Electrical Engineering

Electronic transport in a carbon nanotube (CNT) metal-oxide-semiconductor field effect transistor (MOSFET) is simulated using the non-equilibrium Green's functions method with the account of electron-phonon scattering. For MOSFETs, ambipolar conduction is explained via phonon-assisted band-to-band (Landau-Zener) tunneling. In comparison to the ballistic case, we show that the phonon scattering shifts the onset of ambipolar conduction to more positive gate voltage (thereby increasing the off current). It is found that the subthreshold swing in ambipolar conduction can be made as steep as 40mV/decade despite the effect of phonon scattering. 05.60.+w, 72.80.Rj, 73.40.Gk, 85.30.Tv a Electronic address: dmitri.e.nikonov@intel.com.

We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube MOSFETs. Our central results are: (i) Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. It means that elastic scattering mechanisms are most detrimental to nanotube MOSFETs. (ii) For comparable mean free paths, a lower phonon energy leads to a larger degradation of drive current. Thus for semiconducting nanowire FETs, the drive current will be more sensitive than carbon nanotube FETs because of the smaller phonon energies in semiconductors. (iii) Radial breathing mode phonons cause an appreciable reduction in drive current.

Carbon Nanotube (CNT) Electronics attracts much attention for both basic and applied research. We first review a previously developed atomistic simulator for ballistic carbon nanotube transistors [1]. A recent work shows that our modeling of electrostatics, quantum transport, and contacts is sufficient to describe experiment [2]. A self-consistent atomistic simulation for a CNTFET imposes significant computational challenges. In this paper, we show how to efficiently implement the atomistic simulation by using computational techniques for computing Green’s function, solving 3D Poisson equation, running parallel simulation, and improving convergence. A phenomenological model of metal/CNT contacts suitable for routine device simulation is also discussed in detail.

In this paper, we review our recent work using the nonequilibrium Green's function method to model nanotransistors. After presenting a motivation for the need of quantum mechanical modeling, an account of the equations and implementation is given for both 1-D and 2-D modeling. Examples are given to highlight the use of the developed models. Finally, possible future directions in quantum mechanical modeling of transport in nanotransistors are highlighted along with computational challenges. his group developed the first 2-D quantum simulator for nanotransistors, and he codeveloped an algorithm to compute the charge and current density in nanodevices. He has also worked extensively on the modeling and prediction of electrical and electromechanical properties of nanostructures. Since May 2006, he has been a Professor with the

We calculate the current and electrostatic potential drop in metallic carbon nanotube wires selfconsistently, by solving the Green's 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 non linear 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 1.25−1.75 ). At room temperature, the low bias conductance of large diameter nanotubes is larger than 4e 2 /h due to occupation of non crossing subbands. The physics of conductance evolution with bias due to the transmission Zener tunneling in non crossing subbands is discussed.

In this paper, we present a comparative study between non-equilibrium Green’s function and quantum-corrected Monte Carlo approaches for an ultra-short channel MOSFET. As a result, we have found that the both models are equivalent in the simulation of quantum transport in a nano-scale MOSFET. We will also demonstrate that impurity scattering in the source region and plasmon scattering at the drain-end of the channel especially influence the drain current based on the quantum-corrected Monte Carlo approach.

We present a detailed treatment of dissipative quantum transport in carbon-nanotube field-effect transistors (CNT-FETs) using the nonequilibrium Green's function formalism. The effect of phonon scattering on the device characteristics of CNT-FETs is explored using extensive numerical simulation. Both intraand intervalley scattering mediated by acoustic (AP), optical (OP), and radial-breathing-mode (RBM) phonons are treated. Realistic phonon dispersion calculations are performed using forceconstant methods, and electron-phonon coupling is determined through microscopic theory. Specific simulation results are presented for (16,0), (19,0), and (22,0) zigzag CNTFETs, which are in the experimentally useful diameter range. We find that the effect of phonon scattering on device performance has a distinct bias dependence. Up to moderate gate biases, the influence of high-energy OP scattering is suppressed, and the device current is reduced due to elastic backscattering by AP and low-energy RBM phonons. At large gate biases, the current degradation is mainly due to high-energy OP scattering. The influence of both AP and high-energy OP scattering is reduced for larger diameter tubes. The effect of RBM mode, however, is nearly independent of the diameter for the tubes studied here.

The modeling of carbon nanotube-metal contacts is important from both basic and applied view points. For many applications, it is important to design contacts such that the transmission is dictated by intrinsic properties of the nanotube rather than by details of the contact. In this paper, we calculate the electron transmission probability from a nanotube to a free electron metal, which is side-contacted. If the metal-nanotube interface is sufficiently ordered, we find that k-vector conservation plays an important role in determining the coupling, with the physics depending on the area of contact, tube diameter and chirality. The main results of this paper are: (i) conductance scales with contact length, a phenomena that has been observed in experiments and (ii) in the case of uniform coupling between metal and nanotube, the threshold value of the metal Fermi wave vector (below which coupling is insignificant) depends on chirality. Disorder and small phase coherence length relax the need for k-vector conservation, thereby making the coupling stronger.

Electronic transport in a carbon nanotube (CNT) metal-oxide-semiconductor field effect transistor (MOSFET) is simulated using the non-equilibrium Green's functions method with the account of electron-phonon scattering. For MOSFETs, ambipolar conduction is explained via phonon-assisted band-to-band (Landau-Zener) tunneling. In comparison to the ballistic case, we show that the phonon scattering shifts the onset of ambipolar conduction to more positive gate voltage (thereby increasing the off current). It is found that the subthreshold swing in ambipolar conduction can be made as steep as 40mV/decade despite the effect of phonon scattering. 05.60.+w, 72.80.Rj, 73.40.Gk, 85.30.Tv a Electronic address: dmitri.e.nikonov@intel.com.

We investigate the role of electron-phonon scattering and gate bias in degrading the drive current of nanotube MOSFETs. Our central results are: (i) Optical phonon scattering significantly decreases the drive current only when gate voltage is higher than a well-defined threshold. It means that elastic scattering mechanisms are most detrimental to nanotube MOSFETs. (ii) For comparable mean free paths, a lower phonon energy leads to a larger degradation of drive current. Thus for semiconducting nanowire FETs, the drive current will be more sensitive than carbon nanotube FETs because of the smaller phonon energies in semiconductors. (iii) Radial breathing mode phonons cause an appreciable reduction in drive current.

Carbon Nanotube (CNT) Electronics attracts much attention for both basic and applied research. We first review a previously developed atomistic simulator for ballistic carbon nanotube transistors [1]. A recent work shows that our modeling of electrostatics, quantum transport, and contacts is sufficient to describe experiment [2]. A self-consistent atomistic simulation for a CNTFET imposes significant computational challenges. In this paper, we show how to efficiently implement the atomistic simulation by using computational techniques for computing Green’s function, solving 3D Poisson equation, running parallel simulation, and improving convergence. A phenomenological model of metal/CNT contacts suitable for routine device simulation is also discussed in detail.

In this paper, we review our recent work using the nonequilibrium Green's function method to model nanotransistors. After presenting a motivation for the need of quantum mechanical modeling, an account of the equations and implementation is given for both 1-D and 2-D modeling. Examples are given to highlight the use of the developed models. Finally, possible future directions in quantum mechanical modeling of transport in nanotransistors are highlighted along with computational challenges. his group developed the first 2-D quantum simulator for nanotransistors, and he codeveloped an algorithm to compute the charge and current density in nanodevices. He has also worked extensively on the modeling and prediction of electrical and electromechanical properties of nanostructures. Since May 2006, he has been a Professor with the

We calculate the current and electrostatic potential drop in metallic carbon nanotube wires selfconsistently, by solving the Green's 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 non linear 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 1.25−1.75 ). At room temperature, the low bias conductance of large diameter nanotubes is larger than 4e 2 /h due to occupation of non crossing subbands. The physics of conductance evolution with bias due to the transmission Zener tunneling in non crossing subbands is discussed.

In this paper, we present a comparative study between non-equilibrium Green’s function and quantum-corrected Monte Carlo approaches for an ultra-short channel MOSFET. As a result, we have found that the both models are equivalent in the simulation of quantum transport in a nano-scale MOSFET. We will also demonstrate that impurity scattering in the source region and plasmon scattering at the drain-end of the channel especially influence the drain current based on the quantum-corrected Monte Carlo approach.

We present a detailed treatment of dissipative quantum transport in carbon-nanotube field-effect transistors (CNT-FETs) using the nonequilibrium Green's function formalism. The effect of phonon scattering on the device characteristics of CNT-FETs is explored using extensive numerical simulation. Both intraand intervalley scattering mediated by acoustic (AP), optical (OP), and radial-breathing-mode (RBM) phonons are treated. Realistic phonon dispersion calculations are performed using forceconstant methods, and electron-phonon coupling is determined through microscopic theory. Specific simulation results are presented for (16,0), (19,0), and (22,0) zigzag CNTFETs, which are in the experimentally useful diameter range. We find that the effect of phonon scattering on device performance has a distinct bias dependence. Up to moderate gate biases, the influence of high-energy OP scattering is suppressed, and the device current is reduced due to elastic backscattering by AP and low-energy RBM phonons. At large gate biases, the current degradation is mainly due to high-energy OP scattering. The influence of both AP and high-energy OP scattering is reduced for larger diameter tubes. The effect of RBM mode, however, is nearly independent of the diameter for the tubes studied here.

The modeling of carbon nanotube-metal contacts is important from both basic and applied view points. For many applications, it is important to design contacts such that the transmission is dictated by intrinsic properties of the nanotube rather than by details of the contact. In this paper, we calculate the electron transmission probability from a nanotube to a free electron metal, which is side-contacted. If the metal-nanotube interface is sufficiently ordered, we find that k-vector conservation plays an important role in determining the coupling, with the physics depending on the area of contact, tube diameter and chirality. The main results of this paper are: (i) conductance scales with contact length, a phenomena that has been observed in experiments and (ii) in the case of uniform coupling between metal and nanotube, the threshold value of the metal Fermi wave vector (below which coupling is insignificant) depends on chirality. Disorder and small phase coherence length relax the need for k-vector conservation, thereby making the coupling stronger.

Atomistic simulations using a combination of classical forcefield and Density-Functional-Theory (DFT) show that carbon atoms remain essentially sp2 coordinated in either bent tubes or tubes pushed by an atomically sharp AFM tip. Subsequent Green's-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.

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 non equilibrium Green's function and Poisson equations self-consistently, including the effect of electron-phonon scattering. A transition from ballistic to diffusive regime of electron transport with increase of applied bias is manifested by qualitative changes in 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 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 view point, we find that the current carrying capacity of nanotubes increases with increase in diameter. Finally, we investigate the role of inner tubes in affecting the current carried by the outermost tube of a multiwalled nanotube.