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Articles 1 - 15 of 15
Full-Text Articles in Engineering
Uml Extensions For Real-Time Control Systems, Qimin Gao, Lyndon Brown, Luiz Fernando Capretz
Uml Extensions For Real-Time Control Systems, Qimin Gao, Lyndon Brown, Luiz Fernando Capretz
Electrical and Computer Engineering Publications
The use of object oriented techniques and methodologies for the design of real-time control systems appears to be necessary in order to deal with the increasing complexity of such systems. Recently many object-oriented methods have been used for the modeling and design of real-time control systems. We believe that an approach that integrates the advancements in both object modeling and design methods, and real-time scheduling theory is the key to successful use of object oriented technology for real-time software. Surprisingly several past approaches to integrate the two either restrict the object models, or do not allow sophisticated schedulability analysis techniques. …
Personality Types In Software Engineering, Luiz Fernando Capretz
Personality Types In Software Engineering, Luiz Fernando Capretz
Electrical and Computer Engineering Publications
No abstract provided.
Long-Lasting Photoluminescence In Freestanding Gan Templates, Michael A. Reshchikov, M. Zafar Iqbal, Hadis Morkoç, S. S. Park, K. Y. Lee
Long-Lasting Photoluminescence In Freestanding Gan Templates, Michael A. Reshchikov, M. Zafar Iqbal, Hadis Morkoç, S. S. Park, K. Y. Lee
Electrical and Computer Engineering Publications
We studied time-resolvedphotoluminescence(PL) over a temporal range 10−6–103 s in high-purity freestanding GaN templates. Red, yellow, green, blue, and shallow donor–acceptor emission bands can be resolved in the PLspectrum. Observation of luminescence long after the excitation is switched off is a striking feature of our study. The persistent PL observed for all above bands, except for the green band, is primarily attributed to the donor–acceptor-pair-type recombination. An unusually slow, nonexponential decay of radiative transitions from the conduction band to the shallow acceptor was also observed, pointing to some additional mechanism for the persistent PL. Possible role of the surface states …
Stimulated Emission And Ultrafast Carrier Relaxation In Algan/Gan Multiple Quantum Wells, Ü. Özgür, Henry O. Everitt, Lei He, Hadis Morkoç
Stimulated Emission And Ultrafast Carrier Relaxation In Algan/Gan Multiple Quantum Wells, Ü. Özgür, Henry O. Everitt, Lei He, Hadis Morkoç
Electrical and Computer Engineering Publications
Stimulated emission (SE) and ultrafast carrier relaxation dynamics were measured in two AlxGa1−xN/GaNmultiple-quantum-well(MQW) structures, grown in a Ga-rich environment with x=0.2 and 0.3, respectively. The threshold density for SE (Ith≃100 μJ/cm2) was found to be independent of x. Room-temperature, time-resolved, differential transmission measurements mapped the carrier relaxation mechanisms for above barrier energy excitation. Photoexcited carriers are observed to relax into the QWs in <1 ps, while carrier recombination times as fast as 30 ps were measured. For excitation above Ith, SE is shown to deplete carriers in the barriers through a cascaded refilling of the QW state undergoing SE. Similar behavior is seen in an Al0.3Ga0.7N/GaNMQW grown with a N-rich atmosphere, but the relaxation phenomena of all AlGaN MQWs are significantly faster than observed in InGaN MQWs of similar structure.
4h–Sic Photoconductive Switching Devices For Use In High-Power Applications, S. Doǧan, A. Teke, D. Huang, Hadis Morkoç, C. B. Roberts, J. Parish, B. Ganguly, M. Smith, R. E. Meyers, S. E. Saddow
4h–Sic Photoconductive Switching Devices For Use In High-Power Applications, S. Doǧan, A. Teke, D. Huang, Hadis Morkoç, C. B. Roberts, J. Parish, B. Ganguly, M. Smith, R. E. Meyers, S. E. Saddow
Electrical and Computer Engineering Publications
Siliconcarbide is a wide-band-gapsemiconductor suitable for high-power high-voltage devices and it has excellent properties for use in photoconductive semiconductor switches (PCSSs). PCSS were fabricated as planar structures on high-resistivity 4H–SiC and tested at dc bias voltages up to 1000 V. The typical maximum photocurrent of the device at 1000 V was about 49.4 A. The average on-state resistance and the ratio of on-state to off-state currents were about 20 Ω and 3×1011, respectively. Photoconductivity pulse widths for all applied voltages were 8–10 ns. These excellent results are due in part to the removal of the surface damage by high-temperature H2 …
Improvement Of N-Gan Schottky Diode Rectifying Characteristics Using Koh Etching, J. Spradlin, S. Doğan, M. Mikkelson, D. Huang, L. He, D. Johnstone, Hadis Morkoç, R. J. Molnar
Improvement Of N-Gan Schottky Diode Rectifying Characteristics Using Koh Etching, J. Spradlin, S. Doğan, M. Mikkelson, D. Huang, L. He, D. Johnstone, Hadis Morkoç, R. J. Molnar
Electrical and Computer Engineering Publications
KOH etch was investigated as a means to improve the I–Vcharacteristics of Schottky diodes onn-type GaNgrown by molecular-beam epitaxy on sapphire, or on hydride vapor phase epitaxy templates. Atomic force microscopy images and I–Vcharacteristics are presented. After etching as-grown films in molten KOH, Schottky diodes on c-plane GaN had orders of magnitude reduction in reverse leakage current. The best devices had leakage currents less than 10−12 A (10−8 A/cm2) at −5 V, and ideality factors of 1.04. Measurements on several different sample structures indicate a correlation between surface roughness and saturation current, and an improvement in ideality …
Electric-Field-Induced Heating And Energy Relaxation In Gan, T. A. Eckhause, Ö. Süzer, Ç. Kurdak, F. Yun, Hadis Morkoç
Electric-Field-Induced Heating And Energy Relaxation In Gan, T. A. Eckhause, Ö. Süzer, Ç. Kurdak, F. Yun, Hadis Morkoç
Electrical and Computer Engineering Publications
Electric-field-induced heating is studied using noise measurements in n-type GaN grown on sapphire substrates. The measured electron temperature is found to be an order of magnitude higher than what is expected based on calculations of electron–phonon coupling via acoustic deformation potential scattering processes in GaN. The discrepancy may be explained by a large thermal boundary resistance between the GaN film and the sapphire substrate.
Current Mapping Of Gan Films By Conductive Atomic Force Microscopy, A. A. Pomarico, D. Huang, J. Dickinson, A. A. Baski, R. Cingolani, Hadis Morkoç, R. Molnar
Current Mapping Of Gan Films By Conductive Atomic Force Microscopy, A. A. Pomarico, D. Huang, J. Dickinson, A. A. Baski, R. Cingolani, Hadis Morkoç, R. Molnar
Electrical and Computer Engineering Publications
Conductive atomic force microscopy has been used to investigate the local conductivity in hydride vapor-phase epitaxy and molecular-beam epitaxyGaN films, focusing on the effect of off-axis facet planes. We investigated two different types of samples, in which the facet planes were either present on the perimeters of as-grown islands, or on the edges of etch pits created by post-growth chemical etching. The results show that crystallographic planes tilted with respect to the c-plane growth direction show a significantly higher conductivity than surrounding areas. The n-type (or p-type) samples required a negative (or positive) sample bias for current conduction, …
Observation Of Optical Phonon Instability Induced By Drifting Electrons In Semiconductor Nanostructures, W. Liang, K. T. Tsen, Otto F. Sankey, S. M. Komirenko, K. W. Kim, V. A. Kochelap, Meng-Chyi Wu, Chong-Long Ho, Wen-Jeng Ho, Hadis Morkoç
Observation Of Optical Phonon Instability Induced By Drifting Electrons In Semiconductor Nanostructures, W. Liang, K. T. Tsen, Otto F. Sankey, S. M. Komirenko, K. W. Kim, V. A. Kochelap, Meng-Chyi Wu, Chong-Long Ho, Wen-Jeng Ho, Hadis Morkoç
Electrical and Computer Engineering Publications
We have experimentally proven the Cerenkov generation of optical phonons by drifting electrons in a semiconductor. We observe an instability of the polar optical phonons in nanoscale semiconductors that occurs when electrons are accelerated to very high velocities by intense electric fields. The instability is observed when the electron drift velocity is larger than the phase velocity of optical phonons and rather resembles a “sonic boom” for optical phonons. The effect is demonstrated in p–i–nsemiconductor nanostructures by using subpicosecond Raman spectroscopy.
Conductance Modulation Of Spin Interferometers, Marc Cahay, Supriyo Bandyopadhyay
Conductance Modulation Of Spin Interferometers, Marc Cahay, Supriyo Bandyopadhyay
Electrical and Computer Engineering Publications
We study the conductance modulation of gate controlled electron spin interferometers (also known as spin field effect transistors) based on the Rashba spin–orbit coupling effect. It is found that the modulation is dominated by Ramsauer (or Fabry-Perot) type transmission resonances rather than the Rashba effect in typical structures. These transmission resonances are due to reflections at the interferometer’s contacts caused by large interface potential barriers and effective mass mismatch between the contact material and the semiconductor. They are particularly strong in quasi-one-dimensional structures which, in fact, are preferred for spin interferometers because of the energy independence of the spin precession …
Generation–Recombination Noise In Gallium Nitride-Based Quantum Well Structures, Rolando S. Duran, Grover L. Larkins Jr., Carolyn M. Van Vliet, Hadis Morkoç
Generation–Recombination Noise In Gallium Nitride-Based Quantum Well Structures, Rolando S. Duran, Grover L. Larkins Jr., Carolyn M. Van Vliet, Hadis Morkoç
Electrical and Computer Engineering Publications
Electronic noise has been investigated in AlxGa1−xN/GaN modulation-doped field-effect transistors of submicron dimensions, grown by molecular beam epitaxy techniques. Some 20 devices were grown on a sapphire substrate. Conduction takes place in the quasi-two-dimensional (2D) layer of the junction (xy plane) which is perpendicular to the triangular quantum well (z direction). A nondoped intrinsic buffer layer separates the Si-doped donors in the AlxGa1−xN layer from the 2D transistor plane. Since all contacts must reach through the AlxGa1−xN layer to connect internally to the 2D plane, parallel conduction through this layer is a feature of all modulation-doped devices. The …
Infrared Optical Absorbance Of Intersubband Transitions In Gan/Algan Multiple Quantum Well Structures, Qiaoying Zhou, Jiayu Chen, B. Pattada, M. O. Manasreh, Faxian Xiu, Steve Putigan, K. S. Ramaiah, Hadis Morkoç
Infrared Optical Absorbance Of Intersubband Transitions In Gan/Algan Multiple Quantum Well Structures, Qiaoying Zhou, Jiayu Chen, B. Pattada, M. O. Manasreh, Faxian Xiu, Steve Putigan, K. S. Ramaiah, Hadis Morkoç
Electrical and Computer Engineering Publications
Intersubband transitions in Si-doped molecular beam epitaxygrown GaN/AlGaN multiple quantum wells on c-plane sapphire were investigated using the Fourier-transform infrared optical absorption technique. Several GaN quantum well samples were grown with either AlGaN bulk or GaN/AlGaN short period superlattice barriers. The measurements were made in a waveguide configuration utilizing a facet polished at 45° to the c plane. The integrated area of the intersubband transitions in several waveguides cut from different location of the wafer was measured, from which we estimated the two-dimensional electron gas density (σ). The measured values of σ are about two orders of magnitude larger …
Unusual Luminescence Lines In Gan, M. A. Reshchikov, D. Huang, F. Yun, P. Visconti, L. He, H. Morkoç, J. Jasinski, Z. Liliental-Weber, R. J. Molnar, S. S. Park, K. Y. Lee
Unusual Luminescence Lines In Gan, M. A. Reshchikov, D. Huang, F. Yun, P. Visconti, L. He, H. Morkoç, J. Jasinski, Z. Liliental-Weber, R. J. Molnar, S. S. Park, K. Y. Lee
Electrical and Computer Engineering Publications
A series of sharp intense peaks was observed in the low-temperature photoluminescence spectrum of unintentionally doped GaN in the photon energy range between 3.0 and 3.46 eV. We attributed the majority of these peaks to excitons bound to unidentified structural and surface defects. Most of the structural- and surface-related peaks (at 3.21, 3.32, 3.34, 3.35, 3.38, and 3.42 eV) were observed in Ga polar films. In N polar GaN, we often observed the 3.45 eV peak attributed to excitons bound to the inversion domain interfaces.
Contactless Electroreflectance, In The Range Of 20 K < T < 300 K, Of Freestanding Wurtzite Gan Prepared By Hydride-Vapor-Phase Epitaxy, Y. S. Huang, Fred H. Pollak, S. S. Park, K. Y. Lee, H. Morkoç
Contactless Electroreflectance, In The Range Of 20 K < T < 300 K, Of Freestanding Wurtzite Gan Prepared By Hydride-Vapor-Phase Epitaxy, Y. S. Huang, Fred H. Pollak, S. S. Park, K. Y. Lee, H. Morkoç
Electrical and Computer Engineering Publications
We have performed a detailed contactless electroreflectance study of the interband excitonic transitions on both the Ga and N faces of a 200-μm-thick freestanding hydride-vapor-phase-epitaxy grown wurtzite GaN sample with low defect concentration in the temperature range between 20 and 300 K. The transition energies of the A, B, and C excitons and broadening parameters of the A and B excitons have been determined by least-square fits to the first derivative of a Lorentzian line shape. The energy positions and separations of the excitonic transitions in the sample reveal the existence of residual strain. At 20 K the broadening parameter …
Routing Protocols For Self-Organizing Hierarchical Ad-Hoc Wireless Networks, Suli Zhao, Kemal Tepe, Ivan Seskar, Dipankar Raychaudhuri
Routing Protocols For Self-Organizing Hierarchical Ad-Hoc Wireless Networks, Suli Zhao, Kemal Tepe, Ivan Seskar, Dipankar Raychaudhuri
Electrical and Computer Engineering Publications
—A novel self-organizing hierarchical architecture is proposed for improving the scalability properties of adhoc wireless networks. This paper focuses on the design and evaluation of routing protocols applicable to this class of hierarchical ad-hoc networks. The performance of a hierarchical network with the popular dynamic source routing (DSR) protocol is evaluated and compared with that of a conventional “flat” ad-hoc networks using an ns-2 simulation model. The results for an example sensor network scenario show significant capacity increases with the hierarchical architecture (∼4:1). Alternative routing metrics that account for energy efficiency are also considered briefly, and the effect on user …