Nonequilibrium Green Functions Simulations Research Articles

Inverse Scaling Trends for Charge-Trapping-Induced Degradation of FinFETs Performance

In this paper, we investigate the impact of a single discrete charge trapped at the top oxide interface on the performance of scaled nMOS FinFET transistors. The charge-trapping-induced gate voltage shift is simulated as a function of the device scaling and for several regimes of conduction-from subthreshold to ON-state. Contrary to what is expected for planar MOSFETs, we show that the trap impact decreases with scaling down the FinFET size and the applied gate voltage. By comparing drift-diffusion with nonequilibrium Green functions simulations, we show that quantum effects in the charge distribution and transport can reduce or amplify the impact of discrete traps in simulation of reliability resilience of scaled FinFETs.

Hubbard nanoclusters far from equilibrium

The Hubbard model is a prototype for strongly correlated many-particle systems, including electrons in condensed matter and molecules, as well as for fermions or bosons in optical lattices. While the equilibrium properties of these systems have been studied in detail, the nonequilibrium dynamics following a strong non-perturbative excitation only recently came into the focus of experiments and theory. It is of particular interest how the dynamics depend on the coupling strength and on the particle number and whether there exist universal features in the time evolution. Here, we present results for the dynamics of finite Hubbard clusters based on a selfconsistent nonequilibrium Green functions (NEGF) approach invoking the generalized Kadanoff--Baym ansatz (GKBA). We discuss the conserving properties of the GKBA with Hartree--Fock propagators in detail and present a generalized form of the energy conservation criterion of Baym and Kadanoff for NEGF. Furthermore, we demonstrate that the HF-GKBA cures some artifacts of prior two-time NEGF simulations. Besides, this approach substantially speeds up the numerical calculations and thus presents the capability to study comparatively large systems and to extend the analysis to long times allowing for an accurate computation of the excitation spectrum via time propagation. Our data obtained within the second Born approximation compares favorably with exact diagonalization results (available for up to 13 particles) and are expected to have predictive capability for substantially larger systems in the weak coupling limit.

Large on/off current ratio in hybrid graphene/BN nanoribbons by transverse electric field-induced control of bandgap

We propose an approach to design efficient electronic switches based on Graphene/BN heterostructures. By means of atomistic Tight Binding simulations, we investigate heterostructures made of an armchair BN nanoribbon sided by two armchair graphene ribbons where a significant bandgap can be opened. In particular, we show that a bandgap of about 0.55 eV can be strongly suppressed by applying a relatively weak transverse electric field of 10 mV/A. Additionally, by using non-equilibrium Green's function simulation, we show that this effect can be used to control the current in electron devices even at small bias and small length. An on/off current ratio higher than 104 is achieved at room temperature.

Nonequilibrium green function simulations of graphene-nanoribbon resonant-tunneling transistors

We have performed nonequilibrium Green function simulations on the transport characteristics in ultra-small graphene nanoribbon resonant-tunneling transistors (RTTs). For an ultra-narrow nanoribbon transistor, the current–voltage characteristics resemble those observed in large-size graphene-based RTTs. For a wider nanoribbon transistor, we find that two types of structure due to inter-subband transitions appear in addition to the main peak: one originates from the resonance at kx = 0, while the other is due to resonance at finite kx.

Ballistic performance comparison of monolayer transition metal dichalcogenide MX2 (M = Mo, W; X = S, Se, Te) metal-oxide-semiconductor field effect transistors

We study the transport properties of monolayer MX2 (M = Mo, W; X = S, Se, Te) n- and p-channel metal-oxide-semiconductor field effect transistors (MOSFETs) using full-band ballistic non-equilibrium Green's function simulations with an atomistic tight-binding Hamiltonian with hopping potentials obtained from density functional theory. We discuss the subthreshold slope, drain-induced barrier lowering (DIBL), as well as gate-induced drain leakage (GIDL) for different monolayer MX2 MOSFETs. We also report the possibility of negative differential resistance behavior in the output characteristics of nanoscale monolayer MX2 MOSFETs.

Proposal for all-graphene monolithic logic circuits

Since the very inception of integrated circuits, dissimilar materials have been used for fabricating devices and interconnects. Typically, semiconductors are used for devices and metals are used for interconnecting them. This, however, leads to a “contact resistance” between them that degrades device and circuit performance, especially for nanoscale technologies. This letter introduces and explores an “all-graphene” device-interconnect co-design scheme, where a single 2-dimensional sheet of monolayer graphene is proposed to be monolithically patterned to form both active devices (graphene nanoribbon tunnel-field-effect-transistors) as well as interconnects in a seamless manner. Thereby, the use of external contacts is alleviated, resulting in substantial reduction in contact parasitics. Calculations based on tight-binding theory and Non-Equilibrium Green's Function (NEGF) formalism solved self-consistently with the Poisson's equation are used to analyze the intricate properties of the proposed structure. This constitutes the first NEGF simulation based demonstration that devices and interconnects can be built using the “same starting material” – graphene. Moreover, it is also shown that all-graphene circuits can surpass the static performances of the 22 nm complementary metal-oxide-semiconductor devices, including minimum operable supply voltage, static noise margin, and power consumption.

Hysteresis in the conductance of asymmetrically biased GaAs quantum point contacts with in-plane side gates

We have observed hysteresis between the forward and reverse sweeps of a common mode bias applied to the two in-plane side gates of an asymmetrically biased GaAs quantum point contact (QPC). The size of the hysteresis loop increases with the amount of bias asymmetry ΔVg between the two side gates and depends on the polarity of ΔVg. Our results are in qualitative agreement with Non-Equilibrium Green's Function simulations including the effects of dangling bond scattering on the sidewalls of the QPC. It is argued that hysteresis may constitute another indirect proof of spontaneous spin polarization in the narrow portion of the QPC.

Mode space approach for tight-binding transport simulations in graphene nanoribbon field-effect transistors including phonon scattering

In this paper, we present a mode space method for atomistic non-equilibrium Green's function simulations of armchair graphene nanoribbon FETs that includes electron-phonon scattering. With reference to both conventional and tunnel FET structures, we show that, in the ideal case of a smooth electrostatic potential, the modes can be decoupled in different groups without any loss of accuracy. Thus, inter-subband scattering due to electron-phonon interactions is properly accounted for, while the overall simulation time considerably improves with respect to real-space, with a speed-up factor of 40 for a 1.5-nm-wide device. Such factor increases with the square of the device width. We also discuss the accuracy of two commonly used approximations of the scattering self-energies: the neglect of the off-diagonal entries in the mode-space expressions and the neglect of the Hermitian part of the retarded self-energy. While the latter is an acceptable approximation in most bias conditions, the former is somewhat inaccurate when the device is in the off-state and optical phonon scattering is essential in determining the current via band-to-band tunneling. Finally, we show that, in the presence of a disordered potential, a coupled mode space approach is necessary, but the results are still accurate compared to the real-space solution.

Contact-Induced Negative Differential Resistance in Short-Channel Graphene FETs

In this paper, we clarify the physical mechanism for the phenomenon of negative output differential resistance (NDR) in short-channel graphene FETs through nonequilibrium Green's function simulations and a simpler semianalytical ballistic model that captures the essential physics. This NDR phenomenon is due to a transport mode bottleneck effect induced by the graphene Dirac point in the different device regions, including the contacts. NDR is found to occur only when the gate biasing produces an n-p-n or p-n-p polarity configuration along the channel, for both positive and negative drain-source voltage sweep. In addition, we also explore the impact on the NDR effect of contact-induced energy broadening in the source and drain regions and a finite contact resistance.

Compliant energy and momentum conservation in NEGF simulation of electron-phonon scattering in semiconductor nano-wire transistors

The modelling of spatially inhomogeneous silicon nanowire field-effect transistors has benefited from powerful simulation tools built around the Keldysh formulation of non-equilibrium Green function (NEGF) theory. The methodology is highly efficient for situations where the self-energies are diagonal (local) in space coordinates. It has thus been common practice to adopt diagonality (locality) approximations. We demonstrate here that the scattering kernel that controls the self-energies for electron-phonon interactions is generally non-local on the scale of at least a few lattice spacings (and thus within the spatial scale of features in extreme nano-transistors) and for polar optical phonon-electron interactions may be very much longer. It is shown that the diagonality approximation strongly under-estimates the scattering rates for scattering on polar optical phonons. This is an unexpected problem in silicon devices but occurs due to strong polar SO phonon-electron interactions extending into a narrow silicon channel surrounded by high kappa dielectric in wrap-round gate devices. Since dissipative inelastic scattering is already a serious problem for highly confined devices it is concluded that new algorithms need to be forthcoming to provide appropriate and efficient NEGF tools.

Influence of surface scattering on the anomalous conductance plateaus in an asymmetrically biased InAs/In0.52Al0.48As quantum point contact

We study of the appearance and evolution of several anomalous (i.e., G < G0 = 2e2/h) conductance plateaus in an In0.52Al0.48As/InAs quantum point contact (QPC). This work was performed at T = 4.2 K as a function of the offset bias ΔVG between the two in-plane gates of the QPC. The number and location of the anomalous conductance plateaus strongly depend on the polarity of the offset bias. The anomalous plateaus appear only over an intermediate range of offset bias of several volts. They are quite robust, being observed over a maximum range of nearly 1 V for the common sweep voltage applied to the two gates. These results are interpreted as evidence for the sensitivity of the QPC spin polarization to defects (surface roughness and impurity (dangling bond) scattering) generated during the etching process that forms the QPC side walls. This assertion is supported by non-equilibrium Green function simulations of the conductance of a single QPC in the presence of dangling bonds on its walls. Our simulations show that a spin conductance polarization α as high as 98% can be achieved despite the presence of dangling bonds. The maximum in α is not necessarily reached where the conductance of the channel is equal to 0.5G0.

AC Power Consumption of Single-Walled Carbon Nanotube Interconnects: Non-Equilibrium Green's Function Simulation

AC Power Consumption of Single-Walled Carbon Nanotube Interconnects: Non-Equilibrium Green's Function Simulation

Scaling Length Theory of Double-Gate Interband Tunnel Field-Effect Transistors

A scaling theory of double-gate interband tunnel field-effect transistors (TFETs) using a physics-based 2-D analytical model is presented. Ignoring the mobile charge in the channel, the electrostatic potential profile and electric field are analytically solved, and the current is calculated by integrating the band-to-band tunneling generation rate over the volume of the device. The analytical model has excellent agreement with the numerical results obtained from a commercial simulator and atomistic nonequilibrium Green function simulations for both heterojunction and homojunction TFETs. The analytical model allows us to quantitatively extract the electrostatic scaling lengths in TFETs and compare the short-channel effect of TFETs with that of MOSFETs. We conclude that double-gate TFETs exhibit superior short-channel performance than their MOSFETs counterparts at a longer gate length (greater than four times the scaling length), but the scalability of the TFETs degrades at a faster rate than MOSFETs do at smaller gate lengths (less than four times the scaling length).

AC Power Consumption of Single-Walled Carbon Nanotube Interconnects: Non-Equilibrium Green's Function Simulation

We theoretically investigate the emittance and dynamic dissipation of a nanoscale interconnect consisting of a metallic single-walled carbon nanotube using the non-equilibrium Green's function technique for AC electronic transport. We show that the emittance and dynamic dissipation depend strongly on the contact conditions of the interconnect and that the power consumption can be reduced by adjusting the contact conditions. We propose an appropriate condition of contact that yields a high power factor and low apparent power.

Study of Discrete Doping-Induced Variability in Junctionless Nanowire MOSFETs Using Dissipative Quantum Transport Simulations

The impact of discrete doping in junctionless gate all-around n-type silicon nanowire transistors is studied using 3-D nonequilibrium Green's functions simulations. The studied devices have a 20 nm long gate and cross sections of 4.2 × 4.2 and 6.2 × 6.2 nm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . The average doping concentration is 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">20</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> . The dopant distributions are randomly generated and modeled in a fully atomistic way. Phonon scattering, elastic and inelastic, is also included in the simulations. We show that junctionless nanowire transistors have a much higher subthreshold variability than their inversion mode counterparts for the equivalent geometry and doping level.

NEGF simulations of a junctionless Si gate-all-around nanowire transistor with discrete dopants

We have carried out 3D Non-Equilibrium Green Function simulations of a junctionless gate-all-around n-type silicon nanowire transistor of 4.2×4.2nm2 cross-section. We model the dopants in a fully atomistic way. The dopant distributions are randomly generated following an average doping concentration of 1020cm−3. Elastic and inelastic phonon scattering is considered in our simulation. Considering the dopants in a discrete way is the first step in the simulation of random dopant variability in junctionless transistors in a fully quantum mechanical way. Our results show that, for devices with an “unlucky” dopants configuration, where there is a starvation of donors under the gate, the threshold voltage can increase by a few hundred mV relative to devices with a more homogeneous distribution of dopants. For the first time we have used a quantum transport model with dissipation to evaluate the change in threshold voltage and subthreshold slope due to the discrete random donors in the channel of a junctionless nanowire nMOS transistor. These calculations require a robust convergence scheme between the quantum transport equation and the Poisson equation in order to achieve convergence in the dopant-induced resonance regime.

Transport-Confined Multi-Barrier FETs: A New Paradigm for Low-Leakage High On-Current Transistors

Physics and performances of a new concept of nanoscale MOSFET, the Gate-Modulated Resonant-Tunneling (RT)-FET, are investigated through 3D Non-Equilibrium Green's Function simulations. Owing to the additional barriers and the related longitudinal confinement, the density of states in a RT-FET is reduced in its off state, while remaining comparable, in its on state, to that of a MOS transistor without barriers. The RT-FET thus features both a lower RT-limited off current and a faster increase of the current with gate voltage, i.e. an improved slope characteristic, and hence an improved ION/IOFF ratio, along with high on current and therefore good speed performance. RT-FETs could therefore be promising devices for future generation low power, high speed applications owing to superior delay-power trade-off than a MOSFET. In addition, RT-FETs are intrinsically immune to source-drain tunneling and appear promising candidate for extending the roadmap below 10nm.

Properties of Charged Defects on Unidimensional Polymers

Departamento de Fisica, Universidade Federal do Para, Belem, PA 66075-110, BrazilIn this work is presented a theoretical investigation of the neutral and bipolaronlike ground andexcited states of molecules and polymers isoelectronic composed by Polyacetylene, Polyazine andPolyazoethene. The results obtained, utilizing DFT and ab initio methodologies, reveal that a verygood defects description can be important in the investigation of insulator-metal transition of quasi-unidimensional polymers indicating metallic behavior around the Fermi level as mechanism of con-ductivity of polymers. This result is consistent with experimental data and do not anticipate bySu-Schrieffer-Heeger (SSH) methodology. Our results are consistent with significant features as ananodevice and can be summarized as: (i) it could be used as single directional molecular rectifierwith a conformational geometry with small lead coupling; (ii) our non-equilibrium green functionsimulation present that Polyacetylene, Polyazine and Polyazoethene could rectified without gatecurrent; (iv) based on properties of bonds type ( / , it can be utilized to design devices withapplications in molecular electronics.

Quantum Confinement Effects in Capacitance Behavior of Multigate Silicon Nanowire MOSFETs

3-D nonequilibrium Green's function simulations reveal the presence of oscillations of gate capacitance in multigate silicon nanowire FETs as the gate voltage is increased. These oscillations are due to the filling of successive energy subbands by electrons. The effect is due to both the 1-D distribution of the density of states and a change of position of the charge centroid location with gate voltage in the confined structure. This paper also proposes a model for the gate capacitance that allows one to better understand the nature of the oscillations and shows that the oscillations are mostly due to the particular shape of the 1-D density of states. The change of position of the charge centroid location contributes to a few percents in the total variation of the gate capacitance. The gate-capacitance to oxide-capacitance ratio remains low even at high gate voltages and worsens when the gate oxide thickness is decreased.

Schottky-Barrier Metal–Oxide–Semiconductor Field-Effect Transistors as Resonant Tunneling Devices

The current–voltage characteristics of double-gated Schottky-barrier metal–oxide–semiconductor field-effect transistor (MOSFET) 10 nm in length and with body thickness of 3 nm is numerically studied, illustrating a similarity between the rectangular quantum well cavity in conventional resonant tunneling diodes and the parabolic-like cavity created by a pair of Schottky junctions in scaled Schottky-barrier MOSFETs. Assuming ballistic transport for electrons within effective mass approximation, the appearance of negative differential resistance due to the resonant tunneling effect between the Schottky junctions of 0.75 eV height is confirmed by non-equilibrium Green's function simulation. In such scaled Schottky-barrier MOSFETs, the tunneling electrons by themselves determine the shape of resonant potential, through the charge terms in electrostatic field equations. Using both the Poisson equation and the Laplace equation, we highlight the importance of the self-consistency for realizing successful resonant tunneling operation in scaled Schottky-barrier MOSFETs.