A comment on generalized Schwinger effect

A spatially homogeneous, time-dependent, electric field can produce charged particle pairs from the vacuum. When the electric field is constant, the mean number of pairs which are produced depends on the electric field and the coupling constant in a non-analytic manner, showing that this result cannot be obtained from the standard perturbation theory of quantum electrodynamics. When the electric field varies with time and vanishes asymptotically, the result may depend on the coupling constant either analytically or non-analytically. We investigate the nature of this dependence in detail. We show that the dependence of particle production on coupling constant is non-analytic for a class of time-dependent electric fields which vanish asymptotically when a specific condition is satisfied. We also demonstrate that for another class of electric fields, which vary rapidly, the dependence of particle production on coupling constant is analytic.Comment: published versio

Study of Novel Power Semiconductor Devices for Performance and Reliability.

Power Semiconductor Devices are crucial components in present day power electronic systems. The performance and efficiency of the devices have a direct correlation with the power system efficiency. This dissertation will examine some of the components that are commonly used in a power system, with emphasis on their performance characteristics and reliability. In recent times, there has a proliferation of charge balance devices in high voltage discrete power devices. We examine the same charge balance concept in a fast recovery diode and a MOSFET. This is crucial in the extending system performance at compact dimensions. At smaller device and system sizes, the performance trade-off between the ON and OFF states becomes all the more critical. The focus on reducing the switching losses while maintaining system reliability increases. In a conventional planar technology, the technology places a limit on the switching performance owing to the larger die sizes. Using a charge balance structure helps achieve the improved trade-off, while working towards ultimately improving system reliability, size and cost. Chapter 1 introduces the basic power system based on an inductive switching circuit, and the various components that determine its efficiency. Chapter 2 presents a novel Trench Fast Recovery Diode (FRD) structure with injection control is proposed in this dissertation. The proposed structure achieves improved carrier profile without the need for excess lifetime control. This substantially improves the device performance, especially at extreme temperatures (-40oC to 175oC). The device maintains low leakage at high temperatures, and it\u27s Qrr and Irm do not degrade as is the usual case in heavily electron radiated devices. A 1600 diode using this structure has been developed, with a low forward turn-on voltage and good reverse recovery properties. The experimental results show that the structure maintains its performance at high temperatures. In chapter 3, we develop a termination scheme for the previously mentioned diode. A major limitation on the performance of high voltage power semiconductor is the edge termination of the device. It is critical to maintain the breakdown voltage of the device without compromising the reliability of the device by controlling the surface electric field. A good termination structure is critical to the reliability of the power semiconductor device. The proposed termination uses a novel trench MOS with buried guard ring structure to completely eliminate high surface electric field in the silicon region of the termination. The termination scheme was applied towards a 1350 V fast recovery diode, and showed excellent results. It achieved 98% of parallel plane breakdown voltage, with low leakage and no shifts after High Temperature Reverse Bias testing due to mobile ion contamination from packaging mold compound. In chapter 4, we also investigate the device physics behind a superjunction MOSFET structure for improved robustness. The biggest issue with a completely charge balanced MOSFET is decreased robustness in an Unclamped Inductive Switching (UIS) Circuit. The equally charged P and N pillars result in a flat electric field profile, with the peak carrier density closer to the P-N junction at the surface. This results in an almost negligible positive dynamic Rds-on effect in the MOSFET. By changing the charge profile of the P-column, either by increasing it completely or by implementing a graded profile with the heavier P on top, we can change the field profile and shift the carrier density deeper into silicon, increasing the positive dynamic Rds-on effect. Simulation and experimental results are presented to support the theory and understanding. Chapter 5 summarizes all the theories presented and the contributions made by them in the field. It also seeks to highlight future work to be done in these areas

Understanding Plant Response to Stress Using Gene Model Quality Evaluation and Transcriptome Analysis

The overall aim of the project was to understand how plants reacted to environmental stress and evolved to overcome it. The land plants that we see today evolved from a green algal ancestor around 510 million years ago. Plants had to make significant changes to their cellular, morphological, regulatory and physiological processes during their adaptation to the terrestrial environment from an aquatic environment. The first part of the project was to find out how these changes were reflected on the protein makeup of the early land plants. The gene model sequence data of two early land plants, Physcomitrella patens (moss) and Chlamydomonas reinhardtii(green algae). We specifically focused on the protein family expansion of protein kinases due to their roles in various important functions that would affect the transition from water to land. We developed a gene model quality evaluation method to score the gene models of P. patens and C. reinhardtii using well-studied plants such as Arabidopsis thaliana and Oryza sativa (rice) to improve the poor quality gene models that currently exist. The resulting corrected gene models were analyzed using functional annotation methods to understand how the proteomics of the early land plants varied from modern land plants. The second part of the project was to identify the genes responsible for herbicide resistance in Ambrosia trifida (giant ragweed). Giant ragweed is one of the most competitive annual weeds in corn and soybean production across the eastern Corn Belt in the United States. The use of glyphosate (commercial name: Roundup) and glyphosate-ready crop systems managed to keep giant ragweed populations under control. Glyphosate-ready crop systems consist of seeds that are resistant to glyphosate, which enables farmers to use glyphosate to control the population of weeds. But in the last decade, glyphosate-resistant giant ragweed populations have been reported across the world. It is a huge problem to farmers since it results in unusable glyphosate-ready cropping systems and huge yield losses. Glyphosate-resistant and sensitive plants were identified from across the Midwestern United States and a RNA-seq experiment was performed by isolating the total mRNA from leaf material, and obtaining the expressed messenger RNA sequences. The genetic makeup of the sensitive and resistant strains was thus compared based on their transcriptome data, and a list of potential genes that were differentially expressed between them was identified. We also analyzed how much the quality of the transcriptome can be improved by using the transcriptome and genome of sunflower, a closely-related plant

A clinical study of surgical management of paediatric humeral supracondylar fracture

Background: Injury to the supracondylar region of the humerus and complete displacement of the fragments occurs in many of the cases when children usually fall while playing. This is most common fracture seen in children, makes up to 60% of all elbow injuries. Early intervention results in excellent reduction and avoids complications. Concomitant vascular and nerve damages may occur. Open reduction and internal fixation is a helpful option for the treatment of supracondylar facture of humerus in children.Methods: Forty children who presented with displaced supracondylar fracture of the humerus were treated with open reduction and internal fixation with Kirschner (K) wires after thorough pre-operative investigation during the course of the study. Children below the age of 13 were selected irrespective of the sex.Results: Good results were obtained in 60% of the patients, fair in 30% and poor in 10%. The poor results were due to the open fracture and in two cases the patient presented very late to the hospital. Complications such as nerve injuries, vascular injuries, infections were noticed in the study. Four patients had cubitus varus and twelve patients had flexion loss on follow-up study.Conclusions: Open reduction and internal fixation with K wire is an easy, simple, inexpensive method, which has good outcome

Lead to Can Abrasion (LTCA) Test

Pacemakers are medical devices which regulate a person’s heartbeat through electric pulses delivered via an endocardial lead attached to the heart. When a pacemaker is implanted in a patient, there is usually excess length left on the lead. Surgeons will typically wrap the excess lead length around the pacemaker device and close the wound. However, wrapping the lead around the pacemaker outer surface can open the lead to abrasion and cause the lead’s vital electrical insulation to be compromised. This is currently tested using an existing abrasion fixture at Abbott. However, the current tester does not yield consistent data results for cycles until a lead is electrically compromised. The scope of this project is to design a testing machine that will be able to yield statistically significant data. This scope was met through our results meeting the engineering specifications set out at the start of the project. This document will encapsulate the project’s groundwork, including background research, scope of the problem, project requirements set by the sponsor, concept design and design process, final design, manufacturing plan, design verification, project management, and conclusions and recommendations