Geoff Dunn

Attribution of Campylobacter Infections in Northeast Scotland to Specific Sources by Use of Multilocus Sequence Typing

We show that a higher incidence of campylobacteriosis is found in young children (age, <5 years) living in rural, compared with urban, areas. Association of this difference with particular animal sources was evaluated using multilocus sequence typing. This evaluation was achieved by comparing Campylobacter isolates originating from these children, retail poultry, and a range of animal sources by use of source attribution and phylogenetic analysis methods. The results indicate that chicken is a major source of infection in young urban children, although not in their rural counterparts, for which ruminant and other avian sources are more important.

Does age acquired immunity confer selective protection to common serotypes of Campylobacter jejuni

BackgroundCampylobacter infection is a major cause of bacterial gastrointestinal disease. Exposure to Campylobacter is known to produce an immune response in humans that can prevent future symptomatic infections. Further, studies of the general population have shown that seroprevalence to Campylobacter increases with age.MethodsA large collection of serotyped Campylobacter isolates, obtained from human clinical faecal samples, were analysed by comparing the ratio of uncommon to common serotypes by different age groups, using χ2 tests.ResultsWe have identified that older age groups, as well as having generally lower incidence, are significantly less likely to be infected by the more common serotypes.ConclusionThese results are indicative of acquired immunity, however, further studies are needed to rule out the confounding effects of the variations in exposure pathways experienced by different age groups.

Terahertz oscillations in an In0.53Ga0.47As submicron planar Gunn diode

The length of the transit region of a Gunn diode determines the natural frequency at which it operates in fundamental mode—the shorter the device, the higher the frequency of operation. The long-held view on Gunn diode design is that for a functioning device the minimum length of the transit region is about 1.5 μm, limiting the devices to fundamental mode operation at frequencies of roughly 60 GHz. Study of these devices by more advanced Monte Carlo techniques that simulate the ballistic transport and electron-phonon interactions that govern device behaviour, offers a new lower bound of 0.5u2009μm, which is already being approached by the experimental evidence that has shown planar and vertical devices exhibiting Gunn operation at 600u2009nm and 700u2009nm, respectively. The paper presents results of the first ever THz submicron planar Gunn diode fabricated in In0.53Ga0.47As on an InP substrate, operating at a fundamental frequency above 300u2009GHz. Experimentally measured rf power of 28u2009μW was obtained from a 600u2009nm lo...

Reduction of Impact Ionization in GaAs-Based Planar Gunn Diodes by Anode Contact Design

Impact ionization in GaAs-based planar Gunn diodes is studied through electroluminescence (EL) analysis with the aim of reducing its magnitude by means of contact design and shaping, and thus enhance device performance and reliability. Designs in which the diode ohmic anode has an overhanging Schottky extension (composite anode contact) are shown to result in a significantly reduced amount of impact ionization, as compared with a simple ohmic contact design. The EL results are consistent with Monte Carlo simulations, which show a reduced impact ionization in composite anode contact devices due to a reduced electron density beneath the anode Schottky extension that, on the one hand, weakens the Gunn domain electric field and softens its variations near the anode edge, and, on the other hand, reduces the number of electrons capable of generating holes by impact ionization. A comparison between standard and composite anode contact approaches in terms of radio-frequency operation of the devices is made showing oscillations up to 109 GHz with an output power of -5 dBm in devices featuring the composite anode contact and no oscillations from all-ohmic contact devices. The findings reported in this paper may be useful not only for the design and the fabrication of planar Gunn diodes but also for other devices such as high-electron-mobility transistors where impact ionization can result in reliability limitations.

Impact ionisation electroluminescence in planar GaAs-based heterostructure gunn diodes: spatial distribution and impact of doping non-uniformities

When biased in the negative differential resistance regime, electroluminescence (EL) is emitted from planar GaAs heterostructure Gunn diodes. This EL is due to the recombination of electrons in the device channel with holes that are generated by impact ionisation when the Gunn domains reach the anode edge. The EL forms non-uniform patterns whose intensity shows short-range intensity variations in the direction parallel to the contacts and decreases along the device channel towards the cathode. This paper employs Monte Carlo models, in conjunction with the experimental data, to analyse these non-uniform EL patterns and to study the carrier dynamics responsible for them. It is found that the short-range lateral (i.e., parallel to the device contacts) EL patterns are probably due to non-uniformities in the doping of the anode contact, illustrating the usefulness of EL analysis on the detection of such inhomogeneities. The overall decreasing EL intensity towards the anode is also discussed in terms of the interaction of holes with the time-dependent electric field due to the transit of the Gunn domains. Due to their lower relative mobility and the low electric field outside of the Gunn domain, freshly generated holes remain close to the anode until the arrival of a new domain accelerates them towards the cathode. When the average over the transit of several Gunn domains is considered, this results in a higher hole density, and hence a higher EL intensity, next to the anode.

Improvements in thermionic cooling through engineering of the heterostructure interface using Monte Carlo simulations

A self-consistent Ensemble Monte Carlo (EMC) model was developed to simulate the thermionic effect in heterostructure barrier coolers. The model was validated on an InGaAs-InGaAsP heterostructure device of variable barrier height and width, producing good quantitative agreement with previous literature results. The operation of the cooler was found to be a complex and intricate process depending on the field, conduction band and details of barrier structure. When applied to a GaAs-AlGaAs micro-cooler there was good agreement with the experimental results. Importantly, very small alterations in the barrier structure were found to lead to considerable changes in device performance.

Micro-coolers fabricated as a component in an integrated circuit

The packing density and power capacity of integrated electronics is increasing resulting in higher thermal flux densities. Improved thermal management techniques are required and one approach is to include thermoelectric coolers as part of the integrated circuit. An analysis will be described showing that the supporting substrate will have a large influence on the cooling capacity of the thermoelectric cooler. In particular, for materials with a low ZT figure of merit (for example gallium arsenide (GaAs) based compounds) the substrate will have to be substantially thinned to obtain cooling, which may preclude the use of thermoelectric coolers, for example, as part of a GaAs based integrated circuit. Further, using experimental techniques to measure only the small positive cooling temperature difference (ΔT) between the anode (Th) and the cathode (Tc) contacts can be misinterpreted as cooling when in fact it is heating.

Micro-cooler enhancements by barrier interface analysis

A novel gallium arsenide (GaAs) based micro-cooler design, previously analysed both experimentally and by an analytical Heat Transfer (HT) model, has been simulated using a self-consistent Ensemble Monte Carlo (EMC) model for a more in depth analysis of the thermionic cooling in the device. The best fit to the experimental data was found and was used in conjunction with the HT model to estimate the cooler-contact resistance. The cooling results from EMC indicated that the cooling power of the device is highly dependent on the charge distribution across the leading interface. Alteration of this charge distribution via interface extensions on the nanometre scale has shown to produce significant changes in cooler performance.