computing have been considered to be two competitive candidates for unconventional computing, due to their extraordinary potential for energy-efficient and high-performance computing. Although conventional electronic devices can mimic the topology of the human brain, these require high power consumption and large area. Spintronic devices represented by magnetic tunnel junctions (MTJs) exhibit remarkable high-energy efficiency, non-volatility, and similarity to biological nervous systems, making them one of the promising candidates for unconventional computing. In this work, we review the fundamentals of MTJs as well as the development of MTJ-based neurons, synapses, and probabilistic-bit. In the section on neuromorphic computing, we review a variety of neural networks composed of MTJ-based neurons and synapses, including multilayer perceptrons, convolutional neural networks, recurrent neural networks, and spiking neural networks, which are the closest to the biological neural system. In the section on stochastic computing, we review the applications of MTJ-based p-bits, including Boltzmann machines, Ising machines, and Bayesian networks. Furthermore, the challenges to developing these novel technologies are briefly discussed at the end of each section. [ABSTRACT FROM AUTHOR]

The stable precession region in the spintronic oscillator with an in-plane magnetic tunnel junction is very narrow under small external fields, restricting its applications such as for microwave generators. Here, we show that this region can be greatly enlarged by introducing competing effects between different torques. Moreover, we observe large-angle precessions at zero external field, which leads to large output power. We further evaluate the oscillator performance in a voltage-input device, where the circuit area can be minimized and the difficulty in accurate current control can be resolved. The operating voltage window in the proposed device is over 1.23 V, and its frequency can be adjusted from 1.6 to 4.9 GHz. A maximum output power of 0.28 μW is obtained at an energy consumption of 2.2 mW. This study should provide insights into designing voltage-input spintronic oscillators. [ABSTRACT FROM AUTHOR]

We report the ultimate performance of a double-gate ultrathin body (DG-UTB) FET employing materials from group IV, III-V, and 2-D materials based on International Technology Roadmap for Semiconductors (ITRS) projected specifications for high-performance (HP) and low-power (LP) technologies. The band structures of the channel materials were obtained using the sp ^3\textd^5\texts^\ast tight binding model and the first-principles density functional theory. The ballistic performance of FETs designed based on the ITRS specifications for 2018 and beyond was evaluated via the semiclassical ballistic transport model. The leakage current due to direct source-to-drain tunneling was calculated based on Wentzel–Kramers–Brillouin approximation. For nFET used in the HP technology, GaSb nFET has the best voltage scalability followed by Ge nFET in 2026. It was found that I\mathrm{\scriptscriptstyle ON} of InAs and In0.3 Ga0.7Sb nFETs is among the lowest, and they require larger power supply voltage ( V\mathrm{DD} ) among III-V semiconductors considered to achieve I\mathrm{\scriptscriptstyle ON} of silicon nFET. However, they show lower power delay product (PDP) due to their smaller electron effective mass, resulting in higher carrier velocity. Ge pFET offers the best voltage scalability for pFET used in the HP technology. For the voltage scalability assessment based on the requirements for the LP technology, MOSFETs based on Si offer better performance in terms of I\mathrm{\scriptscriptstyle ON} and PDP than MOSFETs based on 2-D materials. Among the 2-D materials studied, black phosphorus and silicane MOSFETs exhibit higher I\mathrm{\scriptscriptstyle ON} and better voltage scalability and PDP. [ABSTRACT FROM AUTHOR]

A surface potential based compact model for two-dimensional field effect transistors (2D-FETs) is proposed to incorporate the structural disorders induced transition behaviors among variable range hopping (VRH), nearest neighbor hopping (NNH), and band-like transport in most 2D materials. These functions coupled with effective transport energy and multiple trapping and releasing theory enable our developed model to predict the temperature and carrier density dependent current characteristics. Its validity is confirmed by the experimental results such as the metal insulator transition (MIT) in transition metal dichalcogenides and VRH-NNH transition in black phosphorus nanoribbon. Based on this model, the band-tail effects on the crossover gate voltage of MIT behavior are quantitatively investigated. It is found that the transition behavior is closely related to the distribution of the band-tail states. Furthermore, this model is implemented in Verilog-A for circuit-level prediction and evaluation of 2D-FETs to provide deeper insight into the relationship between material properties, device physics, and circuit performances. [ABSTRACT FROM AUTHOR]

Ge/Si core/shell nanowires (NWs) are attractive and flexible building blocks for nanoelectronics ranging from field-effect transistors (FETs) to low-temperature quantum devices. Here we report the first studies of the size-dependent performance limits of Ge/Si NWFETs in the sub-100 nm channel length regime. Metallic nanoscale electrical contacts were made and used to define sub-100 nm Ge/Si channels by controlled solid-state conversion of Ge/Si NWs to NiSixGey alloys. Electrical transport measurements and modeling studies demonstrate that the nanoscale metallic contacts overcome deleterious short-channel effects present in lithographically defined sub-100 nm channels. Data acquired on 70 and 40 nm channel length Ge/Si NWFETs with a drain−source bias of 0.5 V yield transconductance values of 78 and 91 µS, respectively, and maximum on-currents of 121 and 152 µA. The scaled transconductance and on-current values for a gate and bias voltage window of 0.5 V were 6.2 mS/µm and 2.1 mA/µm, respectively, for the 40 nm device and exceed the best reported values for planar Si and NW p-type FETs. In addition, analysis of the intrinsic switching delay shows that terahertz intrinsic operation speed is possible when channel length is reduced to 70 nm and that an intrinsic delay of 0.5 ps is achievable in our 40 nm device. Comparison of the experimental data with simulations based on a semiclassical, ballistic transport model suggests that these sub-100 nm Ge/Si NWFETs with integrated high-κ gate dielectric operate near the ballistic limit. [ABSTRACT FROM AUTHOR]

We investigate thermally induced currents in a zigzag graphene nanoribbon (ZGNR) heterostructure, consisting of hydrogen-terminated ZGNR (ZGNR-H) and oxygen-terminated ZGNR (ZGNR-O), under different electronic and magnetic states. Compared to a pure ZGNR-H system, the heterostructure displays a considerably larger thermally induced current due to its asymmetric transmission spectrum. Moreover, the magnetized ZGNR-H/ZGNR-O shows spin filter and magnetoresistance effects, suggesting potential applications of the ZGNR-H/ZGNR-O heterostructures in thermoelectric and spintronics devices. [ABSTRACT FROM AUTHOR]

This paper presents a theoretical study of the effect of transverse electric potentials on the transport properties of armchair graphene nanoribbons (AGNRs), formed by pairs of asymmetric gates placed along the side of the ribbon. Single-pair and dual-pair configurations are considered. The effect of hollows (spatial regions void of carbon atoms) in the AGNR channels is also examined. The use of hollowed AGNRs in the dual-pair configuration allows for significant modulation of the transport gap when the two pairs have opposite gate bias polarity. Furthermore, for the dual-gate system, hollowed AGNR channels exhibit the optimal ratio of on-state to off-state conductance, due to the smaller off-state conductance compared with spatially homogenous AGNR channels. Our results indicate that transverse gate technology coupled with careful engineering of hollow geometry may lead to possible applications in graphene-based electronic devices. [ABSTRACT FROM AUTHOR]

The analog resistive switching (RS) characteristics in 2-D polycrystalline (poly-) molybdenum disulfide (MoS2) field-effect transistors (FETs) enable new electronic devices capable of emulating biological synaptic behaviors. In 2-D poly-materials, grain boundary (GB)-induced trap states are of major significance to RS behaviors. However, there is still a lack of appropriate compact models that capture accurate physical mechanisms. Therefore, we developed a surface potential-based compact model, based on the theories of the GB energy barrier and space charge limited current (SCLC). By calibrating to experimental data of MoS2, the physical parameters are extracted, and the model explains the scaling behaviors of channel lengths and grain sizes. Due to the electric-field-induced defect redistribution, the energy barrier modulation at a single-GB (e.g., intersecting GB) quantitatively matches the reported experiments. Moreover, the possible SCLC-based RS behavior is also investigated. Furthermore, we have optimized the set/reset process and simulated the postsynaptic current (PSC) with a tunable potentiation (or depression) process, and then the gate voltage dependence and statistical effects on RS and PSC have been investigated. Thus, this model provides important devices physics insights of 2-D poly-materials and it guides device design, fabrication, and material engineering, to meet the requirements of the future neuromorphic computing application. [ABSTRACT FROM AUTHOR]

Computational studies on nanoelectromechanical switches based on bilayer graphene nanoribbons (BGNRs) with different designs are presented in this work. By varying the interlayer distance via electrostatic means, the conductance of the BGNR can be changed in order to achieve ON-states and OFF-states, thereby mimicking the function of a switch. Two actuator designs based on the modified capacitive parallel plate (CPP) model and the electrostatic repulsive force (ERF) model are discussed for different applications. Although the CPP design provides a simple electrostatic approach to changing the interlayer distance of the BGNR, their switching gate bias VTH strongly depends on the gate area, which poses a limitation on the size of the device. In addition, there exists a risk of device failure due to static fraction between the mobile and fixed electrodes. In contrast, the ERF design can circumvent both issues with a more complex structure. Finally, optimizations of the devices are carried out in order to provide insights into the design considerations of these nanoelectromechanical switches. [ABSTRACT FROM AUTHOR]

The possibility of using graphene nanoribbons (GNRs) as the material for resonant tunneling diodes (RTDs) was investigated using a device simulator based on the nonequilibrium Green’s function with the π-orbital tight-binding approach. The double-barrier quantum well (DBQW) requirements of a RTD can be implemented by adjusting the width of a GNR to derive a negative differential resistance (NDR). The implementation of such a device is demonstrated in this paper and its mechanism was also found to be robust regardless of the eventual shape of the GNR patterned. Furthermore, the effects of the shape of the patterned GNR and the operating temperature on the performance of the device were explored by looking at the real space current flux of the device and the temperature dependency of the peak-valley ratio (PVR), respectively. Although the different shapes of GNR RTDs had a similar DBQW structure, their PVRs were different due to their conduction mechanisms which were dependent on the different geometrical shapes of each case. Lastly, the effect of thermal broadening, and width/length dependence of the central GNR between two barriers on the device performance, was further investigated in order to provide insights into the device physics of GNR RTDs for future study on performance optimization. [ABSTRACT FROM AUTHOR]

Silicon CMOS technology has fueled the phenomenal growth in semiconductor electronics over the past several decades. The miniaturization of a transistor, the basic tenet of technology scaling, is at the core of this revolution. Despite several challenges, the innovations in materials, processes, and device architecture have ensured that Moore’s law is still alive and going strong, with 7-nm silicon FinFET technology in volume production. However, over the past couple of decades, there has been a realization that the silicon CMOS technology may reach the end of scaling unless it is augmented and complemented by other semiconductors. For instance, there has been a substantial effort in the research community to integrate other bulk semiconductors such as germanium and III-Vs, into silicon technology. [ABSTRACT FROM AUTHOR]

Klein tunneling refers to the absence of normal backscattering of electrons even under the case of high potential barriers. At the barrier interface, the perfect matching of electron and hole wavefunctions enables a unit transmission probability for normally incident electrons. It is theoretically and experimentally well understood in two-dimensional relativistic materials such as graphene. Here we investigate the Klein tunneling effect in Weyl semimetals under the influence of magnetic field induced by ferromagnetic stripes placed at barrier boundaries. Our results show that the resonance of Fermi wave vector at specific barrier lengths gives rise to perfect transmission rings, i.e., three-dimensional analogue of the so-called magic transmission angles in two-dimensional Dirac semimetals. Besides, the transmission profile can be shifted by application of magnetic field in the central region, a property which may be utilized in electro-optic applications. When the applied potential is close to the Fermi level, a particular incident vector can be selected by tuning the magnetic field, thus enabling highly selective transmission of electrons in the bulk of Weyl semimetals. Our analytical and numerical calculations obtained by considering Dirac electrons in three regions and using experimentally feasible parameters can pave the way for relativistic tunneling applications in Weyl semimetals. [ABSTRACT FROM AUTHOR]

Integration of InxGa1–xAs n-MOSFETs and SiyGe1–y p-MOSFETs could be a key to realize future low-power and high-speed logic circuits. In this paper, monolithic integration of InAs n-MOSFETs and Ge p-MOSFETs on a Si substrate is reported. To address the challenge of integrating materials with large lattice mismatch (InAs and Ge on Si substrate), a sub-120-nm GaSb-on-GaAs buffer on a germanium-on-insulator (GeOI) starting substrate is employed. The strain resulting from the 7.78% lattice mismatch between the GaSb and GaAs layers is mainly relaxed via interfacial misfits at the GaSb/GaAs interface, enabling significant reduction in the buffer thickness. For device fabrication, a self-aligned gate last process flow with Si-CMOS-compatible modules is used. To realize raised source-drain device architecture, a combination of dry and digital etch processes is developed to etch InAs and Ge cap layers. Devices with channel thicknesses less than 5 nm and channel lengths less than 200 nm are realized for both n- and p-MOSFETs, with promising electrical characteristics. [ABSTRACT FROM PUBLISHER]

Black phosphorus (BP) has been proposed as the channel material in the next generation ultrascaled CMOS devices. In order to gain insight into the current characteristics in 2-D layered materials, the current distribution of a few-layer BP Schottky barrier FET is investigated via state-of-the-art quantum device simulations. Approximately 40% of the total current was found to be concentrated in the top layer when the device was switched on, with the remaining current distributed among the other layers. In comparison, ~80% of the current concentrated below the surface in a Si device with the same structure. These features are related to the strength of the intra/interlayer interaction in few-layer BP and are unique to 2-D layered materials. Moreover, the current distribution and the device performance were different for the top- and side-contacted devices, with the side-contacted devices yielding lower resistance compared with the top-contacted devices. [ABSTRACT FROM PUBLISHER]

An improved simulation scheme for investigating the performance of nanoscale FETs is developed in this paper. The total current of the MOSFET consists of two main components: thermionic current above the top of barrier of the channel calculated by ballistic approach and tunneling current computed by Wentzel–Kramer–Brillouoin approximation based on a full-complex-band structure. Furthermore, to get atomic-position-based potential profile in the nanoscale device, we self-consistently solve atomic charges and potentials in the real space along and transverse in the transport direction. The device performance calculated with this model shows excellent agreement with that obtained using non-equilibrium Green’s function solver (full quantum mechanism) but by using only one-tenth of the simulation resource. Moreover, special characteristics of insulating materials integrated with advanced device structures can also be incorporated in this Poisson solver. An example based on the negative capacitance MOSFET is examined with this model, and it shows significantly improved performance than conventional MOSFET. [ABSTRACT FROM PUBLISHER]

A correction to the article "Voltage-input spintronic oscillator based on competing effect for extended oscillation regions" in a 2019 issue is presented.

A simple two step wet etch approach to fabricate nanowires (NWs) with a tapered source/drain (S/D) architecture is presented. Based on the unique NW architecture, gate-all-around junctionless NW FETs with sub-15-nm channel length ( $L_{{\mathrm{CH}}}$ ), NW height ( $H_{{\mathrm{NW}}}$ ), and NW width ( $W_{{\mathrm{NW}}}$ ) were realized. Despite having a large equivalent oxide thickness of $\sim 4.5$ nm, high extrinsic transconductance ( $G_{{m,{\mathrm{ ext}}}}) of 820 \mu \textS/\mu \textm was achieved at V\mathrm{ D} of 0.5 V. Due to the unique tapered S/D structure, the device realized in this paper achieved S/D series resistance ( R\mathrm {SD} ) of 275 \Omega \cdot \mu \mathrm m , which is one of the lowest among the reported 3-D InGaAs MOSFETs. [ABSTRACT FROM PUBLISHER]

Electron transport in Bi 2 Se 3 topological insulator slabs is investigated in the thermal activation regime (>50 K) both in the absence (ballistic) and presence of weak and strong acoustic phonon scattering using the non-equilibrium Green function approach. Resistance of the slab is simulated as a function of temperature for a range of slab thicknesses and effective doping in order to gain a handle on how various factors interact and compete to determine the overall resistance of the slab. If the Bi 2 Se 3 slab is biased at the Dirac point, resistance is found to display an insulating trend even for strong electron–phonon coupling strength. However, when the Fermi-level lies close to the bulk conduction band (heavy electron doping), phonon scattering can dominate and result in a metallic behavior, although the insulating trend is retained in the limit of ballistic transport. Depending on values of the operating parameters, the temperature dependence of the slab is found to exhibit a remarkably complex behavior, which ranges from insulating to metallic, and includes cases where the resistance exhibits a local maximum, much like the contradictory behaviors seen experimentally in various experiments. [ABSTRACT FROM AUTHOR]

The effect of different contact configurations (semi-infinite extended-channel, normal metal and ferromagnetic metal) on quantum transport through thin Bi2Se3 three-dimensional (3D) topological insulator (TI) slab (channel) has been investigated through Non-Equilibrium Green Function. The issue of contact dependent current flow and distribution across quintuple layers of 3D-TI has been addressed in this work and applied to expound the explanation for recent experimental work on electrical detection of spin-momentum locking on topological surface for long channel device. A theoretical model is propounded to develop a microscopic understanding of transport in 3D-TI in which contact type and magnetization concur with helical surface states of the TI channel to manifest seemingly counter-intuitive current distribution across layers. The quantum transport calculations for short channel devices with magnetic source and drain contacts postulate negative surface current for anti-phase magnetization whose axis is transverse to both current and quintuple layers. For in-phase magnetization at the two terminals, it is shown that observations can change fundamentally to result in anomalous current distribution. Such results are explained to stem from the confinement of 3D-TI between ferromagnetic contacts along the transport direction. A simple mechanism to validate topological insulators via quantum transport experiments has also been suggested. [ABSTRACT FROM AUTHOR]

We investigated the effect of body thickness on the electrical performance of GaSb double-gate ultrathin-body (DG-UTB) MOSFET by examining the band structure of the 12- ( $\sim 2$ nm), 24- ( $\sim 4$ nm), 36- ( $\sim 6$ nm), and 48- ( $\sim 8$ nm) atomic-layer (AL) thick GaSb. Two different surface orientations, namely, (100) and (111), were studied. sp3d5s* tight-binding model is used to calculate the band structures of GaSb MOSFET. Ballistic transport was studied using the semiclassical top-of-barrier model with applied self-consistent real-space potential across the body. First, we found that for (100) surface orientation, GaSb DG-UTB FET with body thickness of 24 ALs offered relatively larger ON-state current for various gate dielectric materials studied. However, for (111) surface orientation, 12 ALs GaSb DG-UTB FET showed the best performance due to its reasonably higher injection velocity and larger electron density. Furthermore, for the FET with a body thickness of 48 ALs and HfO2 dielectric, it was observed that the charge occupations shift toward the surface, unlike the cases of FETs with thinner body, leading to the formation of inversion charge on the surface. Finally, we compared the ON-state current of GaSb DG-UTB FET with different channel surface orientations and found that (100) surface generally outperforms (111) surface in terms of ON-state current. [ABSTRACT FROM AUTHOR]

We report the demonstration of a Ge0.95Sn0.05 on silicon (Ge0.95Sn0.05/Si) avalanche photodiode (APD) having a separate-absorption-charge-multiplication structure, wherein a Ge0.95Sn0.05 layer and a Si layer function as an absorption layer and a multiplication layer, respectively. Material characterization was performed by atomic force microscopy, X-ray diffraction, and transmission electron microscopy. The dark current Idark of the APD is dominated by the area-dependent bulk leakage rather than the surface leakage. The temperature dependence of breakdown voltage of the Ge0.95Sn0.05/Si APD was characterized and a thermal coefficient of 0.05% \mathrmK^\mathrm -1 was obtained, achieving a lower thermal sensitivity than the conventional III-V-based APDs. In the wavelength range of 1600–1630 nm, a responsivity of $\sim 1$ A/W (bias voltage {V}_{\rm bias} = -9.8 V) was achieved due to the internal avalanche gain. [ABSTRACT FROM PUBLISHER]

The ballistic transport performance of field-effect transistor (FET) based on hydrogenated silicene and germanene, i.e., silicane and germanane, respectively, is examined. The electronic band structures of silicane and germanane are investigated using the first-principles density functional theory. Subsequently, the ballistic performance of FETs is evaluated via the semiclassical ballistic transport model. We find that silicane n-MOSFET offers a relatively better ON-current performance than transistors made of germanane and 2-D transition metal dichalcogenides (2-D-TMDs) ( MoS{2} , MoSe{2} , WS{2} , and WSe{2} ). Germanane n-MOSFET suffers from the issue of low density of states due to its smaller electron effective mass. P-FETs based on germanane and silicane have higher ON-current than those of 2-D-TMDs p-FETs. Further investigation on other aspects of silicane and germanane MOSFETs, such as gate leakage and contact resistance, is needed to comprehensively assess their overall performance metrics. [ABSTRACT FROM PUBLISHER]

Abstract: We investigated the device performance of graphene nanoribbon tunneling field-effect transistors with heterogeneous channel as a function of the contact doping concentrations. The simulations were carried out based on the non-equilibrium Green’s function, coupled with a Dirac Hamiltonian model, and the roles of symmetric and asymmetric contact doping concentrations on the device performance were identified. It was observed that the device performances such as OFF-state currents (I OFF), ON-state currents (I ON) and subthreshold slopes (SSs) were greatly influenced by the source doping concentrations, while variations in drain doping concentrations changed mainly the I OFF. By applying proper asymmetric source and drain doping concentrations, low SS and large I ON/I OFF ratio can be achieved, indicating that it is an alternative route to effectively enhance the device performance. [Copyright &y& Elsevier]

We present a computational study of the device performance of graphene nanoribbon tunneling field-effect transistors (TFETs) with a heterogeneous channel. By varying the length and the energy bandgap (EG) of the heterogeneous region, the on- and off-state currents ( ION and IOFF) can be effectively optimized independently. Both semiconducting and semimetallic heterogeneous regions are studied to understand the effects of EG engineering on device behaviors. In addition, the effect of gate coverage (GC) over the heterogeneous region is also investigated. We found that device performance is greatly affected by the positioning of the gate to modify the region where band-to-band tunneling occurs. For a given ION/IOFF of eight orders, our results show that, for the semiconducting heterojunction, a higher ION can be obtained by having the gate partially covering the heterogeneous region. This is due to a combination of a short tunneling length and resonant states, which leads to an increase in carrier concentration for the tunneling mechanism. On the other hand, for the semimetallic case, a similar ION/IOFF is only attainable when the heterogeneous region is not covered by the gate. A large I \rm OFF is observed for even small GC due to the valence electrons from the source traveling to the conduction bands of the semimetallic region, enhancing the carrier transport toward the drain. Our study highlights the device design consideration required when optimizing the device performance of heterojunction TFETs. [ABSTRACT FROM PUBLISHER]

Abstract: We investigate the impact of interlayer mismatch on the electronic properties of bilayer graphene nanoribbons (BGNRs) with armchair-edges in terms of the total energy and electronic structures by first principle calculations. Simulation results show that in-plane misalignments require little energy and a large variation in the energy bandgap (E G) can be observed. Based on the resulting atomic configurations due to the misalignments, the details of the observed relationship between bandgap and the lattice mismatch are investigated. It is observed that in general, misalignment in the transverse direction results in a decrease in the interaction between the two layers, giving rise to a larger E G. On the other hand, misalignment in the longitudinal direction, i.e. along the edges, leads to an oscillation in E G due to the periodic change of the GNR stacking order. A combination of these movements results in a complex variation of E G, which introduces great uncertainty in electronic devices. However, such a phenomenon could also be used in various kinds of nanoelectromechanical systems as it provides a large change in electronic properties with a small movement. [Copyright &y& Elsevier]

We present a detailed simulation study on the current–voltage characteristics of ballistic graphene nanoribbon (GNR) tunneling FETs of different widths with varying temperatures and channel length. Our model uses the self-consistent nonequilibrium Green's function and the quasi-2-D Poisson solver with the material details of the GNRs modeled by the uncoupled mode space Dirac equation. We find that, in general, the GNR tunneling FETs from the \3p + \1 family have better ION/IOFF characteristics than those from the \3p family due to smaller effective masses of the former. A lower drain doping concentration relative to that of the source enhances the ION/IOFF. Most significantly, we find that a higher doping concentration at the source enhances ION but degrades the subthreshold swing (SS). As a function of temperature, the SS shows highly nonlinear behaviors. In terms of intrinsic delay and power-delay product, the GNR tunneling FETs show very promising scaling behaviors and can be optimized to meet the International Technology Roadmap for Semiconductors roadmap requirements through adjustment in doping concentrations and other parameters. [ABSTRACT FROM PUBLISHER]

The thermal stability and electronic structures of two dimensional Cx(BN)y compounds are studied using first-principles calculations based on the density functional theory. Although, from total energy calculations, it was well-established that phase-segregated atomic arrangements had the lowest energy, we found that due to the high activation energy required for phase-segregation process, evenly distributed configurations are stable at room temperature. Furthermore, the energy bandgap (EG) of the evenly distributed Cx(BN)y compounds is dependent on the carbon concentration. By controlling the carbon concentration in the compound, the EG of the compound material can be adjusted for electronic applications. [ABSTRACT FROM AUTHOR]

Reproducible current hysteresis is observed in graphene with a back gate structure in a two-terminal configuration. An opposite sequence of switching with different charge carriers, holes, and electrons is found. The charging and discharging effect is proposed to explain this ambipolar bistable hysteretic switching. To confirm this hypothesis, one-level transport model simulations including charging effect are performed and the results are consistent with our experimental data. Methods of improving the on/off ratio of graphene resistive switching are suggested. [ABSTRACT FROM AUTHOR]

Negative differential conductance and tunneling characteristics of two-terminal graphene devices are observed before and after electric breakdown, respectively. The former is caused by the strong scattering under a high E-field, and the latter is due to the appearance of a tunneling barrier in graphene channel induced by a structural transformation from crystalline graphene to disordered graphene because of the breakdown. Using Raman spectroscopy and imaging, the presence of nonuniform disordered graphene is confirmed. A memory switching effect of 100 000% ON/OFF ratio is demonstrated in the tunneling regime, which can be employed in various applications. [ABSTRACT FROM AUTHOR]

Tunneling transport across electrical potential barriers in Weyl semimetals with tilted energy dispersion is investigated. We report that the electrons around different valleys experience opposite direction refractions at the barrier interface when the energy dispersion is tilted along one of the transverse directions. Chirality dependent refractions at the barrier interface polarize the Weyl fermions in angle-space according to their valley index. A real magnetic barrier configuration is used to select allowed transmission angles, which results in electrically controllable and switchable valley polarization. Our findings may pave the way for experimental investigation of valley polarization, as well as valleytronic and electron optic applications in Weyl semimetals. [ABSTRACT FROM AUTHOR]