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Germanium space-charge regions have been studied under conditions of double-depletion achieved by applying a reverse bias to both junctions of a transistor structure. The generation and distribution of carriers between the two junctions are described in terms of models for the thermal generation of carrier pairs and for potential distribution in a cylindrical system. Measurements over a wide range of temperature reveal that generation of carriers occurs through the medium of one or more sets of recombination centers and not by direct transition between the valence and conduction bands even at elevated temperatures. The Shockley-Read recombination-generation theory is applied to obtain the activation energies associated with the recombination centers. The impurity density, base width and junction areas are estimated from measurements of punchthrough voltage and junction capacitance. For some specimens the capacitance measurements made with one junction floating confirm the sharp increase in capacitance at punchthrough noted by Barker. The distribution of current between the junctions when both are equally reverse biased is found to be roughly proportional to their areas. It is also shown that control of the reverse current across one junction may be achieved by variation of the reverse-bias on the other junction. The mechanism of this interaction is considered in terms of the diffusion of carriers between the two space-charge regions. The phenomen of slightly non-saturating reverse current is explained in terms of the expansion with reverse bias of the space-charge region within the base. An expression relating the base current and reverse bias in a cylindrical system shown to give good agreement with experiment for one specimen. For the units in which the increase in current with voltage is appreciable, space-charge expansion cannot account for the increase and two other mechanisms are considered: avalanche multiplication in the bulk and along the surface, and a high generation rate on the surface. The surface conduction channels on the base region are investigated in two ways. Maximum floating junction potentials are calculated from measured values of the common emitter amplification factor (using the Shockley 1949 theory) and these are compared with directly measured potentials. A second method involves direct measurement of a.c. surface conductance between collector and emitter when both junctions are reverse-biased to prevent bulk conduction. Both tests reveal very small degrees of surface conductance on all specimens. / Science, Faculty of / Physics and Astronomy, Department of / Graduate

An automated ellipsometer was used to study three topics associated with the anodic oxide films of tantalum and niobium. The electro-optic effect was measured on tantalum and niobium oxides and was found to be quadratic. The change in refractive index upon application of a field occurred in two phases: an instantaneous change followed by a slower change. The effects on Nb₂O₅ were greater than on Ta₂O₅. The effect of ultra violet light on the two oxides was found to cause a change in the refractive index before appreciable photo-induced growth occurred. The results indicated that it was unlikely that the u.v.-induced change in refractive index occurred uniformly throughout the film. The effects of annealing and temperature are discussed in relation to the constant field current transient. The change in the refractive index during the transient was monitored with the ellipsometer. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

The present thesis is concerned mainly with space charge and high field effects on the electrical properties of thin amorphous films. A theory of space charge contribution to the polarization current in thin dielectric films is proposed. The transient current on short-circuiting a thin dielectric film is believed to consist of two components, one due to the dielectric polarization and the other due to trapped space charge. The space charge contribution is investigated using a model for a film containing distributed traps. Computed results seem to be consistent with experimental results on Ta/Ta₂O₅/Au diodes, so that space charge effects are more important at low preapplied fields. The applicability of step response techniques to determine low frequency dielectric losses is discussed and the effect of space charge on the dielectric losses is analysed. The theory of thermoluminescence and thermally stimulated currents is extended to the case of traps with distributed binding energies to investigate the possibility of distinguishing between distributed and discrete trap levels. It seems possible to distinguish experimentally between distributed and discrete traps by using different doses of optical radiation to obtain initially different amounts of trapped charges, and by varying the frequency of optical excitation over a suitable frequency range to allow only certain energy levels to be occupied by excited electrons. High field electronic conduction through very thin films sandwiched between two metal electrodes is analysed. In view of the fast tunneling time of electrons through very thin films, MIM structures can be used for microwave detection. It is shown that the maximum responsivity-bandwidth product of such detectors is obtained when they are biased at a voltage equal to the anode work function (in volts), and that the presence of invariant positive space charge increases the magnitude of this maximum. In considering high field switching in thin films of semiconducting glasses, it is suggested that Joule heating, which could account for the delay times observed experimentally, serves only to initiate an electronic switching mechanism. A model for current-controlled negative resistance due to space charge formation is proposed and its dc characteristics are computed. Carrier injection from the electrodes is taken to occur either by Schottky thermionic emission or a Fowler-Nordheim tunneling mechanism. The injected carriers develop space charge regions near the electrodes by impact ionization. The position dependent generation-recombination rate is discussed. The small ac signal equivalent circuit of the model is given. The formation of current filaments is analysed. Memory devices are discussed in terms of filament formation and phase change mechanisms due to excessive heating. Filamentary breakdown has been observed in anodic films grown on Ta, Al, Nb and Ti. A detailed experimental study of film growth and the effects of growth conditions, film thickness, counterelectrodes and temperature on breakdown strength has been carried out. A possible mode of breakdown, in which breakdown can result from thermal effects following a non-destructive electron avalanche, is proposed and its limitations are pointed out. It is concluded that breakdown in thin anodic films would occur due to disruption of the chemical bonds as the applied field approaches the formation field. The product of the molecular dissociation and the presence of energetic electrons could start an accumulative process which might end with the formation of a highly conducting channel. The injected electrons, field distortion and thermal runaway could assist in the channel development. Once the channel is developed, the sample's stored energy starts to dissipate through the channel. The voltage collapse has been found experimentally to occur in a time of less than 200 nanoseconds. / Applied Science, Faculty of / Electrical and Computer Engineering, Department of / Graduate

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