This thesis presents a study of distributed majority carrier effects in semiconductor device structures by simulation techniques. Some background material on the theory of finite-difference approximation, essential to simulation work, are given. High-frequency terminal measurements on a diffusion transistor are carried out, the range of frequencies being such that the majority carrier flow dominates the device performance. A passive RCL analogue of the base-region of a transistor is constructed in which parasitics such as lead-inductances are simulated. Measurements show a qualitative similarity of the frequency-variation of the input immittances of the analogue and of the real device. This demonstrates the nature and magnitude of the effect of parasitics on the determination of extrinsic elements. The procedure is then extended to the simulation of the frequency response of distributed RC structures of different shapes by a finite number of lumped components. The transient response of linear and nonlinear semiconductor RC structures is also studied. Results for linear networks are confirmed by measurements on real devices. Par the nonlinear networks, different types of PN junction capacitance have been considered. The effect of a nonlinear structure on the time parameters of pulse is investigated in order to determine conditions for maximum delay and minimum risetime. A theoretical possibility of delaying a pulse by the application of an external bias to a chain of metal-oxide-semiconductor transistors has also been explored. The concluding sections contain suggestions for further work.

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With all the key elements of quantum computing in ion traps demonstrated by the research community, the focus is now placed on building more sophisticated traps with larger numbers of ions to allow practical scale information processing. One promising avenue is to store ions in and shuttle them between many independent traps which serve as potential interaction sites.

This thesis investigates the dynamics of electrons ionized by strong low frequency laser fields, from a semiclassical perspective, developing a trajectory-based formalism to describe the interactions of the outgoing electron with the remaining ion. Trajectory models for photoionization generally arise in the regime known as optical tunnelling, where the atom is subjected to a strong, slow field, which tilts the potential landscape around the ion, forming a potential energy barrier that electrons can then tunnel through. There are multiple approaches that enable the description of the ionized electron, but they are generally limited or models derived by analogy, and the status of the trajectories is unclear. This thesis analyses this trajectory language in the context of the Analytical R-Matrix theory of photoionization, deriving a trajectory model from the fundamentals, and showing that this requires both the time and the position of the trajectory to be complex. I analyse this complex component of the position and I show that it requires careful handling: of the potentials where it appears, and of the paths in the complex plane that the trajectory is taken through. In this connection, I show that the Coulomb potential of the ion induces branch cuts in the complex time plane that the integration path needs to avoid, and I show how to navigate these branch cuts. I then use this formalism to uncover a kinematic mechanism for the recently discovered (Near-)Zero Energy Structures of above-threshold ionization. In addition, I analyse the generation of high-order harmonics of the driving laser that are emitted when the photoelectron recollides with the ion, using a pair of counter-rotating circularly polarized pulses to drive the emission, both in the context of the conservation of spin angular momentum and as a probe of the long-wavelength breakdown of the dipole approximation.

Pulsed terahertz time-domain systems (THz-TDS) offer many applications for spectroscopy and imaging. Typically terahertz generation is achieved by using a photoconductive antenna to generate an electric field (of about 10 kV cm-1) across a semiconductor. By creating such an electric field electro-migration occurs within the photoconductive antenna to eventually bridge the antenna electrodes. As a result photoconductive switches used for terahertz generation have a limited lifetime dependent on the voltage applied to them. This thesis investigates the lateral photo-Dember (LPD) effect as an alternative emitter that does not require an applied electric field. The photo-Dember effect relies on the difference in electron and hole mobility within semiconductors creating a current surge on photo-excitation. The lateral photo-Dember effect works by partially covering regions of the diffusion area to selectively suppresses the terahertz emission radiated by diffusion current. By selecting lateral currents the LPD emitters work in the same configuration as photoconductive antennas while only requiring a metallic boundary near photo-excitation. We investigate the mechanism of the photo-Dember effect and the suppression that causes the LPD effect. Both 1D and 2D models are demonstrated to for calculating diffusion currents within semiconductors and are used within finite element modelling to demonstrate dipole suppression. Optical fluence, beam position and polarisation are characterised within GaAs LPD emitters with SI-GaAs showing a competing generation mechanism from the Schottky barrier at high fluences. We find that the emitter dependence on optical polarisation is due to plasmonic enhancement that occurs on the metal boundary. We demonstrate a simple to fabricate multiplexed LPD emitter based on metals with different reflectivities within the terahertz regime that can be scaled over a large area and propose a design using plasmonic enhancement.

Low energy (7.0eV < ?w) < 21.2eV) photoelectron spectra are presented for the simple metals Ga, Tl, Pb and Bi in the liquid and solid (frozen) states. The results for the solid phase are consistent with other published data and are broadly similar to the results for the liquid phase. Little change is found in the spectra with increasing temperature for Ga and Bi above the melting point. The three step model of photoemission described by Bergland and Spicer is extended to allow for higher order inelastic scattering mechanisms. It is applied to the results for the liquid specimens and optical density of valence states functions are derived. More structure is observed than would be expected from a weak scattering description of these systems; in particular, the separation of the 6s and 6p derived bands in T1, Pb and Bi is clearly evident. The optical density of states function in these cases reflects the prominent features of the calculated density of states curves for the solid crystalline phases. This is attributed to the strength of the scattering potential and the relatively small change in the local ionic arrangement that takes place on melting. By fitting the absolute photoelectron yields with a Monte Carlo simulation of the photoemission process, the elastic electron-ion mean free path has been determined. In all cases it is found to be much larger than the value obtained from transport measurements. The apparent inconsistency between the solid state characteristics of the optical density of states functions and the free electron behavior observed in optical measurements is attributed to an increase in the Drude-like absorption and the influence of the f-sum rule. The calculated interband contribution to the optical conductivity, although much reduced, is similar to that observed for the solid state.

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The invention and development of Radiofrequency Endometrial Ablation (RaFEA) has been, by turns, exciting, frustrating and anxiety - provoking. Although one of the major motives for developing an alternative means of effective endometrial ablation to the hysteroscopically directed modalities was to improve safety, it seems that in using RaFEA, one set of potential dangers may have been exchanged for another set. Whilst the potentially fatal risks of uterine penetration and fluid toxicity are not encountered with RaFEA, the charging of the patient with an electric field for the duration of therapy brings its own risks, requiring very special precautions of their own (see section 2 - safety). At the time of writing, the future role of the technique is still being decided. It may be that the technique requires such specialist monitoring that it is unsuitable for general use, and may be restricted to one or two specialist centres for the treatment of certain patients who cannot be treated easily any other way. What is certain is that safety is of paramount importance, and the adequate training of those concerned and a basic knowledge of RF physics are both essential to safe practice. Practiced safely, the technique is highly successful, and has proved of considerable benefit to hundreds of patients. However, there have been a number of serious complications in other centres, each of which has been analysed in very great detail. These are considered in section 2 - safety.