Tanroku Miyoshi

Simulation of quantum transport in quantum devices with spatially varying effective mass

The authors report progress in quantum-mechanical simulation based on the Wigner function model. An exact nonlocal formulation in the Wigner representation due to a spatially varying effective mass and its discretization for numerical calculation are discussed. To verify the validity of such a formulation, the current-voltage characteristics of resonant tunneling diodes are simulated to compare with the conventional Wigner function model. The authors also point out the importance of self-consistent calculation in the electrostatic potential for precise device simulation. The emphasize that the Wigner function model is superior to the alternative method based on the transmission probability method even for the static simulation of quantum transport. >

Optimum design of coplanar waveguide for LiNbO/sub 3/ optical modulator

In this paper, we first present a novel finite element method combined with the conformal mapping (FEM-CM) for a quasi-static analysis of coplanar waveguides (CPW). Using this approach, the optimum CPW structures for the use in the Ti:LiNbO/sub 3/ optical modulator are discussed in detail to realize optical-microwave phase velocity match and electrode-source characteristic impedance match. Our numerical results reveal that both conditions can be satisfied simultaneously by introducing a SiO/sub 2/ buffer layer and thicker electrodes. The modulator efficiency with respect to the voltage-length product is also evaluated for the optimized structures. Finally, the design guidelines to the optimum CPW structure are presented. >

NONEQUILIBRIUM GREEN'S FUNCTION APPROACH TO HIGH-TEMPERATURE QUANTUM TRANSPORT IN NANOSTRUCTURE DEVICES

A quantum kinetic equation for the Wigner distribution function including collisional broadening effects is formulated based upon nonequilibrium Green’s function technique, and applied to the high-temperature quantum transport in nanostructure devices. The collisional effects due to longitudinal optical phonon scattering are introduced to the transport equation through the electron spectral function. After studying the influence of quantum size confinement on the spectral functions in nanostructures, the nonlinear current–voltage characteristics of quantum wires and the phonon bottleneck phenomena in quantum dot lasers at high temperature are analyzed.

A Quantum-Corrected Monte Carlo Study on Quasi-Ballistic Transport in Nanoscale MOSFETs

In this paper, the authors study a quasi-ballistic transport in nanoscale Si-MOSFETs based upon a quantum-corrected Monte Carlo device simulation to explore an ultimate device performance. It was found that, when a channel length becomes shorter than 30 nm, an average electron velocity at the source-end of the channel increases due to ballistic transport effects, and then, it approaches a ballistic limit in a sub-10-nm regime. Furthermore, the authors elucidated a physical mechanism creating an asymmetric momentum distribution function at the source-end of the channel and the influences of backscattering from the channel region. The authors also demonstrated that an electron injection velocity at a perfectly ballistic transport is independent of the channel length and corresponds well to a prediction from Natoris analytical model

Ferrite Planar Circuits in Microwave Integrated Circuits

The ferrite planar circuit to be discussed in this paper is a general planar circuit using ferrite substrates magnetized perpendicular to the ground conductors. The main subject of this paper is the analysis of an arbitrarily shaped triplate ferrite planar circuit. In particular, the circuit parameters of the equivalent multiport are determined. To analyze ferrite planar circuits in general, two approaches are possible. One approach is based upon a contour-integral solution of the wave equation. In the other approach the fields in the circuit are expanded in terms of orthonormal eigenfunctions. Examples of the application of such analyses are described.

Comparative Study on Drive Current of III–V Semiconductor, Ge and Si Channel n- mosfet s based on Quantum-Corrected Monte Carlo Simulation

Recently, a variety of new channel materials have been intensively studied to achieve a continuous enhancement in drive current of n-channel MOSFETs. In this paper, we performed a quantum-corrected Monte Carlo device simulation to examine advantages of new channel materials such as III-V compound semiconductors and Ge, by considering scattering effects, quantum mechanical effects, and new device structure. Then, we found that all materials converge to the similar current level as the channel length decreases, but Ge-MOSFET with (111) surface orientation and InP-MOSFET provide higher drive current than the other materials under the quasi-ballistic transport. Furthermore, we demonstrated that the reduction of parasitic resistance in source and drain regions will be indispensable to maintain a definite advantage of III-V materials.

Quantum transport simulation of ultrathin and ultrashort silicon-on-insulator metal-oxide-semiconductor field-effect transistors

The quantum transport properties of nanoscale silicon-on-insulator (SOI) metal-oxide-semiconductor field-effect transistors (MOSFETs) are investigated based on a quantum Monte Carlo (MC) device simulation. Quantum mechanical effects are incorporated in terms of a quantum correction of potential in well-developed particle MC computational techniques. The ellipsoidal multivalleys of the silicon conduction band are also considered in the simulation. First, the validity of the quantum MC technique is verified by comparing the simulated results with those calculated by a self-consistent Schrodinger–Poisson method at thermal equilibrium. Then, the nonequilibrium quantum transport characteristics of nanoscale SOI-MOSFETs are demonstrated. Furthermore, a quasi-ballistic behavior of ultrashort-channel devices is studied by evaluating the frequency of carrier scattering events in the channel region.

Finite-element analysis of quantum wires with arbitrary cross sections

A finite-element method is developed for the analysis of eigenstates in the valence band of quantum wires which have arbitrary potential profiles. Our method is basically based on the Galerkin procedure and triangle linear elements are used as finite elements. In our formulation the effect of the band mixing in the valence band is duly taken into account. Boundary conditions at heterointerfaces are also taken into account in the multiband envelope function space. Numerical examples are presented for circular, square, rectangular, and triangular quantum wire structures. The relation is clarified between the degeneracy in the E-ky dispersion curve and the symmetricity of the confinement potential.

Multiband quantum transport with Γ–X valley-mixing via evanescent states

Abstract Calculations of quantum transport of electrons through heterostructures are presented based on an empirical tight-binding model where an evanescent-wave matching at a heterointerface as well as the Γ – X valley-mixing effects are duly taken into account. Our results show, in particular, that current–voltage ( I – V ) characteristics of a GaAs/AlAs/GaAs single barrier diode have extra structures associated with the X valley tunneling, which can illustrate the experimental results. It should also be noted that the matching of evanescent electron modes is essentially necessary to include the valley-mixing effects for GaAs/AlAs heterostructures, since a lattice-translational symmetry is lacking in the growth direction in such structures.

Static and Dynamic Electron Transport in Resonant-Tunneling Diodes

The static and dynamic behavior of resonant-tunneling diodes is studied using the improved Wigner function model. In the study of static current-voltage characteristics, the exponential decrease of the peak current with the barrier thickness and the existence of a proper barrier thickness for which the peak-to-valley current ratio becomes the maximum are demonstrated. In the large signal simulation, the external current response to an abrupt bias switch is analyzed. As a result, it is found that the switching time of resonant-tunneling diodes is principally determined by the electron effective mass of the material system used rather than by such a device structure as the thickness of the double barrier and the doping density of the electrode.