Optimized V-shape design of GaN nanodiodes for the generation of Gunn oscillations

In this work, recent advances in the design of GaN planar Gunn diodes with asymmetric shape, socalled self-switching diodes, are presented. A particular geometry for the nanodiode is proposed, referred as V-shape, where the width of the channel is intentionally increased as approaching the anode. This design, which reduces the effect of the surface-charges at the anode side, is the most favourable one for the onset of Gunn oscillations, which emerge at lower current levels and with lower threshold voltages as compared to the standard square geometry, thus enhancing the power efficiency of the self-switching diode as sub-millimeter wave emitters

Self-consistent electro-thermal simulations of AlGaN/GaN diodes by means of Monte Carlo method

In this contribution we present the results from the simulation of an AlGaN/GaN heterostructure diode by means of a Monte Carlo tool where thermal effects have been included. Two techniques are investigated: (i) a thermal resistance method (TRM), and (ii) an advanced electro-thermal model (ETM) including the solution of the steady-state heat diffusion equation. Initially, a systematic study at constant temperature is performed in order to calibrate the electronic model. Once this task is performed, the electro-thermal methods are coupled with the Monte Carlo electronic simulations. For the TRM, several values of thermal resistances are employed, and for the ETM method, the dependence on the thermal-conductivity, thickness and die length is analyzed. It is found that the TRM with well-calibrated values of thermal resistances provides a similar behavior to ETM simulations under the hypothesis of constant thermal conductivity. Our results are validated with experimental measurements finding the best agreement when the ETM is used with a temperature-dependent thermal conductivity

Searching for THz Gunn oscillations in GaN planar nanodiodes

A detailed study of GaN-based planar asymmetric nanodiodes, promising devices for the fabrication of room temperature THz Gunn oscillators, is reported. By using Monte Carlo simulations, an analysis of the static I-V curves and the time-domain evolution of the current obtained when varying some simulation parameters in the diodes has been made. Oscillation frequencies of hundreds of GHz are predicted by the simulations in diodes with micrometric channel lengths. Following simulation guidelines, a first batch of diodes was fabricated. It was found that surface charge depletion effects are stronger than expected and inhibit the onset of the oscillations. Indeed, a simple standard constant surface charge model is not able to reproduce experimental measurements and a self-consistent model must be included in the simulations. Using a self-consistent model, it was found that to achieve oscillations, wider channels and improved geometries are necessary.ROOTHz (FP7-243845

Operation of GaN planar nanodiodes as THz detectors and mixers

In this paper, we perform, by means of Monte Carlo simulations and experimental measurements, a geometry optimization of GaN-based nano-diodes for broadband Terahertz direct detection (in terms of responsivity) and mixing (in terms of output power). The capabilities of the so-called self-switching diode (SSD) are analyzed for different dimensions of the channel at room temperature. Signal detection up to the 690 GHz limit of the experimental set-up has been achieved at zero bias. The reduction of the channel width increases the detection responsivity, while the reduction in length reduces the responsivity but increases the cut-off frequency. In the case of heterodyne detection an intrinsic bandwidth of at least 100 GHz has been found. The intermediate frequency (IF) power increases for short SSDs, while the optimization in terms of the channel width is a trade-off between a higher non-linearity (obtained for narrow SSDs) and a large current level (obtained for wide SSDs). Moreover, the RF performance can be improved by biasing, with optimum performances reached, as expected, when the DC non-linearity is maximum

Phenotypic and genotypic characterization of meningococcal carriage and disease isolates in Burkina Faso after mass vaccination with a serogroup a conjugate vaccine

BACKGROUND: The conjugate vaccine against serogroup A Neisseria meningitidis (NmA), MenAfriVac, was first introduced in mass vaccination campaigns of the 1-29-year-olds in Burkina Faso in 2010. The aim of this study was to genetically characterize meningococcal isolates circulating in Burkina Faso before and up to 13 months after MenAfriVac mass vaccination. METHODS: A total of 1,659 meningococcal carriage isolates were collected in a repeated cross-sectional carriage study of the 1-29-year-olds in three districts of Burkina Faso in 2010 and 2011, before and up to 13 months after mass vaccination. Forty-two invasive isolates were collected through the national surveillance in Burkina Faso in the same period. All the invasive isolates and 817 carriage isolates were characterized by serogroup, multilocus sequence typing and porA-fetA sequencing. RESULTS: Seven serogroup A isolates were identified, six in 2010, before vaccination (4 from carriers and 2 from patients), and one in 2011 from an unvaccinated patient; all were assigned to sequence type (ST)-2859 of the ST-5 clonal complex. No NmA carriage isolate and no ST-2859 isolate with another capsule were identified after vaccination. Serogroup X carriage and disease prevalence increased before vaccine introduction, due to the expansion of ST-181, which comprised 48.5% of all the characterized carriage isolates. The hypervirulent serogroup W ST-11 clone that was responsible for most of meningococcal disease in 2011 and 2012 was not observed in 2010; it appeared during the epidemic season of 2011, when it represented 40.6% of the serogroup W carriage isolates. CONCLUSIONS: Successive clonal waves of ST-181 and ST-11 may explain the changing epidemiology in Burkina Faso after the virtual disappearance of NmA disease and carriage. No ST-2859 strain of any serogroup was found after vaccination, suggesting that capsule switching of ST-2859 did not occur, at least not during the first 13 months after vaccination

Experimental demonstration of direct terahertz detection at room-temperature in AlGaN/GaN asymmetric nanochannels

The potentialities of AlGaN/GaN nanodevices as THz detectors are analyzed. Nanochannels with broken symmetry (so called Self Switching Diodes) have been fabricated for the first time in this material system using both recess-etching and ion implantation technologies. The responsivities of both types of devices have been measured and explained using Monte Carlo simulations and non linear analysis. Sensitivities up to 100 V/W is obtained at 0.3 THz with a 280 pW/sqrt(Hz) Noise Equivalent Power.ROOTHz (FP7-243845

Effect of Schistosoma mansoni Infection on Innate and HIV-1-Specific T-Cell Immune Responses in HIV-1-Infected Ugandan Fisher Folk.

In Uganda, fisher folk have HIV prevalence rates, about four times higher than the national average, and are often coinfected with Schistosoma mansoni. We hypothesized that innate immune responses and HIV-specific Th1 immune responses might be downmodulated in HIV/S. mansoni-coinfected individuals compared with HIV+/S. mansoni-negative individuals. We stimulated whole blood with innate receptor agonists and analyzed supernatant cytokines by Luminex. We evaluated HIV-specific responses by intracellular cytokine staining for IFN-γ, IL-2, and TNF-α. We found that the plasma viral load and CD4 count were similar between the HIV+SM+ and HIV+SM- individuals. In addition, the TNF-α response to the imidazoquinoline compound CL097 and β-1, 3-glucan (curdlan), was significantly higher in HIV/S. mansoni-coinfected individuals compared with HIV only-infected individuals. The frequency of HIV-specific IFN-γ+IL-2-TNF-α- CD8 T cells and IFN-γ+IL-2-TNF-α+ CD4 T cells was significantly higher in HIV/S. mansoni-coinfected individuals compared with HIV only-infected individuals. These findings do not support the hypothesis that S. mansoni downmodulates innate or HIV-specific Th1 responses in HIV/S. mansoni-coinfected individuals

The evolving SARS-CoV-2 epidemic in Africa: Insights from rapidly expanding genomic surveillance

INTRODUCTION Investment in Africa over the past year with regard to severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) sequencing has led to a massive increase in the number of sequences, which, to date, exceeds 100,000 sequences generated to track the pandemic on the continent. These sequences have profoundly affected how public health officials in Africa have navigated the COVID-19 pandemic. RATIONALE We demonstrate how the first 100,000 SARS-CoV-2 sequences from Africa have helped monitor the epidemic on the continent, how genomic surveillance expanded over the course of the pandemic, and how we adapted our sequencing methods to deal with an evolving virus. Finally, we also examine how viral lineages have spread across the continent in a phylogeographic framework to gain insights into the underlying temporal and spatial transmission dynamics for several variants of concern (VOCs). RESULTS Our results indicate that the number of countries in Africa that can sequence the virus within their own borders is growing and that this is coupled with a shorter turnaround time from the time of sampling to sequence submission. Ongoing evolution necessitated the continual updating of primer sets, and, as a result, eight primer sets were designed in tandem with viral evolution and used to ensure effective sequencing of the virus. The pandemic unfolded through multiple waves of infection that were each driven by distinct genetic lineages, with B.1-like ancestral strains associated with the first pandemic wave of infections in 2020. Successive waves on the continent were fueled by different VOCs, with Alpha and Beta cocirculating in distinct spatial patterns during the second wave and Delta and Omicron affecting the whole continent during the third and fourth waves, respectively. Phylogeographic reconstruction points toward distinct differences in viral importation and exportation patterns associated with the Alpha, Beta, Delta, and Omicron variants and subvariants, when considering both Africa versus the rest of the world and viral dissemination within the continent. Our epidemiological and phylogenetic inferences therefore underscore the heterogeneous nature of the pandemic on the continent and highlight key insights and challenges, for instance, recognizing the limitations of low testing proportions. We also highlight the early warning capacity that genomic surveillance in Africa has had for the rest of the world with the detection of new lineages and variants, the most recent being the characterization of various Omicron subvariants. CONCLUSION Sustained investment for diagnostics and genomic surveillance in Africa is needed as the virus continues to evolve. This is important not only to help combat SARS-CoV-2 on the continent but also because it can be used as a platform to help address the many emerging and reemerging infectious disease threats in Africa. In particular, capacity building for local sequencing within countries or within the continent should be prioritized because this is generally associated with shorter turnaround times, providing the most benefit to local public health authorities tasked with pandemic response and mitigation and allowing for the fastest reaction to localized outbreaks. These investments are crucial for pandemic preparedness and response and will serve the health of the continent well into the 21st century

Studies of Gallium Nitride-based Self Switching Diodes potentialities for Terahertz detection and emission

Le domaine des sciences et technologies Térahertz gagne un intérêt international en raison de ses nombreuses applications allant des systèmes de transmission sans fils ultra rapide au diagnostique médical, au contrôle de qualité et à la sécurité industrielle. Aujourd’hui, cet intérêt pousse la recherche en électronique à se focaliser sur la réduction des composants dans le but d’augmenter leur fréquence de fonctionnement. Les nanotechnologies sont donc au cœur de cette course à la montée en fréquence. La gamme des longueurs d'onde THz ouvre une nouvelle ère de systèmes directement liés à l'information et aux technologies de communication, étendant considérablement ceux déjà existant qui reposent sur des dispositifs optiques et électroniques. Les dispositifs THz à base de semi-conducteurs sont une des voies possibles pour la réalisation de composants à l’interface entre les micro-ondes et la phonique. L'application à grande échelle du domaine des THz pour l'astronomie, l'environnement, les communications, l'imagerie, la sécurité, la biologie et la médecine pourraient conduire à définir la gamme THz comme un champ d’application spécifique pour les chercheurs et les ingénieurs. Par exemple la spectroscopie moléculaire très importante pour l'astronomie (analyse des gaz interstellaires, observations planétaires), l'environnement (surveillance de la pollution), les télécommunications, les communications locales sécurisées (à travers une forte atténuation en dehors de la zone ciblée) à très haut débit de données sera à terme possible. L'imagerie également un élément important contribuant à la sécurité (détection d’armes et de matériel illicite, analyse non destructive et non invasive d’objets).L'un des obstacles pour le développement d’applications pratique dans la gamme du THz est le manque de sources continues, compactes, accordables et puissantes (à faible coût, si possible). Ainsi, ce travail s’inscrit dans le cadre du projet européen ROOTHz et nous proposons d'exploiter les oscillations type Gunn sur un nouveau type de nanodispositifs fabriqués pour la première fois sur du nitrure de Gallium : La diode autocommutante, ou Self switching diode (SSD). Sur la base de simulations Monte-Carlo, la géométrie particulière des SSD favorise l'apparition d'oscillations Gunn à des fréquences Térahertz, en utilisant une dyssymétrie d'un canal de type transistor assez étroit, la SSD peut fournir un comportement de redressement. Cet effet, basé sur les effets de surface et les effets électrostatiques, utilisé sur le GaN permet le fonctionnement comme détecteurs THz à température ambiante. Au cours de cette thèse, des études sur les SSD à base de nitrure de gallium ont été menées afin d'évaluer leurs potentialités comme émetteurs et détecteurs dans la gamme de fréquence du THz.The field of Terahertz Science and Technology is gaining international interest due to its numerous applications ranging from ultra high speed optical transmission systems to medical diagnosys, industrial quality control and security-screening tools. In this field, the efforts of electronics industry are centered on device scaling down to the nanometer range to increase the operational speed.The THz range is an intermediate range of wavelengths that will open a new area of systems directly related to information and communication technologies, significantly extending the present ones based on photonic and electronic devices. Thus, solid-state THz devices can be either considered as belonging to both fields or to none of them. Indeed the wide application area of THz for astronomy, environment, communications, imaging, security, biology and medicine could lead to define the THz range as a specific scientific, engineering and application field. Molecular spectroscopy is very important for astronomy (analysis of the interstellar gas, planetary observations), environment (pollution monitoring), etc. For telecommunications, secure local communications (through high attenuation outside the targeted area) with ultrahigh data rates will be possible. Imaging is an important application for security (weapon and illicit material detection), biology and medicine. The emergence of novel functional THz devices will be of immense interest for all those applications. One of the bottlenecks for the practical development of THz applications is the fabrication of room temperature (RT), continuous wave, compact, tunable and powerful sources (at low cost, if possible). For this sake, in the framework of the EU funded project ROOTHz, we propose to exploit THz Gunn oscillations in novel wide bandgap semiconductor nanodevices, which have been predicted by simulations but not experimentally confirmed yet, the Self Switching Diode (SSD). By breaking the symmetry of a narrow channel, SSD can provide a rectifying behaviour (based on surface and electrostatic effects) and using high-mobility material systems their operation frequency as detectors can approach the THz range at RT. Interestingly, the special geometry of SSDs also benefits the onset of Gunn oscillations.During this thesis, studies on Gallium nitride SSDs have been performed in order to evaluate their potentialities as emitters and detectors in the THz frequency range

Terahertz Detection in Zero-Bias InAs Self-Switching Diodes at Room Temperature

RF characterization of InAs self-switching diodes (SSDs) is reported. On-wafer measurements revealed no roll-off in responsivity in the range 2-315 GHz. At 50 GHz, a responsivity of 17 V/W and a noise-equivalent power (NEP) of 150 pW/sqrt(Hz) was observed for the SSD when driven by a 50 Ω source. With a conjugately matched source, a responsivity of 34 V/W and an NEP of 65 pW/sqrt(Hz) were estimated. An antenna-coupled SSD demonstrated a responsivity of 0.7 V/W at 600 GHz. The results demonstrate the feasibility of zero-bias terahertz detection with high-electron mobility InAs SSDs up to and beyond 100 GHzROOTHz (FP7-243845