Theory of resonant tunneling characteristics in semiconductor heterostructure electron devices

Abstract

In this thesis, the electron transport mechanisms and the current-voltage characteristics in one and two-dimensional structures are described. The resonant tunneling characteristics in double-barrier heterostructure electron devices axe investigated. Physical phenomena due to the reduced size of semiconductor devices are theoretically studied. Electron devices with channel dimensions of the ultra-submicron size or less exhibit many novel quantum effects when the wave length becomes comparable to the channel size. The investigations axe carried out for the III-V compound (GaAs/AlGaAs) material system. Particularly, we investigate a gated resonant tunneling diode (GRTD) as a future quantum device [1]. Numerical solutions to the two- dimensional Poisson equation and continuity equation have been used to calculate the lateral depletion region and carrier concentrations by the finite difference method. Depletion effects on current voltage characteristics are found by the simulation of the GRTD using a self-consistent quantum model. The resulting potentials and charge distributions are dominated by the space change region. It is required that the gate must be deposited close to double- barriers to control I-V curve effectively. We compare our results with calculations obtained for the case of weak lateral confinement (3D to 2D tunneling when the GRTD reaches two dimensional in the well region) and for the case of strong lateral confinement (in the bias conditions whereby the GRTD reaches zero-dimensional in the well region). In both cases, the conduction band structure is different depending on the position of x (lateral) direction. This axises from a lateral variation of the voltage drop across the double-barrier, which leads to an edge effect on the current-voltage characteristics. Using the Breit and Wigner formula in the case of weak lateral confinement, the elastic and inelastic scattering mechanism is studied. As scattering time t(i) is decreased, the current-voltage characteristics are dramatically broadened and the peak-to-valley ratio is reduced. This work also includes the investigation of negative transconductance in the GRTD. For strong lateral confinement (lD to OD tunneling), we examine the effects of elastic scattering by defects or impurities in the GRTD double-barrier structure. Theoretical calculations of tunneling characteristics of the gated resonant tunneling diode axe obtained in low dimensionality using a scattering transfer matrix approach. In the bias conditions whereby the GRTD reaches zero-dimension in the well region, we consider the attractive and repulsive perturbation potential (V.,) of impurity or defect scattering in emitter and well regions. The scattering matrices are described by using the presence of evanescent, or non-propagating, modes in different lateral confinement structures. Electron transport in a double- barrier structure is calculated by a self-consistent approach.