Title

Modeling of mesoscopic structures and analysis of novel quantum interference transistors

Date of Completion

January 1998

Keywords

Engineering, Electronics and Electrical

Degree

Ph.D.

Abstract

Time-dependent analysis of the quantum mechanical wavefunction has been performed on a variety of quantum interference transistor (QUIT) structures, and it has been found that reflections at non-adiabatic or quasi-adiabatic junctions can severely effect the evolution between constructive and destructive interference. This multiple reflections effect is neglected in analytical models, but will be accounted for, in the present work, via a time-dependent propagator method that can be applied to an arbitrary device geometry. Furthermore, the analyzed structures will be assessed in terms of switching speed via Fourier Transformation of the Wavefunction response to an applied bias pulse. Simulations indicate that switching speeds of up to 15 THz, at 4.2K, can be achieved by some of the analyzed structures. ^ Constantly advancing semiconductor fabrication methods have made it possible to construct devices with dimensions so small (mesoscopic) that the quantum mechanical nature of the electron can no longer be neglected in transport calculations. These quantum effect devices include resonant tunneling devices (RTD's), single electron transistors (SET's), and quantum interference transistors (QUIT's), which are the focus of this work. QUIT's rely on the phase coherence of electrons, which undergo ballistic (reduced scattering) rather than ohmic transport, on these size scales. ^ Techniques, present in the literature, for modeling transport in the ballistic regime of semiconductors, include Green Functions, Wigner Functions, Transfer Matrix, and propagator methods. Most are time-independent and provide only steady-state analysis. Green and Wigner function approaches also tend to be cumbersome when non-idealized device geometries are examined. Although the Transfer Matrix technique can handle complex geometries, it cannot handle multiply connected regions. Time-independent propagator methods can handle multiply connected regions but are inaccurate for nonadiabatic junctions. Consequently, the method used in the present work is a time-dependent beam propagation method with monoenergetic transparent boundary conditions. It can accommodate both adiabatic and nonadiabatic junctions, multiply connected regions, and yield both transient and steady-state analysis. This technique generates the single electron wavefunction, in the effective mass approximation, for the x (lateral) and z (propagation) directions and time. Additionally, it is possible to incorporate a self-consistent calculation, at thermal equilibrium, of the y (transverse) direction, which would yield a quasi four-dimensional approach for modeling mesoscopic devices. ^