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Negative differential resistance (NDR) devices are adequate candidates for the functional devices applicable to the next‐generation integrated circuit technology so‐called "Beyond CMOS." Here, a graphene velocity‐modulation‐barrier resonant‐tunneling diode operating at room temperature is proposed. The current–voltage characteristics of the device are analyzed using the non‐equilibrium Green's function technique. It is found that the Fermi velocity barrier in the well/barrier region manipulates the tunneling transmission probability by suppressing the Klein region and improving the resonant tunneling leading to NDR. For special values of velocity barriers, resonant states have maximum alignment with each other which increases peak current with a high peak to valley ratio (PVR). The width and the position of the NDR window are controlled and engineered by the device dimensions and the height of potential barriers. The smaller the device showed the better the NDR properties such as larger current density and maximum PVR. Taken together, the results reveal that adequate magnitude of the Fermi velocity in graphene barrier can be an impressive concept for the fabrication of emerging tunneling devices. [ABSTRACT FROM AUTHOR]

A new compact modeling approach is presented which describes the full current-voltage (I-V) characteristic of high-performance (aggressively scaled-down) tunneling field-effect-transistors (TFETs) based on homojunction direct-bandgap semiconductors. The model is based on an analytic description of two key features, which capture the main physical phenomena related to TFETs: (1) the potential profile from source to channel and (2) the elliptic curvature of the complex bands in the bandgap region. It is proposed to use 1D Poisson's equations in the source and the channel to describe the potential profile in homojunction TFETs. This allows to quantify the impact of source/drain doping on device performance, an aspect usually ignored in TFET modeling but highly relevant in ultra-scaled devices. The compact model is validated by comparison with state-of-the-art quantum transport simulations using a 3D full band atomistic approach based on non-equilibrium Green's functions. It is shown that the model reproduces with good accuracy the data obtained from the simulations in all regions of operation: the on/off states and the n/p branches of conduction. This approach allows calculation of energy-dependent band-to-band tunneling currents in TFETs, a feature that allows gaining deep insights into the underlying device physics. The simplicity and accuracy of the approach provide a powerful tool to explore in a quantitatively manner how a wide variety of parameters (material-, size-, and/or geometry-dependent) impact the TFET performance under any bias conditions. The proposed model presents thus a practical complement to computationally expensive simulations such as the 3D NEGF approach. [ABSTRACT FROM AUTHOR]

The triple heterojunction tunnel field-effect transistor (TFET) has been originally proposed to resolve the TFET’s low ON-current challenge. The carrier transport in such devices is complicated due to the presence of quantum wells and strong scattering. Hence, the full-band atomistic nonequilibrium Green’s function (NEGF) approach, including scattering, is required to model the carrier transport accurately. However, such simulations for devices with realistic dimensions are computationally unfeasible. To mitigate this issue, we have employed the empirical tight-binding mode-space approximation to simulate the triple heterojunction TFETs with the body thickness up to 12 nm. The triple heterojunction TFET design is optimized using the model to achieve a sub-60-mV/decade transfer characteristic under realistic scattering conditions. [ABSTRACT FROM AUTHOR]

Being fundamentally limited to a current–voltage steepness of 60mV/dec, MOSFETs struggle to operate below 0.6 V. Further reduction in ${V}_{\text {DD}}$ and, consequently, power consumption can be achieved with novel devices, such as tunneling transistors (TFETs) that can overcome this limitation. TFETs, however, face challenges with low ON-current leading to slow performance. TFETs made from III-nitride heterostructures are quite promising in this regard. The lattice mismatch induces a piezoelectric polarization field in a nitride heterojunction that can boost the ON-current. However, it is shown here that the carrier thermalization at the heterointerface degrades the subthreshold characteristics. Therefore, a good design should minimize the number of confined quantum well (QW) states at the heterointerface so as not to degrade the subthreshold characteristics while maintaining the lattice mismatch induced polarization to boost the ON-current. We show here that an InAlN QW on an InGaN substrate alloy engineered TFET design is promising to fulfill these requirements. Proper engineering of the alloy mole fractions and the width of the well can eliminate (or at least minimize) the undesired thermalization effects and, at the same time, provide a lattice mismatch to induce a piezoelectric field for boosting the ON-current. We have used a suitable atomistic quantum transport model to simulate these devices. The model accounts for the different mechanisms that are involved, and captures realistic scattering thermalization effects. This model has been benchmarked in our earlier work with experimental measurements of nitride tunneling heterojunction diodes and is used here to optimize the alloy engineered nitride TFET. [ABSTRACT FROM AUTHOR]

In this brief, vertical tunnel field-effect transistors (v-TFETs) based on vertically stacked heretojunctions from 2-D transition metal dichalcogenide materials are studied by atomistic quantum transport simulations. The switching mechanism of a v-TFET is found to be different from previous predictions. As a consequence of this switching mechanism, the extension region where the materials are not stacked over is found to be critical for turning off the v-TFET. This extension region makes the scaling of v-TFETs challenging. In addition, due to the presence of both positive and negative charges inside the channel, v-TFETs also exhibit negative top gate capacitance. As a result, v-TFETs have good energy-delay products and are one of the promising candidates for low-power applications. [ABSTRACT FROM AUTHOR]

Tunneling heterojunctions (THJs) have confined states close to the tunneling region, which significantly affect their transport properties. Accurate numerical modeling of THJs requires combining the nonequilibrium coherent quantum transport through the tunneling region as well as the quasi-equilibrium statistics arising from the strong scattering in the confined states. In this paper, a novel atomistic model is proposed to include both the effects: the strong scattering in the regions around THJ and the coherent tunneling. The new model matches reasonably well with experimental measurements of Nitride THJ and provides an efficient engineering tool for performance prediction and design of THJ-based devices. [ABSTRACT FROM PUBLISHER]

In this paper, an analytic model is proposed, which provides, in a continuous manner, the current–voltage ( $I$ – $V$ ) characteristic of high-performance tunneling FETs (TFETs) based on direct bandgap semiconductors. The model provides the closed-form expressions for $I$ – $V$ based on: 1) a modified version of the well-known Fowler–Nordheim (FN) formula (in the ON-state) and 2) an equation that describes the OFF-state performance while providing continuity at the ON/OFF threshold by means of a term introduced as the continuity factor. It is shown that the traditional approaches, such as FN, are accurate in TFETs only through correct evaluation of the total band bending distance and the tunneling effective mass. General expressions for these two key parameters are provided. Moreover, it is demonstrated that the tunneling effective mass captures both the ellipticity of evanescent states and the dual (electron/hole) behavior of the tunneling carriers, and it is further shown that such a concept is even applicable to semiconductors with nontrivial energy dispersion. Ultimately, it is found that the $I$ – $V$ characteristics obtained by using this model are in close agreement with the state-of-the-art quantum transport simulations both in the ON- and OFF-state, thus providing the validation of the analytic approach. [ABSTRACT FROM PUBLISHER]

Self-assembled quantum dots (QDs) are highly strained heterostructures. The lattice strain significantly modifies the electronic and optical properties of these devices. A universal behavior is observed in atomistic strain simulations (in terms of both strain magnitude and profile) of QDs with different shapes and materials. In this paper, this universal behavior is investigated by atomistic as well as analytic continuum models. Atomistic strain simulations are very accurate but computationally expensive. On the other hand, analytic continuum solutions are based on assumptions that significantly reduce the accuracy of the strain calculations, but are very fast. Both techniques indicate that the strain depends on the aspect ratio (AR) of the QDs, and not on the individual dimensions. Thus, simple closed-form equations are introduced which directly provide the atomistic strain values inside the QD as a function of the AR and the material parameters. Moreover, the conduction and valence band edges EC/V and their effective masses m^*C/V of the QDs are dictated by the strain and AR consequently. The universal dependence of atomistic strain on the AR is useful in many ways. Not only does it reduce the computational cost of atomistic simulations significantly, but it also provides information about the optical transitions of QDs given the knowledge of EC/V and m^*C/V from AR. Finally, these expressions are used to calculate optical transition wavelengths in InAs/GaAs QDs, and the results agree well with the experimental measurements and atomistic simulations. [ABSTRACT FROM AUTHOR]

Artificial semiconductors with manufactured band structures have opened up many new applications in the field of optoelectronics. The emerging two-dimensional (2D) semiconductor materials, transition metal dichalcogenides (TMDs), cover a large range of bandgaps and have shown potential in high performance device applications. Interestingly, the ultrathin body and anisotropic material properties of the layered TMDs allow a wide range modification of their band structures by electric field, which is obviously desirable for many nanoelectronic and nanophotonic applications. Here, we demonstrate a continuous bandgap tuning in bilayer MoS2 using a dual-gated field-effect transistor (FET) and photoluminescence (PL) spectroscopy. Density functional theory (DFT) is employed to calculate the field dependent band structures, attributing the widely tunable bandgap to an interlayer direct bandgap transition. This unique electric field controlled spontaneous bandgap modulation approaching the limit of semiconductor-to-metal transition can open up a new field of not yet existing applications. [ABSTRACT FROM AUTHOR]

Nanoelectronic transistors have reached 3D length scales in which the number of atoms is countable. Truly atomistic device representations are needed to capture the essential functionalities of the devices. Atomistic quantum transport simulations of realistically extended devices are, however, computationally very demanding. The widely used mode space (MS) approach can significantly reduce the numerical cost, but a good MS basis is usually very hard to obtain for atomistic full-band models. In this work, a robust and parallel algorithm is developed to optimize the MS basis for atomistic nanowires. This enables engineering-level, reliable tight binding non-equilibrium Green's function simulation of nanowire metal-oxide-semiconductor field-effect transistor (MOSFET) with a realistic cross section of 10 nm × 10 nm using a small computer cluster. This approach is applied to compare the performance of InGaAs and Si nanowire n-type MOSFETs (nMOSFETs) with various channel lengths and cross sections. Simulation results with full-band accuracy indicate that InGaAs nanowire nMOSFETs have no drive current advantage over their Si counterparts for cross sections up to about 10 nm × 10 nm. [ABSTRACT FROM AUTHOR]

Triple heterojunction (THJ) tunneling field-effect transistors (TFETs) have been proposed to resolve the low ON-current challenge of TFETs. However, the design space for THJ-TFETs is limited by fabrication challenges with respect to device dimensions and material interfaces. This work shows that the original THJ-TFET design with 12-nm body thickness has poor performance because its subthreshold swing (SS) is 50 mV/decade and the ON-current is only 6 μA/μm. To improve the performance, the doping profile of THJ-TFET is engineered to boost the resonant tunneling efficiency. The proposed THJ-TFET design shows an SS of 40 mV/decade over four orders of drain current and an ON-current of 325 μA/μm with VGS = 0.3 V. Since THJ-TFETs have multiple quantum wells and material interfaces in the tunneling junction, quantum transport simulations in such devices are complicated. State-of-the-art mode-space quantum transport simulation, including the effect of thermalization and scattering, is employed in this work to optimize THJ-TFET design. [ABSTRACT FROM AUTHOR]

The 2-D material-based TFETs are among the most promising candidates for low-power electronics applications since they offer ultimate gate control and high-current drives that are achievable through small tunneling distances ($\Lambda $) during the device operation. The ideal device is characterized by a minimized $\Lambda $. However, devices with the thinnest possible body do not necessarily provide the best performance. For example, reducing the channel thickness (${T}_{\text {ch}}$) increases the depletion width in the source, which can be a significant part of the total $\Lambda $. Hence, it is important to determine the optimum ${T}_{\text {ch}}$ for each channel material individually. In this paper, we study the optimum ${T}_{\text {ch}}$ for three channel materials: WSe2, black phosphorus, and InAs using full-band self-consistent quantum transport simulations. To identify the ideal ${T}_{\text {ch}}$ for each material at a specific doping density, a new analytic model is proposed and benchmarked against the numerical simulations. [ABSTRACT FROM AUTHOR]

In recent years, alloy electronic structure calculations based on supercell Brillouin zone unfolding have become popular. There are a number of formulations of the method which on the surface might appear different. Here we show that a discrete real-space description, based on discrete Fourier transforms, is fully general. Furthermore, such an approach can more easily show the effects of alloy scattering. We present such a method for treating the random alloy problem. This treatment features straightforward mathematics and a transparent physical interpretation of the calculated effective (i.e., approximate) energy bands. [ABSTRACT FROM AUTHOR]

Thermal properties are of great interest in modem electronic devices and nanostructures. Calculating these properties is straightforward when the device is made from a pure material, but problems arise when alloys are used. Specifically, only approximate band structures can be computed for random alloys and most often the virtual crystal approximation (VCA) is used. Unfolding methods [Boykin, Kharche, Klimeck, and Korkusinski, J. Phys.: Condens. Matter 19, 036203 (2007)] have proven very useful for tight-binding calculations of alloy electronic structure without the problems in the VCA, and the mathematical analogy between tight-binding and valence-force-field approaches to the phonon problem suggests they be employed here as well. However, there are some differences in the physics of the two problems requiring modifications to the electronic-structure approach. We therefore derive a phonon alloy band-structure (vibrational-mode) approach based on our tight-binding electronic-structure method, modifying the band-determination method to accommodate the different physical situation. Using the method, we study In(x)Ga(1 - x)As alloys and find very good agreement with available experiments. [ABSTRACT FROM AUTHOR]

A high performance triple-heterojunction (3HJ) design has been previously proposed for tunneling FETs (TFETs). Compared with single HJ TFETs, the 3HJ TFETs have both shorter tunneling distance and two transmission resonances that significantly improve the ON-state current ( I\scriptscriptstyle {\text {ON}} ). Coherent quantum transport simulation predicts that I\scriptscriptstyle {\text {ON}} = 460~\mu \textsf {A}/\mu \textsf {m} can be achieved at gate length \text Lg = 15~\textsf nm , supply voltage V\textsf {DD} = 0.3~\textsf V , and OFF-state current I\scriptscriptstyle {\text {OFF}} = 1~\textsf {nA}/\mu \textsf {m} . However, strong electron–phonon and electron–electron scattering in the heavily doped leads implies that the 3HJ devices operate far from the ideal coherent limit. In this paper, such scattering effects are assessed by a newly developed multiscale transport model, which combines the ballistic nonequilibrium Green’s function method for the channel and the drift-diffusion scattering method for the leads. Simulation results show that the thermalizing scattering in the leads both degrades the 3HJ TFET’s subthreshold swing through scattering-induced leakage and reduces the turn-ON current through the access resistance. Assuming bulk scattering rates and carrier mobilities, the I\scriptscriptstyle {\text {ON}} is dropped from 460~\mu \textsf A/\mu \textsf m down to 254~\mu \textsf A/\mu \textsf m , which is still much larger than the single HJ TFET case. [ABSTRACT FROM AUTHOR]