Quantum Hydrodynamic Model by Moment Closure of Wigner Equation

In this paper, we derive the quantum hydrodynamics models based on the moment closure of the Wigner equation. The moment expansion adopted is of the Grad type firstly proposed in \cite{Grad}. The Grad's moment method was originally developed for the Boltzmann equation. In \cite{Fan_new}, a regularization method for the Grad's moment system of the Boltzmann equation was proposed to achieve the globally hyperbolicity so that the local well-posedness of the moment system is attained. With the moment expansion of the Wigner function, the drift term in the Wigner equation has exactly the same moment representation as in the Boltzmann equation, thus the regularization in \cite{Fan_new} applies. The moment expansion of the nonlocal Wigner potential term in the Wigner equation is turned to be a linear source term, which can only induce very mild growth of the solution. As the result, the local well-posedness of the regularized moment system for the Wigner equation remains as for the Boltzmann equation

Reduction of the Dark-Current in Carbon Nanotube Photo-Detectors

Abstract-Carbon nanotubes have been considered in recent years for future opto-electronic applications because of their direct band-gap and the tunability of the band-gap with the CNT diameter. The performance of infra-red photo-detectors based on carbon nanotube field-effect transistors is analyzed, using the non-equilibrium Green's function formalism. The relatively low ratio of the photo-current to the dark current limits the performance of such devices. We show that by employing a double gate structure this ratio can be significantly increased. Carbon nanotubes (CNTs) have been extensively studied in recent years due to their exceptional electronic, optoelectronic, and mechanical properties. CNTs can be considered as a graphene sheet which has been wrapped into a tube. The way the graphene sheet is wrapped is represented by a pair of indices (n, m) called the chiral vector. The integers n and m denote the number of unit vectors along two directions in the honeycomb crystal lattice of graphene. If m = 0, the CNT is called zigzag. If n = m, the CNT is called armchair. Otherwise, it is called chiral. CNTs with n−m = 3 are metals, otherwise they are semiconductors. Semiconducting CNTs can be used as channels for transistors. Depending on the work function difference between the metal contact and the CNT, carriers at the metal-CNT interface encounter different barrier heights. Fabrication of devices with positive [1] and zero Some of the interesting electronic properties of CNTs are quasi-ballistic carrier transport [2], suppression of shortchannel effects due to one-dimensional electron transport IR photo detectors based on carbon nanotube field effect transistors (CNT-FETs) have been reported i

An event bias technique for Monte Carlo device simulation

Abstract In Monte Carlo (MC) simulations of semiconductor devices it is necessary to enhance the statistics in sparsely populated regions of interest. In this work the Monte Carlo method for stationary carrier transport, known as the Single-Particle MC method, is considered. It gives a solution to the stationary boundary value problem defined by the semi-classical Boltzmann equation (BE). Using a formal approach which employs the integral form of the problem and the Neumann series expansion of the solution, the Single-Particle MC method is derived in a formal way. The independent, identically distributed random variables of the simulated process are identified. Estimates of the stochastic error are given. Furthermore, the extension of the MC estimators to the case of biased events is derived. An event bias technique for particle transport across an energy barrier is developed and simulation results are discussed

Monte Carlo Algorithm for Mobility Calculations in Thin Body Field Effect Transistors: Role of Degeneracy and Intersubband Scattering

Abstract. We generalize the Monte Carlo algorithm originally designed for small signal analysis of the three-dimensional electron gas to quasitwo-dimensional electron systems. The method allows inclusion of arbitrary scattering mechanisms and general band structure. Contrary to standard Monte Carlo methods to simulate transport, this algorithm takes naturally into account the fermionic nature of electrons via the Pauli exclusion principle. The method is based on the solution of the linearized Boltzmann equation and is exact in the limit of negligible driving fields. The theoretically derived Monte Carlo algorithm has a clear physical interpretation. The diffusion tensor is calculated as an integral of the velocity autocorrelation function. The mobility tensor is related to the diffusion tensor via the Einstein relation for degenerate statistics. We demonstrate the importance of degeneracy effects by evaluating the low-field mobility in contemporary field-effect transistors with a thin silicon body. We show that degeneracy effects are essential for the correct interpretation of experimental mobility data for field effect transistors in single-and double-gate operation mode. In double-gate structures with (100) crystal orientation of the silicon film degeneracy effects lead to an increased occupation of the higher subbands. This opens an additional channel for elastic scattering. Increased intersubband scattering compensates the volume inversion induced effect on the mobility enhancement and leads to an overall decrease in the mobility per channel in doublegate structures

An Investigation of the Geometrical Effects on the Thermal Conductivity of Graphene Antidot Lattices

In this work we investigate the thermal conductivity of graphene-based antidot lattices. A third nearest-neighbor tight-binding model and a forth nearest-neighbor force constant model are employed to study the electronic and phononic band structures of graphene-based antidot lattices. Ballistic transport models are used to evaluate the electronic and the thermal conductivities. Methods to reduce the thermal conductivity and to increase the thermoelectric figure of merit of such structures are studied. Our results indicate that triangular antidot lattices have the smallest thermal conductivity due to longer boundaries and the smallest distance between the neighboring dots

Subband Engineering in n-Type Silicon Nanowires using Strain and Confinement

Abstract We present a model based on k · p theory which is able to capture the subband structure effects present in ultrathin strained nanowires. The effective mass and valley minima are calculated for different crystal orientations thicknesses and strains. The results show that transport enhancement can be achieved by both confinement and strain which is in agreement with recent experimental findings. Motivation Nanowire based gate-all-around transistors offer a perspective for further device size reduction in microelectronics. Apart from the enhancement of electrostatic control over the channel due to a high surface to volume ratio, nanowires exhibit transport properties which deviate significantly from what is observed in bulk silicon or inversion layers. In a recent experimental study Modeling To understand the transport properties in wires below 10 nm one must carefully take quantization effects into account. A simple treatment using effective masses fails to satisfactorily describe the subband structure of such thin devices. This is due to the energy of the lowest subband already being of the order of 100 meV where nonparabolicity effects become noticeable. In this work we investigate the effects of both two dimensional confinement and strain using a two band k · p model for the conduction band V denotes the conduction band edge; m l = 0.91m e are the longitudinal and m t = 0.19m e the transversal effective mass and a amounts to the distance between a X point and the nearest ∆ valleys; ε l-l and ε t1-t2 are uniaxial and shear strain components and Ξ u and Ξ u the deformation potentials; σ x,z denote the Pauli matrices and I the identity matrix. The Hamiltonian is rotated according to the nanowire axis and quantized in the cross section plane to obtain the subband structure

A numerical study of partial-SOI LDMOSFETs

Abstract The high-voltage and self-heating behavior of partial-SOI (silicon-on-insulator) LDMOSFETs were studied numerically. Different locations of the silicon window were considered to investigate the electrical and thermal effects. It is found that the potential distribution of the partial-SOI LDMOSFET with the silicon window under the drain is similar to that of standard junction isolation devices. With the silicon window under the source the potential distribution is similar to that of the conventional SOI LDMOSFET. Using the two-dimensional numerical simulator MINIMOS-NT, we confirm that the breakdown voltage of partial-SOI LDMOSFETs with a silicon window under the source is higher than that of partial-SOI LDMOSFET with a silicon window under the drain

Study of Thermal Properties of Graphene-Based Structures Using the Force Constant Method

The thermal properties of graphene-based materials are theoretically investigated. The fourth-nearest neighbor force constant method for phonon properties is used in conjunction with both the Landauer ballistic and the non-equilibrium Green's function techniques for transport. Ballistic phonon transport is investigated for different structures including graphene, graphene antidot lattices, and graphene nanoribbons. We demonstrate that this particular methodology is suitable for robust and efficient investigation of phonon transport in graphene-based devices. This methodology is especially useful for investigations of thermoelectric and heat transport applications.Comment: 23 pages, 9 figures, 1 tabl

Numerical study of the thermoelectric power factor in ultra-thin Si nanowires

Low dimensional structures have demonstrated improved thermoelectric (TE) performance because of a drastic reduction in their thermal conductivity, {\kappa}l. This has been observed for a variety of materials, even for traditionally poor thermoelectrics such as silicon. Other than the reduction in {\kappa}l, further improvements in the TE figure of merit ZT could potentially originate from the thermoelectric power factor. In this work, we couple the ballistic (Landauer) and diffusive linearized Boltzmann electron transport theory to the atomistic sp3d5s*-spin-orbit-coupled tight-binding (TB) electronic structure model. We calculate the room temperature electrical conductivity, Seebeck coefficient, and power factor of narrow 1D Si nanowires (NWs). We describe the numerical formulation of coupling TB to those transport formalisms, the approximations involved, and explain the differences in the conclusions obtained from each model. We investigate the effects of cross section size, transport orientation and confinement orientation, and the influence of the different scattering mechanisms. We show that such methodology can provide robust results for structures including thousands of atoms in the simulation domain and extending to length scales beyond 10nm, and point towards insightful design directions using the length scale and geometry as a design degree of freedom. We find that the effect of low dimensionality on the thermoelectric power factor of Si NWs can be observed at diameters below ~7nm, and that quantum confinement and different transport orientations offer the possibility for power factor optimization.Comment: 42 pages, 14 figures; Journal of Computational Electronics, 201