Analytical Formulas for Mean Gain and Excess Noise Factor in InAs Avalanche Photodiodes

It has been known that McIntyre\u27s local multiplication theory for avalanche photodiodes (APDs) does not fully explain the experimental results for single-carrier InAs APDs, which exhibit excess noise factor values below 2. While it has been established that the inclusion of the dead-space effect in the nonlocal multiplication theory resolves this discrepancy, no closed-form formulas for the mean gain and excess noise factor have been specialized to InAs APDs in a nonlocal setting. Upon utilizing prior analytical formulation of single-carrier avalanche multiplication based on age-dependent branching theory in conjunction with nonlocal ionization coefficients and thresholds for InAs, closed-form solutions of the mean gain and the excess noise factor for InAs APDs are provided here for the first time. The formulas are validated against published experimental data from InAs APDs across a variety of multiplication region widths and are shown to be applicable for devices with multiplication widths of 500 nm and larger

Serum anti mullerian hormone and renalase levels in predicting the risk of preeclampsia

Objective: The aim of the study was to explore the association of serum AMH and Renalase with the risk of preeclampsia thereby assessing them as screening tools, reducing the risk of gravid consequences of preeclampsia.Materials and Methods: This cross-sectional study recruited n = 95 pregnant women between 14 and 32 gestational weeks. They were categorized as a) women with gestational hypertension (n = 45); b) women with pre-eclampsia (n = 20) and c) normotensive pregnant women (n = 30) according to the ACOG criteria. Anthropometrics data and blood and urine samples were collected. AMH and Renalase levels were measured by ELISA assay.Results: The mean age of study cohort was 27.3 ± 6.2 year and weight was 65.1 ± 14.1 kg. Blood pressures were significantly higher in pre-eclamptic patients versus both the gestational hypertensive females and controls (p \u3c 0.05). AMH was found to be significantly higher in controls but no difference was observed between gestational hypertensive and pre-eclamptic patients. No difference was seen for serum Renalase among the three groups (p \u3e 0.05). AMH showed a negative weak correlation with diastolic blood pressure (r = -0.272; p = 0.008) that remained significant even after adjustment (r = -0.236; p = 0.023) whereas Renalase did not show any difference (r = -0.051; p \u3e 0.05). Females with low levels of AMH were 1.07 times at risk of developing hypertension even after adjustment for age and BMI (p \u3c 0.05).Conclusion: Low AMH levels may lead to hypertension in pregnancy suggesting a role in detecting vascular diseases as well as its effect on ovarian aging. However, further research is required to establish a causal relationship

On the analytical formulation of excess noise in avalanche photodiodes with dead space

Simple, approximate formulas are developed to calculate the mean gain and excess noise factor for avalanche photodiodes using the dead-space multiplication theory in the regime of small multiplication width and high applied electric field. The accuracy of the approximation is investigated by comparing it to the exact numerical method using recursive coupled integral equations and it is found that it works for dead spaces up to 15% of the multiplication width, which is substantial. The approximation is also tested for real materials such as GaAs, InP and Si for various multiplication widths, and the results found are accurate within ∼ 15% of the actual noise, which is a significant improvement over the local-theory noise formula. The results obtained for the mean gain also confirm the recently reported relationship between experimentally determined local ionization coefficients and the enabled non-local ionization coefficients

Demographic and Clinical Features of Dengue Fever in Pakistan from 2003–2007: A Retrospective Cross-Sectional Study

Background: Demographic features of dengue fever have changed tremendously in Pakistan over the past two decades. Small scale studies from all over the country have reported different aspects of individual outbreaks during this time. However, there is scarcity of data looking at the overall trend of dengue virus infection in the country. In this study, we examined annual trends, seasonality, and clinical features of dengue fever in the Pakistani population.Methods: Demographic information and dengue IgM status of all patients tested for dengue IgM antibody at Aga Khan University Hospital from January 2003 to December 2007 were analyzed to look for trends of IgM-positive cases in Pakistan. In addition, clinical and biochemical parameters were abstracted retrospectively from medical records of all patients hospitalized with IgM-proven dengue fever between January 2006 and December 2007. These patients were categorized into dengue fever and dengue hemorrhagic fever according to the WHO severity grading scale.Results: Out of a total of 15040 patients (63.2% male and 36.8% female), 3952 (26.3%) tested positive for dengue IgM antibody. 209 IgM proven dengue patients were hospitalized during the study period. During 2003, IgM positive cases were seen only during the months of July-December. In contrast, such cases were detected throughout the year from the 2004– 2007. The median age of IgM positive patients decreased every year from 32.0 years in 2003 to 24.0 years in 2007 (p,0.001). Among hospitalized patients, nausea was the most common presenting feature found in 124/209 (59.3%) patients. Children presented with a higher median body temperature than adults (p = 0.010). In addition, neutropenia was seen more commonly in children while raised serum ALT levels were seen more commonly in adults (both p = 0.006). While a low total white cell count was more common in patients with dengue fever as compared to Dengue Hemorrhagic Fever (p = 0.020), neutropenia (p = 0.019), monocytosis (p = 0.001) and raised serum ALT level (p = 0.005) were observed more commonly in the latter group.Conclusions: Dengue virus is now endemic in Pakistan, circulating throughout the year with a peak incidence in the post monsoon period. Median age of dengue patients has decreased and younger patients may be more susceptible. Total and differential leukocyte counts may help identify patients at risk of hemorrhage

Analytical modeling and characterization for avalanche photodiodes

For applications requiring the detection of low light levels, avalanche photodiodes (APDs) are the preferred detector of choice. The reason for this is their internal gain, which improves the APDs sensitivity (up to 5 − 10 dB) when compared to PIN-based photodiodes. The internal gain for an APD comes from the process of impact ionization process; however, due to its stochastic nature, the multiplication gain comes at the expense of extra noise. This multiplication noise, called the excess noise, is a measure of the gain uncertainty. Despite significant progress made in the fabrication process for the APDs, the excess noise remains a challenge for state-of- the-art APDs and limits their performance measures such as signal-to-noise ratio (SNR). Although there is present a formulation of noise based on the local-theory model, it fails to provide accurate predictions for thin (\u3c 1μ m) devices. This is due to the presence of dead space, the distance a carrier must travel before it can impact ionize, which reduces the uncertainty of the stochastic process and the excess noise. Additionally, the buildup time, defined as the time required for all the impact ionizations to complete, increases as the mean gain increases. This leads to the presence of intersymbol interference within the device, which is detrimental to the device performance, especially at higher speeds. There is, therefore, a need to characterize the perfomance measures such as the mean gain and the noise in the presence of dead space for thin devices. Presented in this work are modeling techniques which address these issues. A look is taken at 3D device structures for which the field characteristics as well as the material ionization coefficients vary along the multiplication path; this is the practical culmination of the dead space multiplication theory (DSMT). While the linear mode analysis using DSMT for 3D structures has recently been implemented, we show here the adapted 3D DSMT model for transient and Geiger modes analysis, including the evaluation of the gain-bandwidth product and the photon detection efficiency. A sample 3D Si-Ge lateral APD is used to develop and test this model. Even for 1D APDs, implementation of the DSMT is cumbersome. In an effort to simplify it, a look is taken at the scenario when the ionization coefficients for holes and electrons in the APD multiplier may be taken to be equal. This may be the case when the multiplication width is small and the electric field is high. It is shown, mathematically, that by doing so, the DSMT is simplified considerably. The APD devices are then modeled for three different materials for which this assumption may be true. This provides us practical limits for which the formulations may be used. Another instance in which the DSMT implementation is simplified by assuming that the ionization coefficients are completely disparate. Such is the case in InAs, which has recently been shown to be an electron-only ionization material. For this case, it is verified that the behavior of the APD deviates from that predicted by McIntyre’s local theory and that the noise in such APDs is reduced. In addition, earlier predictions made by Hayat et al. for mean gain using the asymptotic formulas are tested against the results from the recursive integral equations modeling data. The use of such material in IR detection, which has many applications in remote sensing, medical imaging, and environmental monitoring, is also discussed in the context of a novel, low-noise, high-gain, design APDs with a heterojunction design. This dissertation, therefore, extends the capability in modeling the performance of APDs for different configurations. In the presence of the dead space, the characterization of practical 3D devices using the DSMT as well as the simplification in the gain and noise calculation for 1D devices provide an invaluable tool in under- standing the behavior of thin APDs. In addition, the modeling of a new design of heterojunction APDs for mid-IR detection allows for a new breed of designs to be developed with lower customizable noise and improved gain characteristics

Design, Electrical, and Optical Modelling of Bulk Heterojunction Polymer Solar Cell

The energy scenario today is focused on the development and usage of solar cells, especially in the paradigm of clean energy. To readily create electron and hole pairs, solar cells utilize either photoactive or photosensitive components. A bulk heterojunction (BHJ) is a nanolayer consisting of donor and acceptor components with a large interpenetrated acceptor and donor contact area. In this context, a mix of P3HT and PCBM offers novelty for its use as an acceptor as well as a donor. In the work presented here, we address the mechanism of modelling and characterization of a BHJ-based polymer solar cell. Here, a new design of BHJ polymer solar cells have been designed, modelled, using Silvaco TCAD in the Organic Solar module, and matched with an already assembled device having similar features. Using this model, we have been able to estimate key parameters for the modelled devices, such as the short-circuit current density, open-circuit voltage, and fill factor with less than 0.25 error index compared to the fabricated counterpart, paving the way for fabless polymer solar cell design and optimization

Fabrication and characterization of graphene induced Metal Semiconductor Metal (MSM) structure for detection and sensing applications

The demand for miniaturization of electronic devices has lent to the development of graphene-based hybrid structures, which include the Metal-Semiconductor-Metal (MSM) device. In this work, one has developed such a device by growing monolayers of graphene on top of Nickel to form the basic structural matrix. Four different variants of the MSM unit structures have been developed to assess their potential in next generation electronics. The presence of graphene in the original matrix was confirmed via Atomic Force Microscopy, and the optical response of the graphene layer was further studied using Spectroscopic Ellipsometry in UV-Vis-NIR regime; Forouhi-Bloomer model was used to analyze the ellipsometry data. Hall effect and other electrical characterization measurements were conducted to analyze the electrical properties of the fabricated devices

Etiology of acute infectious meningitis and meningoencephalitis in Karachi, Pakistan: Retrospective observational study from a tertiary care center

Meningoencephalitis (ME) is potentially fatal and is caused by a wide array of pathogens. Diagnostic and health-care access gaps prevent accurate estimation of the pathogen-specific burden in low-resource settings. We present pathogen-specific etiologies among patients hospitalized with ME in Karachi, Pakistan. We performed a retrospective hospital database evaluation of pathogen etiology and outcomes of community-acquired infectious ME at a single tertiary care center in Karachi, Pakistan. Annual rates of hospitalization (ARH) were calculated by adjusting for missed cases and are reported per 100,000 population. From May 2017 to April 2020, 522 episodes of infectious ME were identified in 514 patients. The overall ARH from ME was 5.7/100,000 population (95% CI, 5.1-6.1). Among children younger than 5 years, the ARH was 9.8/100,000 population (95% CI, 8.1-11.8). Unknown causes of ME resulted in the greatest burden, with an ARH of 1.9/100,000 population (95% CI, 1.7-2.2). Among known causes, the greatest burden of hospitalizations resulted from tuberculous ME (0.8/100,000; 95% CI, 0.6-0.97), followed by pneumococcal and enteroviral ME (both 0.6/100,000 population; 95% CI, 0.5-0.8). The burden of ME caused by pathogens preventable through vaccination or public health measures outweighed that of ME from other causes (P = 0.0092, Fisher\u27s exact test). We report a broad range of pathogens causing ME in southern Pakistan and show a high burden of preventable illness. Synergistic actions to improve diagnostic strategies, increase vaccinations, and introduce measures to reduce water-borne and vector-borne diseases are required to reduce the ME burden in Pakistan and prevent future outbreaks