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Item Open Access Efficiency and harmonic enhancement trends in GaN-based Gunn diodes: Ensemble Monte Carlo analysis(American Institute of Physics, 2004) Sevik, C.; Bulutay, C.Show more Gallium nitride can offer a high-power alternative for millimeter-wave Gunn oscillators. Hence, an ensemble Monte Carlo-based comprehensive theoretical assessment of efficiency and harmonic enhancement in n-type GaN Gunn diodes is undertaken. First, the effects of doping notch/mesa and its position within the active channel are investigated which favors a doping notch positioned next to cathode. It is then observed that the width of the notch can be optimized to enhance the higher-harmonic operation without degrading its performance at the fundamental mode. Next, the effects of dc bias and channel doping density are investigated. Both of these have more significant effects on the higher-harmonic efficiency than the fundamental one. The lattice temperature is observed to have almost no influence up to room temperature but severely degrades the performance above room temperature. As a general behavior, the variations of temperature, channel doping, and the notch width primarily affect the phase angle between the current and voltage wave forms rather than the amplitude of oscillations. Finally, the physical origin of these Gunn oscillations is sought which clearly indicates that the intervalley scattering mechanism is responsible rather than the Γ valley nonparabolicity or the effective mass discrepancy between the Γ and the lowest satellite valleys.Show more Item Open Access Electric breakdown in polycrystalline semiconductors with highly nonlinear I-V characteristics: Simulations for simple barrier height models(1997) Yildirim H.E.; Tanatar, Bilal; Canessa, E.Show more An extension of the Canessa and Nguyen binary model for the nonlinear current-voltage (I-V) characteristics of polycrystalline semiconductors, based on the electrical properties of individual grains, is presented. Simple analytical models for the nonuniform distribution of barrier heights at grain boundaries are assumed. The set of nonlinear Kirchhoff equations, that determine the macroscopic current across the specimen, and the nonlinearity coefficient α are solved numerically. The applied voltage dependence of the barrier height models gives α values reaching ≈ 50, indicating high nonlinearity as required by potential commercial applications. © Tübi̇tak.Show more Item Open Access Electronic transport through a kink in an electron waveguide(Institute of Electrical and Electronics Engineers, 1994) Yalabik, M. C.Show more The current-voltage denendence correspondinp to electronic transport through a kink in an electronic waveguide is analyzed. No phase breaking dissipation mechanisms are considered, but the effects of the Coulomb interaction are included through a self consistent approximation. The results indicate very nonlinear transport properties, including negative differential resistance and bistability. © 1994 IEEEShow more Item Open Access Fast direct (noniterative) solvers for integral-equation formulations of scattering problems(IEEE, 1998) Gürel, Levent; Chew, W. C.Show more A family of direct (noniterative) solvers with reduced computational complexity is proposed for solving problems involving resonant or near-resonant structures. Based on the recursive interaction matrix algorithm, the solvers exploit the aggregation concept of the recursive aggregate T-matrix algorithm to accelerate the solution. Direct algorithms are developed to compute the scattered field and the current coefficient, and invert the impedance matrix. Computational complexities of these algorithms are expressed in terms of the number of harmonics P required to express the scattered field of a larger scatterer made up of N scatterers. The exact P-N relation is determined by the geometry.Show more Item Open Access High-performance solar-blind photodetectors based on AlxGa 1_xN heterostructures(IEEE, 2004) Özbay, Ekmel; Bıyıklı, Necmi; Kimukin, I.; Kartaloglu, T.; Tut, T.; Aytür, O.Show more Design, fabrication, and characterization of high-performance AI xGa1-xN-based photodetectors for solar-blind applications are reported. AlxGa1-xN heterostructures were designed for Schottky. p-i-n, and metal-semicondnctor-metal (MSM) photodiodes. The solar-blind photodiode samples were fabricated using a microwave compatible fabrication process. The resulting devices exhibited extremely low dark currents. Below 3 fA, leakage currents at 6-V reverse bias were measured on p-i-n samples. The excellent current-voltage (I-V) characteristics led to a detectivity performance of 4.9×1014 cmHz1/2W -1. The MSM devices exhibited photoconductive gain, while Schottky and p-i-n samples displayed 0.09 and 0.11 A/W peak responsivity values at 267 and 261 nm, respectively. A visible rejection of 2×104 was achieved with Schottky samples. High-speed measurements at 267 nm resulted in fast pulse responses with greater than gigahertz bandwidths. The fastest devices were MSM photodiodes with a maximum 3-dB bandwidth of 5.4 GHz.Show more Item Open Access In-situ focused ion beam implantation for the fabrication of a hot electron transistor oscillator structure(1996) Kaya I.I.; Dellow, M.W.; Bending, S.J.; Linfield, E.H.; Rose P.D.; Ritchie, D.A.; Jones G.A.C.Show more Recent advances using in situ focused ion beam implantation during an MBE growth interruption have been exploited to fabricate planar GaAs hot electron structures without the need for shallow ohmic contacts. This novel fabrication route shows a very high yield and has been used to demonstrate a prototype high-frequency oscillator structure based on electron multiplication in the base layer. Existing devices show transfer factors in excess of unity as well as reversal of the base current at high injection levels, which are the prerequisites for oscillator action. Future improvements in device design are discussed.Show more Item Open Access ITO-schottky photodiodes for high-performance detection in the UV-IR spectrum(IEEE, 2004) Bıyıklı, Necmi; Kimukin, I.; Butun, B.; Aytür, O.; Özbay, EkmelShow more High-performance vertically illuminated Schottky photodiodes with indium-tin-oxide (ITO) Schottky layers were designed, fabricated, and tested. Ternary and quarternary III-V material systems (AlGaN-GaN, AlGaAs-GaAs, InAlGaAs-InP, and InGaAsP-InP) were utilized for detection in the ultraviolet (UV) (λ < 400 nm), near-IR (λ ∼ 850 nm), and IR (λ ∼ 1550 nm) spectrum. The material properties of thin ITO films were characterized. Using resonant-cavity-enhanced (RCE) detector structures, improved efficiency performance was achieved. Current-voltage, spectral responsivity, and high-speed measurements were carried out on the fabricated ITO-Schottky devices. The device performances obtained with different material systems are compared.Show more Item Open Access Low dark current metal-semiconductor-metal photodiodes based on semi-insulating GaN(AIP Publishing LLC, 2006) Bütün, S.; Gökkavas, M.; Yu, H.; Özbay, EkmelShow more Metal-semiconductor-metal photodetectors on semi-insulating GaN templates were demonstrated and compared with photodetectors fabricated on regular GaN templates. Samples were grown on a metal organic chemical vapor deposition system. Devices on semi-insulating template exhibited a dark current density of 1.96 × 10-10 A/cm2 at 50 V bias, which is four orders of magnitude lower compared with devices on regular template. Device responsivities were 101.80 and 88.63 A/W at 50 V bias for 360 nm ultraviolet illumination for semi-insulating and regular templates, respectively. Incident power as low as 3 pW was detectable using the devices that were fabricated on the semi-insulating template. © 2006 American Institute of Physics.Show more Item Open Access On the elemination of infinities in the PO component of equivalent edge currents(Elsevier, 1993) Oguzer, T.; Altıntaş, A.; Büyükdura, O. M.Show more Equivalent edge currents for the Physical Optics (PO) contribution to the edge diffraction are derived by choosing a proper coordinate system for a half plane. It is seen that these expressions for the PO component are free from singularities except at shadow and reflection boundaries on the Keller Cone. To find the total diffracted field, the expressions for the fringe contribution given earlier by Michaeli should be used along with the PO contribution obtained here. The procedure is applied to the problems of backscattering from square and triangular plates. Higher order diffractions are also accounted for by combining the equivalent currents with the Uniform Theory of Diffraction (UTD). The results obtained are compared with previous solutions and measured data.Show more Item Open Access A parametric analysis of finite phased arrays of printed dipoles on large circular cylinders and comparisons with the planar case(IEEE, 2004-06) Güner, B.; Ertürk Vakur B.; Bakır, O.Show more A parametric study of finite phased arrays of printed dipoles on electrically large coated cylinders is performed using a spatial domain hybrid MoM/Green's function technique. Dipole currents and several performance metrics like active reflection coefficient and input impedances of the dipoles are calculated and compared with their planar counterparts. The effects of the curvature, dipole orientation and changes in the array and host body parameters are observed and discussed.Show more Item Open Access Paraxial space-domain formulation for surface fields on dielectric coated circular cylinder(IEEE, 2002-11) Ertürk, V. B.; Rojas, R. G.Show more A new method to evaluate the surface fields excited within the paraxial (nearly axial) region of an electrically large dielectric coated circular cylinder is presented. This representation is obtained by performing the Watson’s transformation in the standard eigenfunction solution and using the fact that the circumferentially propagating series representation of the appropriate Green’s function is periodic in one of its two variables. Therefore, it can be approximated by a Fourier series where the two leading terms of the expansion yield engineering accuracy in most cases. This work can be used in conjunction with a method of moments solution for the design/analysis of conformal microstrip antennas and arrays. Numerical results are presented and compared with a standard eigenfunction expansion.Show more Item Open Access A self-consistent microscopic model of Coulomb interaction in a bilayer system as an origin of Drag Effect Phenomenon(Elsevier B.V., 2008) Güven, K.; Siddiki, A.; Krishna, P. M.; Hakioǧlu T.Show more In this work we implement the self-consistent Thomas-Fermi model that also incorporates a local conductivity model to an electron-electron bilayer system, in order to describe novel magneto-transport properties such as the Drag Phenomenon. The model can successfully account for the poor screening of the potential within the incompressible strips and its impact on the inter-layer Coulomb interaction. An externally applied current in the active layer results in the tilting of the Landau levels and built-up of a Hall potential across the layer, which, in turn, induces a tilted potential profile in the passive layer as well. We investigate the effect of the current intensity, temperature, magnetic field, and unequal density of layers on the self-consistent density and potential profiles of the bilayer system.Show more Item Open Access Spectrally accelerated biconjugate gradient stabilized method for scattering from and propagation over electrically large inhomogeneous geometries(John Wiley & Sons, 2005) Babaoglu, B.; Altintas, A.; Ertürk, V. B.Show more Scattering from and propagation over rough-terrain profiles, as well as reentrant surfaces are investigated using an integral equation (IE)-based spectrally accelerated biconjugate gradient stabilized (SA-BiCGSTAB) method, with a storage requirement and a computational cost of O(N) per iteration, where N is the surface unknowns in the discretized IE. Numerical results in the form of current and path loss are presented and compared with previously published as well as measured results in order to assess the accuracy and efficiency of this method.Show more Item Open Access Spontaneous and persistent currents in mesoscopic Aharonov-Bohm loops: Static properties and coherent dynamic behavior in crossed electric and magnetic fields(M A I K Nauka - Interperiodica, 2005) Kulik, I. O.Show more Mesoscopic or macromolecular conducting rings with a fixed number of electrons are shown to support persistent currents due to the Aharonov-Bohm flux, and the "spontaneous" persistent currents without the flux when structural transformation in the ring is blocked by strong coupling to the externally azimuthal-symmetric environment. In the free-standing macromolecular ring, symmetry breaking removes the azimuthal periodicity, which is further restored at the increasing field, however. The dynamics of the Aharonov-Bohm loop in crossed electric and magnetic fields is investigated within the tight-binding approximation; we show that transitions between discrete quantum states occur when static voltage pulses of prescribed duration are applied to the loop. In particular, the three-site ring with one or three electrons is an interesting quantum system that can serve as a qubit (quantum bit of information) and a qugate (quantum logical gate) because in the presence of an externally applied static electric field perpendicular to a magnetic field, the macromolecular ring switches between degenerate ground states mimicking the NOT and Hadamard gates of quantum computers.Show more Item Open Access Thermally stimulated currents in n-InS single crystals(Elsevier Science, 2003) Gasanly, N. M.; Aydınlı, Atilla; Yuksek, N. S.Show more Thermally stimulated current measurements are carried out on as-grown n-InS single crystals in the temperature range of 10-125 K. Experimental evidence is found for four trapping centers present in InS. They are located at 20, 35, 60 and 130meV. The trap parameters have been determined by various methods of analysis, and they agree well with each other.Show more Item Unknown Time-domain equivalent edge currents for transient scattering(IEEE, 2001) Altıntaş, A.; Russer, P.Show more Time-domain equivalent edge currents (TD-EEC) are developed for the transient scattering analysis. The development is based on the Fourier inversion of frequency domain equivalent edge current expressions. The time-domain diffracted fields are expressed in terms of a contour integral along the diffracting edges for any arbitrary input pulse shape, thereby yielding finite results at the caustics of diffracted rays. The approach also eliminates the need for the evaluation of a convolution integral in the time domain geometrical theory of diffraction (GTD) analysis. The results are compared with the first order GTD results for the transient scattering analysis for a circular disk.Show more Item Open Access A Wired-AND Current-Mode Logic Circuit Technique in CMOS for Low-Voltage, High-Speed and Mixed-Signal VLSIC(1997) Ungan I.E.; Aşkar, M.Show more A wired-AND current-mode logic (WCML) circuit technique in CMOS technology for low-voltage and high-speed VLSI circuits is proposed, and a WCML cell library is developed using standard 0.8 micron CMOS process. The proposed WCML technique applies the analog circuit design methodologies to the digital circuit design. The input and output logic signals are represented by current quantities. The supply current of the logic circuit is adjustable for the required logic speed and the switching noise level. The noise is reduced on the power supply lines and in the substrate by the current-steering technique and by the smooth swing of the reduced node potentials. Precise analog circuits and fast digital circuits can be integrated on the same silicon substrate by using the low noise property of the WCML. It is shown by the simulations that at low supply voltages, the WCML is faster and generates less switching noise when compared to the static-CMOS logic. At high speeds, the power dissipation of the WCML is less than that of the static-CMOS logic.Show more