Poor Aspect Ratio Research Articles

2.5 A/mm/350 GHz aggressively scaled gate engineered Fe-doped AlN/GaN channel HEMT with graded InGaN Backbarrier on SiC-wafer for next generation RF power electronics applications

This study intends to investigate the effects of barrier scaling, a crucial RF GaN-HEMT design element, which is imperative to achieve enhanced performance at nano-scale dimensions. From the performance characterization of the designed 50 nm recessed gated Fe-doped AlN/GaN/SiC HEMT featuring graded InGaN-back barrier (BB), the highest transconductance of 464.7 mS/mm, a maximum fT of 351.5 GHz, peak ID of 1.596 A/mm, & high drain saturation current of 2.527 A/mm at VGS = 3 V is observed for a 6 nm thin AlN barrier due to better gate control, and aided quantum well confinement because of Fe-doped buffer & graded InGaN BB. We observed that as barrier thickness increases, threshold voltage (Vth) becomes more negative and performance is deteriorated due to poor aspect ratio. This work also examines the role of gate-engineering on the electrical properties of the designed HEMT by employing different metals. The TCAD analysis indicates that the Al-gate HEMT had better DC/RF performance because of its lower ϕm (work-function). The performance declines and Vth increases as the Schottky barrier raises due touplifting of the conduction band edge. Since, it is crucial to understand the contribution of Rs & Rd in RF design, so in addition, we have done simulations by scaling source-gate gap (LGS), & drain-gate gap (LGD). It was found that the HEMT delivered better performance at the smallest LGS & LGD due to minimal Rs & Rd. The incredible performance of the novel HEMT reflects enhanced electron transport properties and implies that it possesses the capability of being extremely desirable for future aviation, military, and space applications.

Electrochemical Investigation of Light-Induced Plating of Nickel on Si Solar Cells and Study of Nickel Silicide Formation

Front-contact metallization plays an important role in the fabrication of a solar cell as it adds up to 40% cost to the total production cost.1 Silver (Ag) screen printing is the commercialized technology for the front contact metallization. On the other hand, silver screen printing causes higher Si wafer breakage and results in conversion loss due to high line resistivity, poor aspect ratio and large printing spot size. Moreover, the high price of silver also drives up the cost of a solar cell. Copper, more than a hundred times cheaper than silver, is an alternate option for the front contact as its conductivity is comparable to that of silver.2 However, copper has a tendency to diffuse through Si and create recombination sites, which decreases the cell performance. To avoid diffusion of copper in silicon, a metal film has to be deposited that acts as a barrier layer. Among various diffusion barrier materials, Ni is the most promising because of its low bulk resistivity that makes nickel silicide compatible with the semiconductor technology.In this work, a nickel seed layer is deposited on silicon samples with light-induced plating (LIP) to obtain the low-resistive NiSi (1:1) phase. In-situ X-ray diffraction (XRD) is carried out along with annealing to understand the phase changes associated with silicide formation (Figure 1). The obtained XRD patterns of nickel deposited by LIP were not comparable with that of EM (electroless metal deposition). Mott-Schottky analysis is carried out to investigate the interface properties of Ni deposited by LIP and electroless methods.3, 4 XPS (X-ray photoelectron spectroscopy) data confirms the interfacial oxide which hindered the nickel diffusion to form silicide. References M. Razykov; C.S. Ferekides; D. Morel; E. Stefanakos; H.S. Ullal; H.M. Upadhyaya., Sol. Energy, 85, 1580 (2011). Kamp; J. Bartsch; S. Nold; M. Retzlaff; M. Hörteis; S.W. Glunz., Energy Procedia, 8, 558 (2011). Priyadarshani, P. Leuaa, R. Maurya, A. Kottantharayil and M. Neergat, J. Phys. Chem. C, 124, 19990 (2020).B. Bera, A. Chakraborty, T. Kar, P. Leuaa and M. Neergat, J. Phys. Chem. C, 121, 20850−20856 (2017). Figure 1

Generalized Layerings for Arbitrary and Fixed Drawing Areas

The Directed Layering Problem (DLP) solves a step of the widely used layer-based approach to automatically draw directed acyclic graphs. To cater for cyclic graphs, usually a preprocessing step is used that solves the Feedback Arc Set Problem (FASP) to make the graph acyclic before a layering is determined. Here we present the Generalized Layering Problem (GLP), which solves the combination of DLP and FASP simultaneously, allowing general graphs as input. We present an integer programming model and a heuristic to solve the NP-complete GLP and perform thorough evaluations on different sets of graphs and with different implementations for the steps of the layer-based approach. We observe that GLP reduces the number of dummy nodes significantly, can produce more compact drawings, and improves on graphs where DLP yields poor aspect ratios. The drawings resulting from GLP also turn out to be more suitable for making the best possible use of a given drawing area. However, we show that a specialized variant of GLP can yield considerable improvements w.r.t. this particular optimization goal.

Fundamental approach to anisotropic heat conduction using the element-based finite volume method

ABSTRACTThis work describes the fundamentals of the element-based finite volume method for anisotropic heat conduction within the framework of the finite element space. Patch tests indicate no element inconsistencies or deficiencies when facing mesh distortion and poor aspect ratio. Convergence and accuracy assessments show that the method presents asymptomatic rate of convergence with discretization errors approaching a second-order scheme. Anisotropic heat conduction in a periodical solid lattice illustrates the application of the method. Application of an optimization technique demonstrates that the choice of a proper material orientation when manufacturing the solid lattice can increase the global heat transfer coefficient.

Buckling Characteristics of Microdrills by Using Finite Element Method

In recent years, microdrill becomes a very important element in the process of drilling apertures of PCB integrated circuits. But different material property will often generate collapse or fracture of the drill tip. This study is about to establish a finite element model (FEM) and to get the buckling stress analysis of the microdrill by ANSYS software. Since the microdrill is a structural brittle material and easy to be broken, force over buckling critical load is employed to explore the structural weakness and to analyze its stress damage. In addition, vibration, occurred at high-speed micro drilling, could easily lead to the excessive deflection or feeding amount of the drill tip, cause the collapse or rupture of the microdrill, and lower the quality by reaming the machining object. Poor geometric aspect ratio of the drill tip will generate different loads in different parts. How to manage the pressure for the microdrill to apply the force stably and to avoid the breakage and destruction is very important both for the tool life and the cost control. Therefore, how to overcome the buckling of tiny structure component under loading becomes a great issue to be explored.

Dedicated finite elements for electrode thin films on quartz resonators

The accuracy of the finite element analysis for thickness shear quartz resonators is a function of the mesh resolution; the finer the mesh resolution, the more accurate the finite element solution. A certain minimum number of elements are required in each direction for the solution to converge. This places a high demand on memory for computation, and often the available memory is insufficient. Typically the thickness of the electrode films is very small compared with the thickness of the resonator itself; as a result, electrode elements have very poor aspect ratios, and this is detrimental to the accuracy of the result. In this paper, we propose special methods to model the electrodes at the crystal interface of an AT cut crystal. This reduces the overall problem size and eliminates electrode elements having poor aspect ratios. First, experimental data are presented to demonstrate the effects of electrode film boundary conditions on the frequency-temperature curves of an AT cut plate. Finite element analysis is performed on a mesh representing the resonator, and the results are compared for testing the accuracy of the analysis itself and thus validating the results of analysis. Approximations such as lumping and Guyan reduction are then used to model the electrode thin films at the electrode interface and their results are studied. In addition, a new approximation called merging is proposed to model electrodes at the electrode interface.

Investigations on mechanical properties of poly(propylene) and poly(lactic acid) reinforced by miscanthus fibers

Microscopic, nanoindentation and tensile tests were carried out to determine the mechanical behavior of PP and PLLA/reed fiber composites. As a first step, nanoindentation investigations were carried out to obtain longitudinal modulus and hardness of vegetal fibers. This study has allowed us to place the mechanical properties of reed fibers between those of sisal and hemp fibers. Optical microscopic observations have exposed the short length and the poor aspect ratio of the reed fibers used. Complementary SEM pictures have shown an improvement of the bonding between fibers and matrix with the addition of PP-g-MA or PLA-g-MA and the poor dispersion of reed fiber into composites by showing an important number of bundles. Tensile tests have highlighted an important improvement of the tensile modulus and a decrease of the tensile strength at yield with the addition of vegetal fibers. The presence of maleated matrix induces an improvement of modulus and strength for PP/reed composites.

Oxidation behaviour of Fe‐Cr‐Al alloys during resistance and furnace heating

AbstractThe behaviour of thin Fe‐Cr‐Al heating element strips was investigated with respect to the oxidation limited life times and geometrical changes during resistance and furnace heating. For this purpose, isothermal and cyclic oxidation tests varying in their total exposure time and cycle duration were performed in the temperature range 1050–1200 °C. Specimens subjected to rapid cyclic, resistance heating revealed shorter life times than calculated for specimens subjected to isothermal exposure. The life times were found to increase with increasing cycle duration and hence decreasing number of cycles for a given time at temperature. This life time decrease is related to an “hour glass” waviness of the specimens, which develops during prolonged thermal cycling. The development of this plastic deformation also occurred during furnace heated, thermal cycling tests. A two‐step mechanism is introduced combining an oxidation kinetics related time to the onset of significant waviness with an enhancement of this waviness as a result of a ratcheting effect. The latter seems to strongly depend on the number of cycles and on the plastic deformation generated during each cycle rather than on the total time at temperature. The development of an “hour glass” waviness leads to an enhanced aluminium depletion due to an increase of the specimen surface area.Additional deformation phenomena like “hot tube” or “corkscrew” behaviour occur during the resistance heating tests. These are related to a temperature gradient that develops over the specimen width due to the poor aspect ratio of the specimens.

Sub-domain scaling for finite element analysis of electrical machines

Finite element modelling of geometries with large dimensional differences between adjacent sub-domains or domains which have poor aspect ratios can impose significant demands on meshing algorithms and computational resources. A simple sub-domain scaling technique for electromagnetic analysis of electrical machines by finite element analysis is proposed. The technique can be used to improve aspect ratios and is useful in modelling electrical machines with small air gap lengths.

A superferric quadrupole for use in an SRF cryomodule

A superferric quadrupole for use in a superconducting radio frequency (SRF) cryomodule has been designed and constructed. Even though the quad has a poor aspect ratio of length-to-aperture, the external fringe field is required to be small so that the SRF can operate at the high gradients required for the ISCL. Calculations of the external fringe fields required an iron flux clamp to obtain the necessary attenuation. Room temperature testing of the magnet confirmed the magnetic length.

Geometry and Topology for Mesh Generation

1R2. Geometry and Topology for Mesh Generation. Cambridge Monographs on Applied and Computational Mathematics. - H Edelsbrunner (Dept of Comput Sci, Duke Univ, Durham NC). Cambridge UP, Cambridge, UK. 2001. 177 pp. ISBN 0-521-79309-2. $44.95.Reviewed by DJ Benson (Dept of Appl Mech and Eng Sci, UCSD, 9500 Gilman, La Jolla CA 92093-0411).This textbook is a slim 177 pages that focuses on automatic triangular and tetrahedral mesh generation. Although one might expect from the title that it would cover a broader range of topics, eg, hexahedral meshes and blending functions, the preface clearly states the author’s intentions. The book is broken into seven chapters, with the first six chapters representing two weeks of lecture each. The sections within the chapter each represent one 75-minute lecture. The book starts with a discussion of Vornoi and Delaunay triangulations, moves on to algorithms for constructing them, and then discusses algorithms for surface simplification and eliminating/transforming elements with poor aspect ratios. The final chapter discusses unresolved problems, a nice feature that gives those new to the field a perspective on current research. The subject and author indices are thorough. The book is clearly written with plenty of figures to illustrate its concepts. Each chapter ends with a bibliography and exercises. The bibliography also contains the author’s comments on the different references, a feature rarely found in books and which this reviewer found to be very helpful. Some of the algorithms are expressed in a pseudo-code format with some features particular to C. Those unfamiliar with C should be able to figure out expressions like “n- -” based on their context. While the target audience is upper division seniors and graduate students, Geometry and Topology for Mesh Generation, should also appeal to those who want to automatically generate triangular and tetrahedral meshes and are looking for more than a cookbook of algorithms. Those seeking a how-to book for commercial mesh generators should look elsewhere. Individuals seeking information on generating logically regular mesh blocks for computational fluid dynamics would be better off with Joe Thompson’s classic book, Handbook of Grid Generation.

Study on contact algorithm of dynamic explicit FEM for sheet forming simulation

Based on existing algorithms, a newly developed contact search algorithm is proposed. The new algorithm consists of global search, local searching, local tracking and penetration calculation processes. It requires no iteration steps. It can deal with not only general tool surfaces with vertical walls, but also tool surfaces meshed with elements having very poor aspect ratios. It is demonstrated that the FE code employing this new contact search algorithm becomes more reliable, efficient and accurate for sheet metal forming simulation than conventional ones.

Regularised marching tetrahedra: improved iso-surface extraction

Marching cubes is a simple and popular method for extracting iso-surfaces from implicit functions or discrete three-dimensional (3-D) data. However, it does not guarantee the surface to be topologically consistent with the data, and it creates triangulations which contain many triangles of poor aspect ratio. Marching tetrahedra is a variation of marching cubes, which overcomes this topological problem, but further degrades the triangle aspect ratios. Improvement in triangle aspect ratio has generally been achieved by mesh simplification, a group of algorithms designed mainly to reduce the triangle count. Vertex clustering is one of the simplest, but does not necessarily maintain the topology of the original mesh. We present a new algorithm, regularised marching tetrahedra (RMT), which combines marching tetrahedra and vertex clustering to generate iso-surfaces which are topologically consistent with the data and contain a number of triangles appropriate to the sampling resolution (typically 70% fewer than marching tetrahedra) with significantly improved aspect ratios. This improvement in aspect ratio greatly enhances smooth shaded displays of the surface. Surface triangulations are shown for implicit surfaces, thresholded medical data, and surfaces created from object cross-sections — implementations of RMT appropriate to each of these situations are available. 1 http://svr-www.eng.cam.ac.uk/∼gmt11/software.html. 1 The application to data sampled on non-parallel planes is also considered.

An application for line elements embedded in a 2D or 3D finite element mesh

Objects with poor aspect ratios are difficult to model with 2D or 3D finite elements because they require a very fine, dense mesh which leads to large coefficient matrices. One-dimensional line elements embedded in a 2D or 3D grid would mitigate this problem. Embedded line elements are economical and, if used properly, yield good solutions to electrostatic problems. The paper demonstrates their utility with a practical electrostatic application.

Surface elements for modelling 3D fields around thin iron sheets

A method that allows thin iron sheets to be included in 3-D finite element models of magnetostatic fields is introduced. This method avoids having to model the sheet using volume elements. This is advantageous, as very fine meshes are conventionally required to model sheets which are thin compared to the other dimensions if elements with a poor aspect ratio are to be avoided. The technique has been available in the MEGA package and has proved useful in modeling electromagnetic compatibility problems, in shielding for magnetic resonance imaging magnets, and for the effects of ships and dockyards on the Earth's magnetic field. >

OBSERVATIONS ON THE BEHAVIOUR OF A PILE-RAFT FOUNDATION ON LONDON CLAY.

The Paper adds greatly to the understanding of the behaviour of such complex foundations. However, the Author’s analysis is only a class B prediction. Has this method been used on projects to provide a class A prediction of settlements? 69. Close inspection of the chosen finite element mesh reveals points not covered in the Paper. Does the program allow a variation of soil properties across each element? The mesh appears excessively fine in view of the neglect of stress values distant from the raft, and many elements have poor aspect ratios. Would the Authoi comment on permissible distortions for this element? 70. The choice of the position and type of boundaries (i.e. smooth, rough, rigid or elastically restrained) at the sides and base of the model are of paramount importance to the settlement values. Indeed, the choice of boundaries may have as much influence on the computed settlements as may the choice of other factors normally considered of greater significance, such as the chosen value of Poisson’s ratio. 71. In this context the use of an undrained Poisson’s ratio of 0.47 is noted. Since the chosen element is capable of simulating much less compressible behaviour if a minimum integrating order is used would the Author comment on the choice of the low value? (In such a case one may not always trust nodal stresses, but that is rarely a great loss.) 72. In 27 it is suggested that shear tractions at the soil-raft interface are negligible. It is certainly obvious that in the model with its concentric cylindrical reinforcements (piles) shear strains are prevented, but it is equally as evident that this is not so in the prototype. The results of reference 10 are important in this context. 73. The Author’s final comments on the rate of settlement are also important. Does his settlement analysis take into account radial drainage? If not, it is hardly surprising that calculated settlement rates are inaccurate (even taking into account factors such as the true consolidation behaviour of the London Clay). It is probable that radial drainage has a greater influence with this geometric configuration than the influence of preferential horizontal drainage paths. Would a coupled (i.e. Biot) type of consolidation theory not provide a more elegant solution to this problem and would it yield the same undrained and drained settlements or would load transfers during the consolidation process lead to a different final settlement picture?