Electrical and Computer Engineering Commons^™

Theoretical Estimation Of Optical Absorption And Photoluminescence In Nanostructured Silicon; An Approach To Improve Efficiency In Nanostructured Solar Cells, Jaspreet S. Nayyar

Dissertations, Master's Theses and Master's Reports - Open

Renewable energy is growing in demand, and thus the the manufacture of solar cells and photovoltaic arrays has advanced dramatically in recent years. This is proved by the fact that the photovoltaic production has doubled every 2 years, increasing by an average of 48% each year since 2002.

Covering the general overview of solar cell working, and its model, this thesis will start with the three generations of photovoltaic solar cell technology, and move to the motivation of dedicating research to nanostructured solar cell. For the current generation solar cells, among several factors, like photon capture, photon reflection, carrier generation …

Fadaptive Backstepping Control Of Active Magnetic Bearings, Silu You

ETD Archive

A new control methodology, adaptive backstepping control (ABC), is applied to a linearized model of an active magnetic bearing (AMB). Our control objective is to regulate the deviation of the magnetic bearing from its equilibrium position in the presence of an external disturbance. The control approach is based on adaptive backstepping control, which is a combination of a recursive Lyapunov controller and adaptive laws. In this thesis, two types of adaptive backstepping methods are used. The first method is based on full-state feedback, for which all three states in the linearized AMB model (velocity, position, and current) are used to …

Evaluation And Comparison Of Mac Protocols In Wireless Sensor Networks, Sharmila Kollipara

ETD Archive

Wireless sensor network applications call for different kinds of network protocols at different levels of the network stack based on application requirements. A number of medium access control (MAC) protocols have been proposed in the literature. Evaluation of most of these MAC protocols have typically been based on simulation, and while such simulation provides interesting insight into the behavior of these protocols, artifacts caused by behavior of hardware is ignored. Further more, MAC protocols are usually evaluated by comparing the new protocol with others based on one or two metrics, the ones that determined the design decisions for the protocol …

Implementation And Evaluation Of A Tdma Based Protocol For Wireless Sensor Networks, Robert M. Fiske

ETD Archive

When evaluating MAC layer network protocols for wireless sensor networks performing simulations of a protocol's operation can provide great insight into the performance of the protocol. In order to prove that a protocol will work in a real setting and not just at the theoretical level, however, there is no substitute for evaluation with a physical implementation. This thesis discusses a physical implementation and evaluation of the Many-to-One-Sensor-to-Sink (MOSS) MAC layer protocol for sink based wireless sensor networks using the MAC Layer Architecture for TinyOS. MOSS is a Time Division Multiple Access (TDMA) based protocol first proposed in an earlier …

A Light Weight Fault Tolerance Framework For Web Services, Srikanth Dropati

ETD Archive

The increased usage of web services by many of the corporate industries to exchange their critical information over World Wide Web has directly impacted the need for high availability of web services. So in this work of ours we designed and developed a light weight fault tolerance framework for web services, by strictly biding ourselves to the design specifications of web services. We developed our framework by extending the open source implementation of Web services reliable messaging specifications. Our framework provides fault tolerance capability using the replication strategy, and can easily be reverted back to basic point to point reliable …

Two-Subband Conduction In A Gated High Density Inaln/Aln/Gan Heterostructure, H. Cheng, C. Kurdak, J. H. Leach, M. Wu, Hadis Morkoç

Electrical and Computer Engineering Publications

Magnetotransport measurements on an In0.16Al0.84N/AlN/GaN gated Hall bar sample have been performed at 0.28 K. By the application of a gate voltage we were able to vary the total two-dimensional electron gas density from 1.83×1013 to 2.32×1013 cm−2. Two frequency Shubnikov–de Haas oscillations indicate occupation of two subbands by electrons. The density of electrons in the first and second sublevels are found to increase linearly with gate voltage with a slope of 2.01×1012 cm−2/V and 0.47×1012 cm−2/V, respectively. And the quantum lifetimes for the first and second subbands ranged from 0.55 to 0.95×10−13 s and from 1.2 to 2.1×10−13 s.

Surface Plasmon Enhanced Uv Emission In Algan/Gan Quantum Well, J. Lin, A. Mohammadizia, A. Neogi, Hadis Morkoç, M. Ohtsu

Electrical and Computer Engineering Publications

The surface plasmon (SP) energy for resonant enhancement of light has shown to be modified by the epitaxial substrate and the overlying metalthin film. The modification of SP energy in AlGaN/GaN epitaxial layers is studied using spectroscopic ellipsometry for enhanced UV-light emission. Silver induced SP can be extended to the UV wavelength range by increasing the aluminum concentration in AlxGa1−xN epilayer. A threefold increase in the UV-light emission is observed from AlGaN/GaN quantum well due to silver induced SP. Photoluminescence lifetime measurements confirm the resonant plasmon induced increase in Purcell factor as observed from the PL intensity measurements.

Ingan Staircase Electron Injector For Reduction Of Electron Overflow In Ingan Light Emitting Diodes, X. Ni, X. Li, J. Lee, S. Liu, Vitaliy Avrutin, Ü. Özgür, Hadis Morkoç, A. Matulionis, T. Paskova, G. Mulholland, K. R. Evans

Electrical and Computer Engineering Publications

Ballistic and quasiballistic electron transport across the active InGaN layer are shown to be responsible for electron overflow and electroluminescence efficiency droop at high current levels in InGaN light emitting diodes both experimentally and by first-order calculations. An InGaN staircase electron injector with step-like increased In composition, an “electron cooler,” is proposed for an enhanced thermalization of the injected hot electrons to reduce the overflow and mitigate the efficiency droop. The experimental data show that the staircase electron injector results in essentially the same electroluminescence performance for the diodes with and without an electron blocking layer, confirming substantial …

Effect Of Hot Phonon Lifetime On Electron Velocity In Inaln/Aln/Gan Heterostructure Field Effect Transistors On Bulk Gan Substrates, J. H. Leach, C. Y. Zhu, M. Wu, X. Ni, X. Li, J. Xie, Ü. Özgür, Hadis Morkoç, J. Liberis, E. Šermukšnis, A. Matulionis, T. Paskova, E. Preble, K. R. Evans

Electrical and Computer Engineering Publications

We report on electron velocities deduced from current gain cutoff frequency measurements on GaN heterostructurefield effect transistors(HFETs) with InAlN barriers on Fe-doped semi-insulating bulk GaN substrates. The intrinsic transit time is a strong function of the applied gate bias, and a minimum intrinsic transit time occurs for gate biases corresponding to two-dimensional electron gas densities near 9.3×1012 cm−2. This value correlates with the independently observed density giving the minimum longitudinal optical phonon lifetime. We expect the velocity, which is inversely proportional to the intrinsic transit time, to be limited by scattering with non equilibrium (hot) phonons at the high fields …

Carrier Velocity In Inaln/Aln/Gan Heterostructure Field Effect Transistors On Fe-Doped Bulk Gan Substrates, J. H. Leach, M. Wu., X. Ni, X. Li, J. Xie, Ü. Özgür, Hadis Morkoç, T. Paskova, E. Preble, K. R. Evans, Chang-Zhi Lu

Electrical and Computer Engineering Publications

We report microwave characteristics of field effect transistors employing InAlN/AlN/GaN heterostructures grown on low-defect-density bulk Fe-doped GaN substrates. We achieved unity current gain cutoff frequencies of 14.3 and 23.7 GHz for devices with gate lengths of 1 and 0.65 μm, respectively. Measurements as a function of applied bias allow us to estimate the average carrier velocity in the channel to be ∼1.0×107 cm/sec for a 1 μm device. Additionally, we found nearly no gate lag in the devices, which is considered a precondition for good performance under large signal operation.

Dominant Spin Relaxation Mechanism In Compound Organic Semiconductors, Supriyo Bandyopadhyay

Electrical and Computer Engineering Publications

Despite the recent interest in “organic spintronics,” the dominant spin relaxation mechanism of electrons or holes in an organic compound semiconductor has not been conclusively identified. There have been sporadic suggestions that it might be hyperfine interaction caused by background nuclear spins, but no confirmatory evidence to support this has ever been presented. Here, we report the electric-field dependence of the spin-diffusion length in an organic spin-valve structure consisting of an Alq3 spacer layer, and argue that these data, as well as the available data on the temperature dependence of this length, contradict the notion that hyperfine interactions relax spin. …

Electrically And Magnetically Tunable Phase Shifters Based On A Barium Strontium Titanate-Yttrium Iron Garnet Layered Structure, J. H. Leach, H. Liu, V. Avrutin, E. Rowe, Ü. Özgür, H. Morkoç, Y.-Y. Song, M. Wu

Electrical and Computer Engineering Publications

We report on the tuning of permittivity and permeability of a ferroelectric/ferromagnetic bilayer structure which can be used as a microwavephase shifter with two degrees of tuning freedom. The structure was prepared by the growth of a yttrium iron garnet (YIG) layer on a gadolinium gallium garnet substrate by liquid phase epitaxy, the growth of a barium strontium titanate (BST) layer on the YIG layer through pulsed laser deposition, and then the fabrication of a coplanar waveguide on the top of BST through e-beam evaporation and trilayer liftoff techniques. The phase shifters exhibit a differential phase shift of 38°/cm at …

Large Dielectric Tuning And Microwave Phase Shift At Low Electric Field In Epitaxial Ba0.5sr0.5tio3 On Srtio3, J. H. Leach, H. Liu, V. Avrutin, B. Xiao, Ü. Özgür, H. Morkoç, J. Das, Y. Y. Song, C. E. Patton

Electrical and Computer Engineering Publications

Dielectric properties of annealed and as-grown ferroelectricBa0.5Sr0.5TiO3 (BST) grown by pulsed laser deposition on sputtered BST seed layers on strontium titanate (STO) substrates were investigated at microwave frequencies in the realm of tunability of its dielectric constant as well as phase shifters based on this material. The as-grown layers were nearly fully relaxed with measured lattice parameters nearly identical to those of bulk BST. The tuning of the relative dielectric constant (∼1750 at zero bias at 10 GHz) of the annealed BST was found to be as high as 59% and 56% at 10 and 19 GHz, respectively. The analysis …

Bias Dependent Two-Channel Conduction In Inaln/Aln/Gan Structures, J. H. Leach, X. Ni, X. Li, M. Wu, Ü. Özgür, H. Morkoç, L. Zhou, D. A. Cullen, D. J. Smith, H. Cheng, Ç. Kurdak, J. R. Meyer, I. Vurgaftman

Electrical and Computer Engineering Publications

Due to growth temperature differences during deposition of GaN heterostructures utilizing InAlN barriers, an inadvertent parasitic GaN layer can form in the InAlN barrier layer. In structures utilizing AlN spacer layers, this parasitic layer acts as a second conduction channel with a carrier density dependent upon polarization charges and lattice strain as well as the surface potential. The effect of an additional GaN spacer layer in InAlN/AlN/GaN structures is assessed using simulations, electron-microscopy observations, magnetoconductivity measurements with gated Hall bar samples, and with quantitative mobility spectrum analysis. We propose a possible formation mechanism for the parasitic layer, and note that …

Ti/Al/Ni/Au Ohmic Contacts For Alinn/Aln/Gan-Based Heterojunction Field-Effect Transistors, Lin Zhou, Jacob H. Leach, Xianfeng Ni, Hadis Morkoç, David J. Smith

Electrical and Computer Engineering Publications

The microstructure of AuNiAlTi/Al0.84In0.16N/AlN/GaNOhmic contacts annealed from 700 to 900 °C has been determined using transmission electron microscopy and associated analytical techniques. Intermixing and phase separation of the metal contact layers was observed to degrade the surface roughness. An optimal contact performance was obtained for contacts annealed at 800 °C and was attributed to the formation of TiN contact inclusions that had penetrated through the AlInN layers into the GaN layers underneath. These TiN contact inclusions had an inverted mushroom shape with a density of ∼108 cm−2, and they were invariably located at the positions of mixed-type threading dislocations. These …

Hot Electron Effects On Efficiency Degradation In Ingan Light Emitting Diodes And Designs To Mitigate Them, X. Ni, X. Li, J. Lee, S. Liu, V. Avrutin, Ü. Özgür, H. Morkoç, A. Matulionis

Electrical and Computer Engineering Publications

Hot electrons and the associated ballistic and quasiballistic transport, heretofore neglected endemically, across the active regions of InGaN light emitting diodes (LEDs) have been incorporated into a first order simple model which explains the experimental observations of electron spillover and the efficiency degradation at high injection levels. The model is in good agreement with experiments wherein an adjustable barrier hot electron stopper, commonly called the electron blocking layer (EBL), is incorporated. The model is also in agreement with experiments wherein the electrons are cooled, eliminating hot electrons, inside a staircase electron injector (SEI) prior to their injection into the active …

On The Use Of A Dynamic Hybrid Tempo Detection Model For Beat Tracking, Mikel Gainza

Conference papers

In this paper, an approach that estimates the times at which musical beats occur is presented. The system uses a hybrid multi-band decomposition in order to estimate the music tempo. Following this, beat events are tracked by using a dynamic programming approach, which is updated by using short time tempo estimates. The hybrid decomposition is used in order to calculate the tempo by using different onset detection functions in different frequency bands. In addition, a method that estimates which frequency bands provide reliable periodicities is also presented. The accuracy of the model is evaluated by comparing the presented system against …

Electromagnetic Fields, Power Losses, And Resistance Of High-Frequency Magnetic Devices, Daniel Joseph Whitman

Browse all Theses and Dissertations

The operation of dc-dc PWM converters requires magnetic components that operate at high frequencies and carry large currents. At such frequencies the skin and proximity effects cause a substantial increase in power losses and ac resistance. This thesis reviews existing one dimensional analytical work on the prediction of these losses and, beginning with an inductor, derives expressions for the electromagnetic fields and power losses within the winding as well as the ac resistance of the device. The results of the inductor analysis are then extended to examine these quantities in transformers of various winding configurations. It is shown that proximity …

Efficacy Of A Final Lab Practicum And Lab Reports For Assessment In A Fundamentals Electric Circuits Laboratory, Edward Carl Greco Jr., James D. Reasoner, Daniel W. Bullock, Carlos Castillo, Patricia Buford, Gill Richards

Faculty Publications - Electrical Engineering

In a fundamental electrical engineering laboratory course, the traditional team based approach to laboratory structure with two or more members per lab team was not as effective for teaching basic laboratory skills and knowledge as a lab structure that allowed students to perform laboratory exercises individually throughout the semester. The laboratory report was insensitive and insufficient to measure the difference in the laboratory abilities and knowledge from these two groups. In order to assess the students' laboratory knowledge and abilities, it was necessary to develop a new assessment instrument that would be sufficient for this purpose. The students' performance on …

Response-Calibration Techniques For Antenna-Coupled Infrared Sensors, Peter Krenz

Electronic Theses and Dissertations

Infrared antennas are employed in sensing applications requiring specific spectral, polarization, and directional properties. Because of their inherently small dimensions, there is significant interaction, both thermal and electromagnetic, between the antenna, the antenna-coupled sensor, and the low-frequency readout structures necessary for signal extraction at the baseband modulation frequency. Validation of design models against measurements requires separation of these effects so that the response of the antenna-coupled sensor alone can be measured in a calibrated manner. Such validations will allow confident extension of design techniques to more complex infrared-antenna configurations. Two general techniques are explored to accomplish this goal. The extraneous …