Metal-Insulator-Metal (MIM), Metal-Insulator-Insulator-Metal (MIIM), and Metal-Insulator-Insulator-Insulator-Metal (MIIIM) quantum tunneling diodes have been designed, fabricated, and characterized. The key interest of this work was to develop tunneling diodes capable of operating and detecting THz radiation up to 30THz, which is well beyond the operation ranges of other semiconductor-based diodes. Al2O3, HfO2 and TiO2 metal oxides were employed for studying the behavior of metal-insulator-metal (MIM) and metal-insulator-insulator-metal (MIIM) quantum tunneling diodes. Specifically, ultra-thin films of these oxides with varied thicknesses were deposited by atomic layer deposition (ALD) as the tunneling junction material that is sandwiched between platinum (Pt) and titanium (Ti) electrodes, with dissimilar work functions of 5.3 eV and 4.1 eV, respectively. Due to the unique and well-controlled tunneling characteristic of the ALD ultra-thin films, reproducible MIM and MIIM diode devices have been developed. The DC characteristics of MIM and MIIM tunneling junctions with different junction areas and materials were investigated in this work. The effects of the different compositions and thicknesses of the tunneling layer on the diodes were studied systematically. Through the introduction of stacked dual tunneling layers, it is demonstrated that the MIIM and MIIIM diodes exhibited a high degree of asymmetry (large ratio between forward and reverse currents) and a strong nonlinearity in their I-V characteristics. The characterization was performed on diodes with micro and nano-scale junction areas. The MIM diodes reported herein exhibited lower junction resistances than those reported by prior works. Moreover, a study was conducted to numerically extract the average barrier heights by fitting the analytical model of the tunneling current to the measured I-V responses, which were evaluated with respect to the thickness of the constituent tunneling layer. RF characterization was performed on the MIM diodes up to 65GHz, and its junction impedance was extracted. A rigorous procedure was followed to extract the diode equivalent circuit model to obtain the intrinsic lumped element model parameters of the MIM diodes.

The foot-and-mouth disease (FMD) virus poses a considerable threat both to farmers and to the wider economy should there be a future incursion into the UK. The most recent large-scale FMD epidemic in the UK was in 2001. Mathematical models were developed and used during this epidemic to aid decision-making about how to most effectively control and eliminate it. While the epidemic was eventually brought to a halt, it resulted in a huge loss of livestock and is estimated to have cost the UK economy around ¿6 billion. The mathematical models predicted the overall spatial spread of FMD well, but had low predictive ability for identifying precisely which farm premises became infected over the course of the epidemic. This will in part have been due to the stochastic nature of the models. However, the transmission probability between two farm premises was represented as the Euclidean distance between their point locations, which is a crude representation of FMD transmission. Additionally, the premises' point location data contain inaccuracies, sometimes identifying the farmer's residential address rather than the farm itself which may be a long way away. Local FMD transmission occurs via contaminated fomites carried by people or vehicles between premises, or by infected particles being blown by wind between proximal fields. Given that these transmission mechanisms are thought to be related to having close field boundaries, it is possible that some of the inaccuracy in model predictions is also due to imprecisely representing such transmission. In this thesis I use fine-scale geographical data of farm premises' field locations to study the contiguity of premises (where contiguous premises (CPs) are defined as having field boundaries < 15m apart). I demonstrate that the distance between two premises' point locations does not accurately represent when they are CPs. Using an area of southern Scotland containing 4767 livestock premises, I compare the predictions of model simulations using two different model formulations. The first is one of the original models based on the 2001 outbreak, and the second is a new model in which transmission probability is related to whether or not premises were contiguous. The comparison suggests that the premises that became infected during the course of the simulations were more predictable using the new model. While it cannot be concluded that this will translate into more accurate predictions until this can be validated during a future outbreak, it does suggest that the new model is more predictable in its route through the landscape, and therefore that it may better reflect local transmission routes than the original model. Networks based on contiguity of premises were constructed for the same area of southern Scotland, and showed that 90.6% (n=4318) of the premises in the area were indirectly connected to one another as part of the Giant Component (GC). The network metric of 'betweenness' was used to identify premises acting as bridges between otherwise disconnected sub-populations of premises. It was found that removing 100 premises with highest betweenness served to fragment the GC. Model simulations indicated that, even with some longer-range transmission possible, removing these premises from the network resulted in a large decrease in mean number of infected premises and outbreak duration. In real terms, premises removal from the network would mean ensuring these premises did not become infected by enhanced biosecurity and/or vaccination depending on policy. In this thesis I also considered the role of biosecurity practices in shaping FMD spread. A sample of 200 Scottish farmers were interviewed on their biosecurity practices, and their biosecurity risk quantified using a biosecurity 'risk score' developed during the 2007 FMD outbreak in Surrey. Using Moran's I and network assortativity measures it was found that there did not appear to be any clustering of biosecurity risk scores on premises. Statistical analysis found no association between biosecurity risk and the mathematical model's premises' susceptibility term (which describes the increase in a premises' susceptibility with increasing numbers of livestock). This suggests that the model's susceptibility term is not indirectly capturing a general pattern in biosecurity on different sized farm premises. Thus, this body of work shows that incorporating a more realistic representation of premises location into mathematical models, in terms of area (i.e. as fields) rather than a point, alters predictions of spatial spread. It also demonstrates that targeted control at a relatively small number of farms could effectively fragment the farming landscape, and has the potential to considerably reduce the size of an FMD outbreak. It also demonstrates that variations in premises' FMD biosecurity risks are unlikely to be indirectly affecting the spatial or demographic components of the model. This increase in understanding of how geographic, social and demographic factors relate to FMD spread through the landscape may enable more effective control of an outbreak, should there be an incursion in the UK in future.

When a nanostructure is coupled between two leads, the electron transmission probability as a function of energy, E, is used in the Landauer formula to obtain the electrical conductance of the nanodevice. The electron transmission probability as a function of energy, T (E), is calculated from the appropriate solution of the time independent Schrödinger equation. Recently, a large class of nanostructures called quantum dragons has been discovered. Quantum dragons are nanodevices with correlated disorder but still can have electron transmission probability unity for all energies when connected to appropriate (idealized) leads. Hence for a single channel setup, the electrical conductivity is quantized. Thus quantum dragons have the minimum electrical conductance allowed by quantum mechanics. These quantum dragons have potential applications in nanoelectronics. It is shown that for dimerized leads coupled to a simple two-slice (l = 2, m = 1) device, the matrix method gives the same expression for the electron transmission probability as renormalization group methods and as the well known Green's function method. If a nanodevice has m atoms per slice, with l slices to calculate the electron transmission probability as a function of energy via the matrix method requires the solution of the inverse of a (2 + ml) (2 + ml) matrix. This matrix to invert is of large dimensions for large m and l. Taking the inverse of such a matrix could be done numerically, but getting an exact solution may not be possible. By using the mapping technique, this reduces this large matrix to invert into a simple (l + 2) (l + 2) matrix to invert, which is easier to handle but has the same solution. By using the map-and-tune approach, quantum dragon solutions are shown to exist for single-layer planar rectangular crystals with different boundary conditions. Each chapter provides two different ways on how to find quantum dragons. This work has experimental relevance, since this could pave the way for planar rectangular nanodevices with zero electrical resistance to be found. In the presence of randomness of the single-band tight-binding parameters in the nanodevice, an interesting quantum mechanical phenomenon called Fano resonance of the electron transmission probability is shown to be observed.

The support of multimedia traffic over IEEE 802.11 wireless local area networks (WLANs) has recently received considerable attention. This dissertation has proposed a new framework that provides efficient channel access, service differentiation and statistical QoS guarantees in the enhanced distributed channel access (EDCA) protocol of IEEE 802.11e. In the first part of the dissertation, the new framework to provide QoS support in IEEE 802.11e is presented. The framework uses three independent components, namely, a core MAC layer, a scheduler, and an admission control. The core MAC layer concentrates on the channel access mechanism to improve the overall system efficiency. The scheduler provides service differentiation according to the weights assigned to each Access Category (AC). The admission control provides statistical QoS guarantees. The core MAC layer developed in this dissertation employs a P-Persistent based MAC protocol. A weight-based fair scheduler to obtain throughput service differentiation at each node has been used. In wireless LANs (WLANs), the MAC protocol is the main element that determines the efficiency of sharing the limited communication bandwidth of the wireless channel. In the second part of the dissertation, analytical Markov chain models for the P-Persistent 802.11 MAC protocol under unsaturated load conditions with heterogeneous loads are developed. The Markov models provide closed-form formulas for calculating the packet service time, the packet end-to-end delay, and the channel capacity in the unsaturated load conditions. The accuracy of the models has been validated by extensive NS2 simulation tests and the models are shown to give accurate results. In the final part of the dissertation, the admission control mechanism is developed and evaluated. The analytical model for P-Persistent 802.11 is used to develop a measurement-assisted model-based admission control. The proposed admission control mechanism uses delay as an admission criterion. Both distributed and centralized admission control schemes are developed and the performance results show that both schemes perform very efficiently in providing the QoS guarantees. Since the distributed admission scheme control does not have a complete state information of the WLAN, its performance is generally inferior to the centralized admission control scheme. The detailed performance results using the NS2 simulator have demonstrated the effectiveness of the proposed framework. Compared to 802.11e EDCA, the scheduler consistently achieved the desired throughput differentiation and easy tuning. The core MAC layer achieved better delays in terms of channel access, average packet service time and end-to-end delay. It also achieved higher system throughput than EDCA for any given service differentiation ratio. The admission control provided the desired statistical QoS guarantees.