The basic requirements of an image detector are outlined and the advantages of the photoemissive cathode as a primary detector are set out. Several methods of image intensification using a photoemissive cathode as the primary detector are reviewed. In particular the advantages for use in astronomy of the thin end-window tube with a phosphor output in which the light image produced in the phosphor screen is recorded by contact-photography and the Lenard window electronographic image tube in which the electron image penetrates the end.-window to be recorded directly on an electron-sensitive emulsion are emphasized. The need to exclude caesium from the accelerating space and the advantages of an electron-optical system of uniform coaxial electric and magnetic fields are described. The theoretical performance of the thin window tube is estimated and the development of a successful design is described. Early tubes suffered from very high back-ground. A mechanism for the production of this background is postulated and a method for preventing it is proposed by using conducting annular baffles. This was found to be successful. The performance obtained agrees well with the theoretical predictions. Work was then started on a Lenard window electronographic image tube because this tube can have, in principle, a far superior performance to the conventional thin end— window tube. The theoretical performance of such a tube is estimated and its development from the thin window tube with a phosphor output is described. A special method for mounting the very thin mica window required for this tube is also described. Results obtained are found to agree reasonably well with the theoretical predictions. However the background is excessive and methods of reducing this are proposed. Finally the advantages of the above tubes in astronomy are emphasized and in particular the use of the Lenard window tube on a spectrograph.

The growing demand for submillimetre wave (SMW ) circuits is spurred not only by the needs of traditional science applications such as radio astronomy and atmospheric remote sensing but also by military and commercial applications such as compact range radar. Several approaches to device and circuit fabrication have been developed and in the past 20 years, many successful SM W components have been built for operation up to THz frequencies. However, they all suffer one or more drawback, such as low reliability and repeatability, lengthy and difficult assembly and reliance on mechanical tuners. The first part of the work described here has been the development of high yield, high quality and high repeatability antiparallel Schottky diodes entirely fabricated at the University of Bath. After producing Schottky diodes having state of the art characteristics, the main focus of the research was the development of a process to monolithically integrate the anti-parallel Schottky diode in a 200 GHz RF mixer circuit. Throughout this research, advantage has been taken of the recent development of advanced computer aided design tools that are suitable for simulating both linear and nonlinear parts of the submillimetre mixer to design and fabricated a fixed-tuned integrated mixer at 183 GHz.

Semiconductors have revolutionised the field of electronics due to the nature of their readily engineerable bandstructure. Additionally, the study of the 2D charge systems hosted by devices made from these materials has led to the discovery of fundamental physics of these systems. Silicon has been at the forefront of developments in this field owing to its natural abundance, ease of processing and naturally occurring oxide. In this thesis a double gated, metal oxide semiconductor field effect transistor (MOSFET) is used to investigate a number of transport properties of electrons and holes. These include the transport properties of a single layer within a bilayer system, the control of the hole g*m* with the MOSFET gates and probing of the effects of giant valley splitting on the electrons. Bilayer systems are composed of two single, physically separate, layers of charge carriers contained within the same quantum well. Their study has provided insight into carrier interactions between the layers with the potential for use in vertically coupled systems of charges in novel electronic devices. Here the effects of one layer on the I-V characteristics of the other are described. Direct control of the electron or hole spin via the gates of a transistor was first proposed by Datta and Das. Although such a device has yet to be fully realised, control of the g*m* has been demonstrated in a number of materials. Here the control of the hole g*m* is demonstrated on the (001) silicon plane for the first time. Giant valley splitting is an effect whereby the valley degeneracy of the electrons in silicon is lifted. In this thesis, the effects of giant valley splitting is shown to have a similar phenomenological effect on the resistivity of a 2D electron gas as spin splitting does on the same, valley degenerate system.

Endohedral functionalization via supercritical CO2 was undertaken in order to produce encapsulation of organometallic systems that are difficult to encapsulate otherwise due to either their large size or extreme air sensitivity. Organometallic molecular systems from the prophyrin and phthalocyanine families (such as NiPc, ClAlPc and NiTPP) were successfully encapsulated inside of nanotubes with relatively large diameters (centred around 2 nm). This was assessed by a combination of high resolution transmission electron microscopy (HRTEM) and Raman spectroscopy. HRTEM revealed previously unreported ordering of NiTPP, a large planar molecule, in row-like assemblies inside nanotubes of diameters that match best the geometrical size of the molecule (2 nm), highlighting the role of confinement in promoting assembly. Using both endohedral and exohedral functionalization with NiTPP, ClAlPc and NiPc molecules provided a set of systems differing by only one specific parameter (e.g. central ion or body type, or size of the HOMO-LUMO gap), or comparatively affected the ability to bind to the nanotubes - the associated changes in the electronic properties of the nanotubes were revealed by resonant Raman spectroscopy. These changes were interpreted in terms of ability of the guest molecular species to produce charge transfer to/from the nanotube, and/or induce structural strain.

III-Nitride semiconductor materials have been widely used for light emitting didoes (LEDs) for a wide range of lighting applications due to their excellent chemical stability and direct wide band gap energy nature. InGaN/GaN LEDs are conventionally grown on sapphire substrates with lateral contact device structure. However, LEDs fabricated with lateral contact device structure suffer from current crowding issue at the mesa boundary and contact edge when current flowing from the p-type GaN to n-type GaN, resulting in localized heating which reduces the internal quantum efficiency. In addition, sapphire substrates are relatively expensive and have a low thermal conductivity of 35 W/mK. Therefore, domestic LED lighting manufacturers have recently changed their interests to growing InGaN/GaN LEDs on Silicon (Si) substrates rather than sapphire substrates. Si substrates not only have higher thermal conductivity (i.e. 149 W/mK), but are also cheaper than sapphire substrates. However, Si substrates are light absorbing; if LEDs fabricated with conventional lateral contact structure the emitted light, which travels toward the Si growth substrate, is absorbed and wasted. To resolve this issue, the vertical LED (VLED) structure is adopted for InGaN/GaN LED grown on Si substrates. The original Si growth substrate is removed and the InGaN/GaN LED epitaxy is bonded on to a Si carrier substrate with a reflectivity p-type ohmic contact. In this thesis, a complete fabrication process of InGaN/GaN grown on Si VLEDs with a novel reflective p-contact was developed and characterized. A reflective p-type ohmic contact of Ni/Ag/Ni was developed to act as a mirror, which prevents the light absorption by the Si carrier substrate. This contact has a high reflectivity of 75% and a low contact resistivity of 6.3x10-5 Ωcm2 after annealing in an oxygen environment for 2 minutes at 450 ℃ to form an ohmic contact to p-GaN. The light output power of our VLED using Ni/Ag/Ni contact is approximately 20 mW higher than the VLEDs without any reflectors fabricated by Xiong et. al. Although the forward voltage of our VLEDs is 0.5 V higher than VLEDs made by Xiong et. al., this can be attributed to our Si carrier substrates do not have any thickens reduction process. The impact of the thickness of Si carrier substrate on the electrical and optical performance of the VLED was also characterized. By using a Si carrier substrate with a thickness of 475 μm rather than 675 μm, the quantum confined stark effect was reduced and the external quantum efficiency was improved by 4 %. Also, the operational voltage was reduced to 5.7 V from 6.7 V at the drive current of 300 mA. Finally, a KOH surface roughen process for the N face n-GaN was devised to resolve the total internal reflection issue at the interface between the n-GaN and air. The light output power of the VLED increased from 53 mW to 97 mW after etching in 4M KOH solution at 100 ℃ for 3 minutes.

Single crystals of the ternary chalcopyrite semiconductors, ZnGeP2 and CdGeP2, space group I42d, were grown by slow cooling in a 20 mol% lead solution. Room temperature lattice parameters were determined for both compounds, and the variations of the lattice parameters with temperature up to the melting point of ZnGeP2 were studied. Hall effect and resistivity measurements showed ZnGeP2 to be p-type while CdGeP2 changed from p-type to n-type conduction at 11°C. Polarised infrared reflectivity measurements were made in the range 40 to 700 cm-1. The spectra were analysed using both classical dispersion theory and Kramers-Kronig integration, yielding four modes in ZnGeP2 and five in CdGeP2 with E irreducible representation and two modes in both materials with B2 irreducible representation. Raman measurements in the backscattering configuration using a Krypton laser determined all thirteen group theoretically predicted modes in CdGeP2 while two were unobserved in ZnGeP2. These modes were interpreted in terms of the atomic motions of the III-V zinc blende analogues. The fundamental absorption edges of the two compounds were studied at room and liquid nitrogen temperatures, and the lowest direct transitions were determined at room temperature by electroreflectance. The polarisation modulation technique was also used to study the higher energy structure at 5K and 300K. It was deduced that CdGeP2 has a direct gap at 1.7eV, while ZnGeP2 is either indirect or "pseudo-direct" at about 2.0eV.

Wide-band gap semiconducting oxides combine high electrical conductivity with optical transparency and find important applications as transparent contacts in solar cells, energy-efficient smart windows and thin film transistors used in flat-panel displays for smartphones and tablets. Among the material family of transparent conducting oxides, In2O3, SnO2, ZnO and CdO are the most prominent examples which all exhibit excellent n-type conductivity. Transition metal oxides such as Cu2O, CuAlO2 and NiO were investigated for their p-type conductivity, which remains a great challenge to achieve in oxide semiconductors. This thesis presents an investigation of the electronic structure and transport properties of NixMg1-xO, a fully miscible solid solution of NiO and MgO which allows for band gap tuning in the deep ultraviolet spectral region. Both materials crystallize in the cubic rock-salt structure with nearly identical lattice parameters, but their alloys exhibit an unusually large, non-parabolic band gap bowing which has led to controversial discussions in the literature. The second part of this thesis focusses on the thin film crystal growth, the electron transport properties and doping in the ideal cubic perovskite BaSnO3, which recently attracted significant interest as high-mobility electron transport material for oxide electronics composed of earth-abundant elements. An effective solid phase epitaxy method for preparation of high-mobility La:BaSnO3 films is developed, employing the room temperature-deposited nanocrystalline films as precursor. Furthermore, the first experimental report of interstitial H-doping of BaSnO3 films is demonstrated, presenting a promising n-type doping alternative for achieving excellent conductivity. Finally, the origin of the exceptionally high room temperature mobility is investigated through an in-depth characterization of the electron effective mass and carrier scattering mechanisms in epitaxial La:BaSnO3 thin films.

Using a light beam modulated at frequencies up to 4 Mc/s with a Kerr cell, the phase shift method for measurement of carrier lifetime in semiconductors has been developed to detect photosignals as small as 10-8 V and to measure directly carrier lifetimes down to at least 10-8 sec.. The combination of high sensitivity and high lifetime resolution makes the technique a useful tool for investigating recombination processes in short lifetime materials such as indium antimonide, with which this work has been concerned principally. The validity of the technique has been checked by comparing preliminary lifetime measurements in germanium and silicon with those obtained by the photoconductivity decay method. The lifetime data obtained from 250°K to 84°K for p-type InSb samples are interpreted on the basis of a model consisting of two sets of recombination centres, each present in the same concentration and having the same properties in all specimens. The two sets are located at 0.07 eV and 0.11 eV from the valence band, and have capture coefficients for electrons considerable greater than those for holes, indicating a strong, positive charge on the centres. The same model explains the lifetime behaviour in n-type samples except where this is complicated by surface effects, The lifetime behaviour in a heat-treated sample, which has been thermally converted from n- to p-type, shows little difference from that in a p-type sample with about the same impurity concentration, suggesting that any flaws arising from heat treatment are far less effective for recombination than those centres already present before heat treatment. The findings that the recombination centres are similar in properties and in concentrations in all the specimens, and that furthermore they do not differ greatly from those investigated elsewhere, support the belief that they arise from fundamental structural defects.

Ambitions for transparent, lightweight, flexible and inexpensive electronic technologies that can be printed over large area substrates have driven substantial advances in the field of organic/printed electronics in recent years. Amongst the various technologies investigated, solution-processed, organic thin-film transistors (OTFTs) have received extraordinary attention, primarily due to the enormous potential for simple, cost-effective manufacturing. Two exciting research areas relevant to OTFT development that offer tremendous potential are those of the small molecule/polymer organic semiconducting blends and the science and engineering of molecular doping. However, the lack of organic semiconducting blends that surpass the benchmark charge carrier mobility of 10 cm2/Vs, and the numerous challenges associated with the practical utilisation of molecular doping, have prevented adaptation of OTFTs as a viable technology for application in the emerging sector of plastic electronics. The work in this thesis focuses on an organic semiconducting system for OTFTs that addresses these two points. The first part of this thesis describes the development of advanced organic semiconducting blends, the so-called 3rd generation (3G) blend systems. Specifically, a new blend based on the small-molecule C8-BTBT and the conjugated polymer C16DT-BT is introduced. A third component, the molecular p-dopant, C60F48, is then added to the blend system and it is found to have remarkably positive effects on OTFT performance. The ternary blend system is then combined with a solvent-mixing approach, resulting in devices with an exceptional hole mobility value exceeding 13 cm2/Vs. Through the use of complementary characterisation techniques, it is shown that key to this achievement is the unusual three-component material distribution, hinting at the existence of an unconventional doping mechanism. Furthermore, by considering alternative processing techniques, the maximum mobility of the resulting OTFTs is improved further to a value in excess of 23 cm2/Vs. The second part of the thesis focuses on the impact of p-doping in the ternary C8 BTBT:C16IDT BT:C60F48 blend on other important operating characteristics of the OTFTs. The intentional and simple to implement doping process is shown to improve key device parameters such as bias-stress stability, parasitic contact resistance, threshold voltage and the overall device-to-device parameter variation (i.e. narrowing of the parameter spread). Importantly, the inclusion of the dopant is not found to adversely affect the nature of the C8 BTBT crystal packing at the OTFT channel. The final part of this thesis describes the incorporation of 3G blend-based OTFTs into fully functional logic electronic circuits. Hybrid inverter circuits (i.e. NOT gates) are fabricated at low temperatures from solution-phase by combining the high hole mobility C8-BTBT:C16IDT-BT:C60F48 blend OTFTs as the p-channel device and a novel In2O3/ZnO heterojunction metal oxide semiconducting system as the n-channel transistor. The resulting complementary inverters exhibit excellent signal gain and high noise margins, making this hybrid circuitry a promising contender for application in the emerging field of printed microelectronics.

This thesis is concerned with the application of transistors to the process of frequency conversion, First, a transistor frequency mixer excited by an external "pump" or local oscillator is analyzed at frequencies ranging from much below to much above the transistor cut-off frequency, fa. At low frequencies the mixing non-linearity is assumed to be a pure variable conductance, but at high frequencies the effect of the non-linear storage and depletion capacitance is also taken into consideration. Diffusion and drift type transistors are considered separately at high frequencies, where a significant difference in operation of the two occurs. A theoretical analysis predicts useful conversion power gains (6 db or more) for the drift transistor up to frequencies of the order of 10 to 20 times f as at corresponding frequencies, a diffusion transistor mixer is shown to suffer a conversion power loss of 5 - 10 db, and therefore to have no advantage over a conventional diode mixer. Both of these theoretical predictions are verified by experimental measurement. The requirements of an oscillator supplying the "pump" signal to a frequency converter are discussed, and the design of frequency-stable transistor oscillators is developed from a study of the variation of output terminal susceptance with frequency. Generalized feedback elements for optimum frequency stability are derived, suitable for application in a wide variety of oscillator configurations. Finally, a study is made of a "self-excited" frequency converter, consisting of a transistor oscillator to which provisions are made for applying a high-frequency input signal and extracting the converted low-frequency output signal. It is shown that oscillation conditions for maximum conversion efficiency differ from the conditions for maximum frequency stability, and that in practice the latter should usually be chosen.