170,053 research outputs found
Leakage Current Mechanisms in SiGe HBTs Fabricated Using Selective and Nonselective Epitaxy
No full textSiGe heterojunction bipolar transistors (HTBs) have been fabricated using selective epitaxy for the Si collector, followed in the same growth step by nonselective epitaxy for the p+ SiGe base and n-Si emitter cap. DC electrical characteristics are compared with cross-section TEM images to identify the mechanisms and origins of leakage currents associated with the epitaxy in two different types of transistor . In the first type, the polysilicon emitter is smaller than the collector active area, so that the extrinsic base implant penetrates into the single-crystal Si and SiGe around the perimeter of the emitter and the polycrystalline Si and SiGe exrtrinsic base. In these transistors, the Bummel plots are near-ideal and there is no evidence of emitter/collector leakage. In the second type, the collector active area is smaller than the polysilicon emitter, so the extrinsic base implant only penetrates into the polysilicon extrinsic base. In these transistors, the leakage currents observed depend on the base doping level. In transistors with a low doped base, emitter/collector and emitter/base leakage is observed, whereas in transistors with a high doped base only emitter/base leakage is observed. The emitter/collector leakage is explained by punch through o fhte base caused by thinning of the SiGe base at the emitter perimeter. The emitter/base leakeage is shown to be due to Poole-Frenkel mechanism and is explained by penetration of the emitter/base depletion region into the p+ polysilicon extrinsic base at the emitter periphery. Variable collector/base reverse leakage currents are observed and a variety of mechanisms are observed, including Shockley-Read-Hall recombination, trap assisted tunneling, Poole Frenkel and band to band tunneling. These result s are explained by the presence of polysilicon grains on the sidewalls of the field oxide at the collector perimeter
Combined (18)F-FDG-PET/CT Imaging in Radiotherapy Target Delineation for Head-and-Neck Cancer.
No full textPURPOSE: To evaluate the effect of the use of (18)F-fluorodeoxyglucose (FDG)-positron emission tomography (PET)/computed tomography (CT) in radiotherapy target delineation for head-and-neck cancer compared with CT alone. METHODS AND MATERIALS: A total of 38 consecutive patients with head-and-neck cancer were included in this study. The primary tumor sites were as follow: 20 oropharyngeal tumors, 4 laryngeal tumors, 2 hypopharyngeal tumors, 2 paranasal sinuses tumors, 9 nasopharyngeal tumors, and 1 parotid gland tumor. The FDG-PET and CT scans were performed with a dedicated PET/CT scanner in one session and then fused. Subsequently, patients underwent treatment planning CT with intravenous contrast enhancement. The radiation oncologist defined all gross tumor volumes (GTVs) using both the PET/CT and CT scans. RESULTS: In 35 (92%) of 38 cases, the CT-based GTVs were larger than the PET/CT-based GTVs. The average total GTV from the CT and PET/CT scans was 34.54 cm(3) (range, 3.56-109) and 29.38 cm(3) (range, 2.87-95.02), respectively (p < 0.05). Separate analyses of the difference between the CT- and PET/CT-based GTVs of the primary tumor compared with the GTVs of nodal disease were not statistically significant. The comparison between the PET/CT-based and CT-based boost planning target volumes did not show a statistically significant difference. All patients were alive at the end of the follow-up period (range, 3-38 months). CONCLUSION: GTVs, but not planning target volumes, were significantly changed by the implementation of combined PET/CT. Large multicenter studies are needed to ascertain whether combined PET/CT in target delineation can influence the main clinical outcomes
Sequential Three-Way Decisions for Reducing Uncertainty in Dropout Prediction for Online Courses
No full textMassive Open Online Courses (MOOCs) allow accessing qualitative online educational resources for huge amounts of online students. In this context, the dropout phenomenon is known as a nasty problem faced by several existing studies proposing methods and techniques to make predictions on students who are at risk of dropping out. Although the majority of such studies adopt traditional classification algorithms based on supervised methods, the present work proposes a sequential approach based on Three-Way Decisions and Neighborhood Rough Sets. The underlying idea is to exploit weekly data in order to classify, with high levels of precision, students who are likely going towards dropout or not. In cases of uncertainty, the classification decision is deferred to the next week, when new data is available. Such an approach has the advantage to preserve resources and avoiding wasting them with students erroneously classified at risk of dropout. The sequential application of the approach makes the recall increase as new data is gathered
A time-driven FCA-based approach for identifying students' dropout in MOOCs
No full textIn online learning, the dropout phenomenon is a relevant issue to address with practical solutions. Several data sets stimulate original, and resolutive data analysis approaches, demonstrating the importance of the dropout phenomenon. This study proposes a novel approach to predicting massive online open course (MOOC) students at risk of dropout stressing the need to consider the temporal dimension in the data log. The proposal aims to build a data-driven decision support system able to identify students at risk of dropout based on the conceptualization of such students' behavior and its evolution along the time dimension. The primary theoretical model behind the proposed method is the formal concept analysis, and its temporal extension (i.e., temporal concept analysis) for analyzing timestamped data and carrying out a timed lattice. The main result of the paper is a method to extract behavioral patterns of MOOC students at risk of dropout. Such patterns are defined as Time-based Behavior Rules extracted from the aforementioned timed lattice obtained through the preprocessing of MOOC platform log files. The resulting rule set can be easily integrated for implementing educational DSS, as shown in the last part of the paper. The conducted experiments reveal promising results in terms of F-score and students' monitoring time
Management of serous cystic neoplasms of the pancreas
No full textPancreatic serous cystadenomas are uncommon benign tumours that are often found incidentally on routine imaging examinations. Radiological imaging techniques alone have proven to be suboptimal to fully characterize cystic pancreatic lesions. Endoscopic ultrasound, with the addition of fine-needle aspiration in difficult cases, has showed greater diagnostic accuracy than conventional imaging techniques. The best management strategy of these neoplasms is still debated. Surgery should be limited only to symptomatic and highly selected cases and most of the patients should only be strictly monitored. In the current paper, we provide an updated overview on pancreatic serous cystadenomas, focusing our attention on epidemiology, clinical characteristics and diagnostic evaluation; finally, we also discuss different management strategies and areas for future research
Regarding “Fecal Hemoglobin Levels in Prior Negative Screening and Detection of Colorectal Neoplasia: A Dose-Response Meta-Analysis”
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