A Self-Consistent Full 3-D Real-Space NEGF Simulator for Studying Nonperturbative Effects in Nano-MOSFETs

As these As transistors are scaled to nanometer dimensions, the discreteness of the dopants becomes increasingly important. Transistors of 3 times 3 nm2 cross section contain, on average, approximately one dopant atom per nanometer of length, making any self-averaging impossible. The individual random dopants act as localized scatterers whose distribution, and therefore, impact on the electron transport, varies from device to device. This is complemented by electrostatic variation in the potential that controls the threshold voltage and the dominant current paths. The current density is greatly influenced by resonances associated with the attractive potential of the donors and screening effects. In this paper, for the first time, a full 3-D nonequilibrium Green's function (NEGF) simulation in the effective mass approximation has been used to study the influence of individual discrete donors in the source/drain on the I-V characteristics of a narrow n-channel Si nanowire transistor. We have compared devices with microscopically different configuration of dopants. The simulated variations in the I-V curves are analyzed with reference to the behavior of the transmission coefficients. We have highlighted the importance of resonance states when solving the NEGF and Poisson equations self-consistently.

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.

Recent focus of the VLSI industry has been on performance enhancement of semiconductors by introducing alternate channel materials. In this work, we have studied electronic properties of 2 dimensional hexagonal shaped aluminum nitride (2D h-AIN) as a channel material in n-MOS transistor. With Density Functional Theory (DFT), we have calculated 2D h-AIN to understand it's electronic properties and carried out non-equilibrium Green's function (NEGF) simulations to study device performance. Our studies reveal good MOS device properties in terms of drain current (86.45 (μA/μm), transconductance (296 (iS/jim) and ON/OFF ratio (1.57 × 104), which enhances the scope of further research on 2D h-AIN FET.

The recent advancement in the fabrication of narrow graphene nanoribbon with smooth edges has catalyzed the growth of nanoribbon based transistors. Motivated by these advances, we suggest bilayer graphene nanoribbon tunnel field-effect transistors (BLGNR-TFETs) for low voltage digital and analog applications. The device performance is analyzed by quantum transport simulation, based on self-consistent solutions of two-dimensional Poisson's equation and nonequilibrium Green's function formalism. The results indicate that the use of BLGNR-TFET instead of monolayer graphene nanoribbon TFET with very narrow width, increases the ON current and reduces the intrinsic device delay without lowering the ON/OFF current ratio. The BLGNR-TFET for analog/RF applications has provided higher intrinsic gain at very narrow ribbon width; however, very narrow width reduces the cutoff frequency. Furthermore, RF figure of merits are investigated in the presence of external parasitics.

Herein, a highly practical first‐principles approach for studying phonon transport across various materials, ranging from low‐dimensional systems to three‐dimensional bulk structures, as well as nanostructures, is presented. This method integrates the nonequilibrium Green's function (NEGF) formalism with ab initio calculations of dynamical matrices generated by Quantum Espresso. A detailed technical approach for extracting atomistic force constants and forming dynamical matrices that adhere to the tri‐diagonal technique within the NEGF framework is provided. Additionally, an efficient method for constructing dynamical matrices of conventional cells from those of primary cells is introduced, remarkably reducing the computational costs associated with density‐functional theory calculations. The robustness and applicability of this methodology are validated through several case studies.

We propose to check and validate the approximations made in dissipative quantum transport (QT) simulations solved in the Non-equilibrium Green's Function formalism by comparing them with the exact solution of the linearized Boltzmann Transport Equation (LB) in the stationary regime. For that purpose, we calculate the phonon-limited electron and hole mobility in bulk Si and ultra-scaled Si nanowires for different crystal orientations ⟨100⟩, ⟨110⟩, and ⟨111⟩. In both QT and LB simulations, we use the same sp3d5s* tight-binding model to describe the electron/hole properties and the same valence-force-field approach to account for the phonon properties. It is found that the QT simplifications work well for electrons, but are less accurate for holes, where a renormalization of the phonon scattering strength is proved useful to improve the results.

This paper describes the point defect treatment in nanowire MOSFETs. The 3D Poisson equation is self-consistently coupled to the Schrodinger equation. We adopt the mode-space representation in which the 3D Schrodinger equation is divided into a 2D equation, including the confinement of the cross-section, and an ID equation describing the ballistic transport based on the nonequilibrium Green's function formalism (NEGF). We use the simplest form of the tight-binding theory, with one orbital per atom, and considering only the first-neighbor-interactions. In this framework, equivalent to the ellipsoidal energy band approximation, influence of point defects on electronic transport is discussed.

We discuss within the nonequilibrium Green's function formalism the self-consistent Born approximation of the electronic self-energy for concurrent coupling of charge carriers to photons and phonons. The expansion of the charge carrier Green's function is shown to provide a description for a range of physical processes with relevance for the photovoltaic device operation, such as indirect optical transitions, thermally assisted photocarrier escape from quantum wells or phonon-mediated interband transfer in tunnel junctions. The nonequilibrium Green's function picture of these processes is reviewed with focus on the evolution of the system towards the steady state in the convergence of the self-consistency iteration process and on the impact of inelastic scattering on device operation.

In this paper, we present the results of a numerical study on the influence of discrete dopant atom distribution and crystal orientation on the electrical characteristics of p-channel silicon nanowire-based transistors using 3-D quantum simulations. The valence band was modeled employing a three-band k.p Hamiltonian with optimized Lüttinger parameters, while the device characteristics were obtained by modeling hole carrier transport through a self-consistent solution of Poisson's equation and the nonequilibrium Green's function formalism. Simulation of various discrete impurity configurations show that impurities located near the center of the channel region have the greatest impact on threshold voltage. It is shown that the effect of donor-like impurities on device threshold voltage is virtually independent of the nanowire's crystallographic orientation, whereas discrete acceptor-like impurities induce smaller threshold voltage shifts in nanowire transistors oriented along the [110] and [111] directions; thus suggesting that the devices oriented along these directions would be relatively less sensitive to the effects of unintentional impurities on threshold voltage.

Aided by density functional theory with nonequilibrium Green's functions simulations, the thermoelectric (TE) properties of nanoporous carbon nanotube junctions formed by covalently binding carbon nanotubes via benzene bridges with either meta‐ or para‐connections are investigated. The results show that the TE performances of meta‐connected nanoporous carbon nanotube junctions are significantly improved due to the influence of destructive quantum interference (DQI) effect. Moreover, the TE properties of meta‐connected carbon nanotube junctions can be further improved by substitution of nitrogen atom and gate voltage. The reason is that the quantum interference patterns can be transformed from DQI to constructive quantum interference (or Fano resonance). The theoretical analysis shows that the TE figure of merit (ZT) for meta‐connected carbon nanotube junctions at room temperature is close to 0.5 near the Fermi level, ≈20 times higher than that of the perfect carbon nanotube junction. These results reveal that quantum interference effect can indeed be used to enhance the TE performance of molecular junctions.

We propose broadband quantum cascade detectors based on a dual-upper-state active region design. The dual-upper-state concept is known to produce broad gain in quantum cascade lasers and is also expected to provide a wide spectral response for quantum cascade detectors. Compared with quantum cascade detectors employing a bound-to-bound design, dual-upper-state devices are anticipated to exhibit broader bandwidth and can achieve higher responsivity. To validate this, we designed and fabricated quantum cascade detectors for each mid-wave (∼5 μm) and long-wave (∼10 μm) infrared regions, implementing anti-crossed dual-upper states in the main absorbing transition. At 300 K, the fabricated devices exhibited responsivities of ∼13.5 mA/W at λpeak ∼5.1 μm and ∼7.1 mA/W at λpeak ∼10 μm. The measured relative bandwidth of the devices Δλ/λcenter are approximately ∼20.8% and ∼21.1%. These results indicate that the dual-upper-state not only enables a wide spectral response but also may provide enhanced responsivity. The experimental findings are further supported by non-equilibrium Green's function simulations. Overall, our results highlight the potential of the dual-upper-state approach for broadband, high-performance infrared detection.

We propose an efficient method to quantum mechanically treat anharmonic interactions in the atomistic nonequilibrium Green's function simulation of phonon transport. We demonstrate that the so-called lowest-order approximation, implemented through a rescaling technique and analytically continued by means of the Pad\'e approximants, can be used to accurately model third-order anharmonic effects. Although the paper focuses on a specific self-energy, the method is applicable to a very wide class of physical interactions. We apply this approach to the simulation of anharmonic phonon transport in realistic Si and Ge nanowires with uniform or discontinuous cross sections. The effect of increasing the temperature above 300 K is also investigated. In all the considered cases, we are able to obtain a good agreement with the routinely adopted self-consistent Born approximation, at a remarkably lower computational cost. In the more complicated case of high temperatures ($\ensuremath{\gg}300$ K), we find that the first-order Richardson extrapolation applied to the sequence of the Pad\'e approximants $N\ensuremath{-}1/N$ results in a significant acceleration of the convergence.

Two-dimensional (2D) polymeric carbon nitride (PCN) materials have drawn broad attention as promising candidates for photocatalytic hydrogen evolution. However, it remains a significant challenge to simultaneously improve the visible light absorption, separation of photogenerated carriers, and activity. Herein, alkali metals doping PCN (e.g., C6N6 and C2N) are systematically investigated based on density functional theory. Different from the conventional notion of doping atoms as active sites, the actual active site is the intrinsic pyridine nitrogen surrounding the alkali metal. Compared to the change of Gibbs free energy value of −0.45 eV (−0.60 eV) for pristine C6N6 (C2N), Li or Cs doped PCN decreases to −0.03 eV (−0.10 eV) or 0.06 eV (−0.11 eV), respectively, benefitting from the adjustment of the 3p electronic state occupation for N atoms by charges transfer from alkali metal. Meanwhile, Li or Cs doping not only broadens the absorption of visible light by narrowing the band edge position but also promotes the separation of photogenerated electrons and holes by regulating their spatial separation, which is further confirmed by significant photocurrents for Li or Cs doped PCN based on nonequilibrium Green's function simulation. Our work could provide interesting insights into the mechanistic understanding and the highly efficient design of PCN materials in photocatalysis.

Intraband source-drain tunneling is predicted to limit leakage current in sub-10 nm field-effect transistors (FETs). We use non-equilibrium Green's function simulations to study this effect in III-V multigate FETs and derive simple, accurate intraband tunneling formulas suitable for use in compact models or semiclassical device simulators. Band nonparabolicity effects are shown to be quantitatively important and incorporated using band gap scaling. We also supply a general subthreshold electrostatic model including source and drain depletion effects for analytical modeling. Our results enable rapid modeling and simulation of ultimately scaled III-V FETs for device evaluation and optimization.

The effect of inelastic scattering on quantum electron transport through double-barrier resonant-tunneling structures with large cross-sectional areas is studied numerically using the approach based on the nonequilibrium Green's-function formalism of Keldysh, Kadanoff, and Baym. The Markov assumption is not made, and the energy coordinate is retained. This makes the inclusion of the phonon-energy spectrum straightforward both conceptually and in practice. The electron-phonon interaction is treated in the self-consistent first Born approximation (SCFBA). The Pauli exclusion principle is taken into account exactly within the SCFBA. The retention of the energy coordinate allows the calculation of a number of quantities that give insight into the effect of inelastic scattering on electron transport: The effect of inelastic scattering on the occupation of the energy levels, the density of states, the energy distribution of the current density, and the power density is calculated from a quantum kinetic equation for actual device structures under high bias.