A. Svizhenko

Two-dimensional quantum mechanical modeling of nanotransistors

Quantization in the inversion layer and phase coherent transport are anticipated to have significant impact on device performance in “ballistic” nanoscale transistors. While the role of some quantum effects have been analyzed qualitatively using simple one-dimensional ballistic models, two-dimensional (2D) quantum mechanical simulation is important for quantitative results. In this paper, we present a framework for 2D quantum mechanical simulation of a nanotransistor/metal oxide field effect transistor. This framework consists of the nonequilibrium Green’s function equations solved self-consistently with Poisson’s equation. Solution of this set of equations is computationally intensive. An efficient algorithm to calculate the quantum mechanical 2D electron density has been developed. The method presented is comprehensive in that treatment includes the three open boundary conditions, where the narrow channel region opens into physically broad source, drain and gate regions. Results are presented for (i) dr...

Role of scattering in nanotransistors

We model the influence of scattering along the channel and extension regions of dual gate nanotransistors. It is found that the reduction in drain current due to scattering in the right half of the channel is comparable to the reduction in drain current due to scattering in the left half of the channel, when the channel length is comparable to the scattering length. This is in contrast to a popular belief that scattering in the source end of a nanotransistor is significantly more detrimental to the drive current than scattering elsewhere. As the channel length becomes much larger than the scattering length, scattering in the drain-end is less detrimental to the drive current than scattering near the source-end of the channel. Finally, we show that for nanotransistors, the classical picture of modeling the extension regions as simple series resistances is not valid.

Electronic transport through carbon nanotubes: Effects of structural deformation and tube chirality

Atomistic simulations using a combination of classical force field and density-functional theory (DFT) show that carbon atoms remain essentially sp(2) coordinated in either bent tubes or tubes pushed by an atomically sharp atomic-force microscope (AFM) tip. Subsequent Greens-function-based transport calculations reveal that for armchair tubes there is no significant drop in conductance, while for zigzag tubes the conductance can drop by several orders of magnitude in AFM-pushed tubes. The effect can be attributed to simple stretching of the tube under tip deformation, which opens up an energy gap at the Fermi surface.

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

In this paper, we present a full 3-D real-space quantum-transport simulator based on the Greens function formalism developed to study nonperturbative effects in ballistic nanotransistors. The nonequilibrium Green function (NEGF) equations in the effective mass approximation are discretized using the control-volume approach and solved self-consistently with the Poisson equation in order to obtain the electron and current densities. An efficient recursive algorithm is used in order to avoid the computation of the full Green function matrix. This algorithm, and the parallelization scheme used for the energy cycle, allow us to compute very efficiently the current-voltage characteristic without the simplifying assumptions often used in other quantum-transport simulations. We have applied our simulator to study the effect of surface roughness and stray charge on the ID-VG characteristic of a 6-nm Si-nanowire transistor. The results highlight the distinctly 3-D character of the electron transport, which cannot be accurately captured by using 1-D and 2-D NEGF simulations, or 3-D mode-space approximations.

Effect of scattering and contacts on current and electrostatics in carbon nanotubes

We computationally study the electrostatic potential profile and current carrying capacity of carbon nanotubes as a function of length and diameter. Our study is based on solving the nonequilibrium Green’s function and Poisson equations self-consistently, including the effect of electron-phonon scattering. A transition from the ballistic to diffusive regime of electron transport with an increase of applied bias is manifested by qualitative changes in the potential profiles, differential conductance, and electric field in a nanotube. In the low-bias ballistic limit, most of the applied voltage drop occurs near the contacts. In addition, the electric field at the tube center increases proportionally with diameter. In contrast, at high biases, most of the applied voltage drops across the nanotube, and the electric field at the tube center decreases with an increase in diameter. We find that the differential conductance can increase or decrease with bias as a result of an interplay of nanotube length, diameter, and a quality factor of the contacts. From an application viewpoint, we find that the current carrying capacity of nanotubes increases with an increase in diameter. Finally, we investigate the role of inner tubes in affecting the current carried by the outermost tube of a multiwalled nanotube.

Ballistic transport and electrostatics in metallic carbon nanotubes

We calculate the current and electrostatic potential drop in metallic carbon nanotube wires self-consistently by solving the Greens function and electrostatics equations in the ballistic case. About one-tenth of the applied voltage drops across the bulk of a nanowire, independent of the lengths considered here. The remaining nine-tenths of the bias drops near the contacts, thereby creating a nonlinear potential drop. The scaling of the electric field at the center of the nanotube with length (L) is faster than 1/L (roughly 1/L/sup 1.25-1.75/). At room temperature, the low bias conductance of larger-diameter nanotubes is larger than 4e/sup 2//h due to occupation of noncrossing subbands. The physics of conductance evolution with bias due to Zener tunneling in noncrossing subbands is discussed.

Analysis of band-gap formation in squashed armchair carbon nanotubes

The electronic properties of squashed arm-chair carbon nanotubes are modeled using constraint free density functional tight binding molecular dynamics simulations. Independent from CNT diameter, squashing path can be divided into {\it three} regimes. In the first regime, the nanotube deforms with negligible force. In the second one, there is significantly more resistance to squashing with the force being

The Impact of Random Dopant Aggregation in Source and Drain on the Performance of Ballistic DG Nano-MOSFETs: A NEGF Study

\sim 40-100

The fundamental 1/f noise and the Hooge parameter in semiconductor quantum wires

nN/per CNT unit cell. In the last regime, the CNT looses its hexagonal structure resulting in force drop-off followed by substantial force enhancement upon squashing. We compute the change in band-gap as a function of squashing and our main results are: (i) A band-gap initially opens due to interaction between atoms at the top and bottom sides of CNT. The

Simulation of direct source-to-drain tunnelling using the density gradient formalism: Non-Equilibrium Greens Function calibration

\pi-