Browsing by Author "Virginia Tech. Bradley Department of Electrical and Computer Engineering"

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    Control of thermal runaway in microwave resonant cavities
    Wu, X.; Thomas, J. R.; Davis, William A. (American Institute of Physics, 2002-09-15)
    This article reports direct experimental evidence of the so-called "S curve" of temperature versus electrical field strength when materials with positive temperature dependence of dielectric loss are heated in a microwave resonant cavity applicator. A complete discussion of how the experimental results were achieved is presented. From the experimental results, we believe the S curve theory provides an incomplete explanation of thermal runaway in microwave heating. To understand microwave heating in a resonant cavity, cavity effects must be considered. To explain the experimental results, a theoretical model based on single-mode waveguide theory is developed. Finally, a method to control thermal runaway is described. (C) 2002 American Institute of Physics.
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    Defect assistant band alignment transition from staggered to broken gap in mixed As/Sb tunnel field effect transistor heterostructure
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, Amy K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-11-01)
    The compositional dependence of effective tunneling barrier height (E-beff) and defect assisted band alignment transition from staggered gap to broken gap in GaAsSb/InGaAs n-channel tunnel field effect transistor (TFET) structures were demonstrated by x-ray photoelectron spectroscopy (XPS). High-resolution x-ray diffraction measurements revealed that the active layers are internally lattice matched. The evolution of defect properties was evaluated using cross-sectional transmission electron microscopy. The defect density at the source/channel heterointerface was controlled by changing the interface properties during growth. By increasing indium (In) and antimony (Sb) alloy compositions from 65% to 70% in InxGa1-xAs and 60% to 65% in GaAs1-ySby layers, the E-beff was reduced from 0.30 eV to 0.21 eV, respectively, with the low defect density at the source/channel heterointerface. The transfer characteristics of the fabricated TFET device with an E-beff of 0.21eV show 2x improvement in ON-state current compared to the device with E-beff of 0.30 eV. On contrary, the value of E-beff was decreased from 0.21 eV to -0.03 eV due to the presence of high defect density at the GaAs0.35Sb0.65/In0.7Ga0.3As heterointerface. As a result, the band alignment was converted from staggered gap to broken gap, which leads to 4 orders of magnitude increase in OFF-state leakage current. Therefore, a high quality source/channel interface with a properly selected E-beff and well maintained low defect density is necessary to obtain both high ON-state current and low OFF-state leakage in a mixed As/Sb TFET structure for high-performance and lower-power logic applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4764880]
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    Design, fabrication, and analysis of p-channel arsenide/antimonide hetero-junction tunnel transistors
    Rajamohanan, Bijesh; Mohata, Dheeraj K.; Zhu, Yan; Hudait, Mantu K.; Jiang, Zhengping; Hollander, Matthew; Klimeck, Gerhard; Datta, Suman (American Institute of Physics, 2014-01-23)
    In this paper, we demonstrate InAs/GaSb hetero-junction (hetJ) and GaSb homo-junction (homJ) p-channel tunneling field effect transistors (pTFET) employing a low temperature atomic layer deposited high-kappa gate dielectric. HetJ pTFET exhibited drive current of 35 mu A/mu m in comparison to homJ pTFET, which exhibited drive current of 0.3 mu A/mu m at V-DS = -0.5V under DC biasing conditions. Additionally, with pulsing of 1 mu s gate voltage, hetJ pTFET exhibited enhanced drive current of 85 mu A/mu m at V-DS = -0.5 V, which is the highest reported in the category of III-V pTFET. Detailed device characterization was performed through analysis of the capacitance-voltage characteristics, pulsed current-voltage characteristics, and x-ray diffraction studies. (C) 2014 AIP Publishing LLC.
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    ECE 2013 Annual Report
    Virginia Tech. Bradley Department of Electrical and Computer Engineering (Bradley Department of Electrical and Computer Engineering, 2013)
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    A game-theoretic framework to investigate the conditions for cooperation between energy storage operators and wind power producers
    Bhela, Siddharth; Tam, Kwa-sur (Virginia Tech, 2015-06)
    Energy storage, has widely been accepted as a means to provide capacity firming service to renewable sources of energy due to its capability to quickly start and shut down and its ability to have flexible ramping rates. Lithium Ion batteries in particular are of interest as their production cost is expected to significantly decrease over the next few years. In addition, Li-Ion batteries have high efficiency, high energy density and high cycling tolerance. These batteries are also used in electric vehicles whose penetration is expected to grow rapidly in the coming years. The social benefit of energy storage to provide energy balancing service to renewable producers is evident, especially in the context of a micro-grid where deviations from distributed generation sources can be handled locally. However, co-operation with renewable producers may not be automatically guaranteed and would depend on the amount of revenue generated by balancing such deviations. Storage may derive more benefit from choosing to operate independently. Balancing wind deviations would take capacity away from providing other high value services to the micro-grid community such as arbitrage and regulation service. The decision to enter the market and balance deviations for the wind producer is highly intertwined with the strategy adopted by the wind producer. Interactive problems in which the outcome of a rational agent's action depends on the actions of other rational players are best studied through the setup of a game-theoretic framework. A case-study is presented here using wind and electricity market data for a site in west Texas. Historical data is used to calculate expected pay-offs for the month of January. The columns in the following table are the available strategies for the wind producer and the rows are the available strategies for the energy storage. There are four possible combination of strategies, which are discussed next: The pay-off table provides the net revenues of the wind producer in the upper right corner and the net revenues of the energy storage in the lower left corner of each cell. Note that revenue from Production Tax Credits (PTC) is not included for the wind producer.
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    A Markov process model of ocean sediments
    Sockell, Michael; Besieris, Ioannis M.; Kohler, Werner E.; Freese, Herbert (Acoustical Society of America, 1985-01-01)
    Monochromatic plane-wave illumination of a randomly stratified, laterally homogeneous sediment layer is considered. The deposition process creating the stochastic layering is assumed to be a continuous parameter, finite state Markov chain. A Riccati equation for the plane-wave reflection coefficient is formulated and first-order partial differential equations for relevant probability density functions are subsequently obtained. These equations are solved numerically for a two-material turbidite model similar to the one considered by Gilbert [J. Acoust. Soc. Am. 68, 1454-1458 (1980)]. Statistical moments of the reflection coefficient are computed at 25 and 250 Hz as a function of overall sediment thickness. These equations are also used to derive the nonrandom or "smooth" geoacoustic model that is appropriate in the low-frequency limit.
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    Quasi-zero lattice mismatch and band alignment of BaTiO3 on epitaxial (110)Ge
    Hudait, Mantu K.; Zhu, Yizheng; Jain, Nikhil; Maurya, Deepam; Zhou, Y.; Priya, Shashank (American Institute of Physics, 2013-07-14)
    Growth, structural, and band alignment properties of pulsed laser deposited amorphous BaTiO3 on epitaxial molecular beam epitaxy grown (110) Ge layer, as well as their utilization in low power transistor are reported. High-resolution x-ray diffraction demonstrated quasi-zero lattice mismatch of BaTiO3 on (110) Ge. Cross-sectional transmission electron microscopy micrograph confirms the amorphous nature of BaTiO3 layer as well as shows a sharp heterointerface between BaTiO3 and Ge with no traceable interfacial layer. The valence band offset, Delta E-v, of 1.99 +/- 0.05 eV at the BaTiO3/(110) Ge heterointerface is measured using x-ray photoelectron spectroscopy. The conduction band offset, Delta E-c, of 1.14 +/- 0.1 eV is calculated using the bandgap energies of BaTiO3 of 3.8 eV and Ge of 0.67 eV. These band offset parameters for carrier confinement and the interface chemical properties of the BaTiO3/(110) Ge system are significant advancement towards designing Ge-based p-and n-channel metal-oxide semiconductor field-effect transistors for low-power application. (C) 2013 AIP Publishing LLC.
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    Role of InAs and GaAs terminated heterointerfaces at source/channel on the mixed As-Sb staggered gap tunnel field effect transistor structures grown by molecular beam epitaxy
    Zhu, Yizheng; Jain, Nikhil; Vijayaraghavan, S.; Mohata, Dheeraj K.; Datta, Suman; Lubyshev, Dmitri; Fastenau, Joel M.; Liu, W. K.; Monsegue, Niven; Hudait, Mantu K. (American Institute of Physics, 2012-07-15)
    The structural, morphological, defect properties, and OFF state leakage current mechanism of mixed As-Sb type-II staggered gap GaAs-like and InAs-like interface heterostructure tunnel field effect transistors (TFETs) grown on InP substrates using linearly graded InxAl1-xAs buffer by molecular beam epitaxy are investigated and compared. Symmetric relaxation of >90% and >75% in the two orthogonal < 110 > directions with minimal lattice tilt was observed for the terminal GaAs0.35Sb0.65 and In0.7Ga0.3As active layers of GaAs-like and InAs-like interface TFET structures, respectively, indicating that nearly equal numbers of alpha and beta dislocations were formed during the relaxation process. Atomic force microscopy reveals extremely ordered crosshatch morphology and low root mean square roughness of similar to 3.17 nm for the InAs-like interface TFET structure compared to the GaAs-like interface TFET structure of similar to 4.46 nm at the same degree of lattice mismatch with respect to the InP substrates. The GaAs-like interface exhibited higher dislocation density, as observed by cross-sectional transmission electron microscopy, resulting in the elongation of reciprocal lattice point of In0.7Ga0.3As channel and drain layers in the reciprocal space maps, while the InAs-like interface creates a defect-free interface for the pseudomorphic growth of the In0.7Ga0.3As channel and drain layers with minimal elongation along the Delta omega direction. The impact of the structural differences between the two interface types on metamorphic TFET devices was demonstrated by comparing p(+)-i-n(+) leakage current of identical TFET devices that were fabricated using GaAs-like and InAs-like interface TFET structures. Higher OFF state leakage current dominated by band-to-band tunneling process due to higher degree of defects and dislocations was observed in GaAs-like interface compared to InAs-like interface where type-II staggered band alignment was well maintained. Significantly lower OFF state leakage current dominated by the field enhanced Shockley-Read-Hall generation-recombination process at different temperatures was observed in InAs-like TFET structure. The fixed positive charge at the source/channel heterointerface influences the band lineup substantially with charge density greater than 1 x 10(12)/cm(2) and the band alignment is converted from staggered gap to broken gap at similar to 6 x 10(12)/cm(2). Clearly, InAs-like interface TFET structure exhibited 4x lower OFF state leakage current, which is attributed primarily to the impact of the layer roughness, defect properties on the carrier recombination rate, suggesting great promise for metamorphic TFET devices for high-performance, and ultra-low power applications. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4737462]
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    Time resolved measurements of spin and carrier dynamics in InAs films
    Kini, Rajeev N.; Nontapot, Kanokwan; Khodaparast, Giti A.; Welser, Roger E.; Guido, Louis J. (American Institute of Physics, 2008-03-15)
    We report time resolved measurements of spin and carrier relaxation in InAs films with carrier densities of 1.3 x 10(16) and 1.6 x 10(16) cm(-3) grown on (001) and (111) GaAs, respectively. We used standard pump-probe and magneto-optical Kerr effect spectroscopy at different excitation wavelengths, power densities, and temperatures. We observed sensitivity of carrier and spin relaxation time to the photoinduced carrier density but not to the variation in temperature. We explain our results using the Elliot-Yafet picture of spin relaxation process in narrow gap semiconductors. (c) 2008 American Institute of Physics.
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    A transport-theoretic analysis of pulse propagation through ocean sediments
    Besieris, Ioannis M.; Kohler, Werner E.; Freese, Herbert (Acoustical Society of America, 1982-09-01)
    The reflection of pulsed acoustic plane waves from ocean sedimentary layers is studied using a stochastic transport theory originally introduced by Barabanenkov et. al. [Izv. Vyssh. Uchebn. Zaved., Radiofiz 15, 1852 (1972)]. The sediments are assumed to be a random medium in which the density and sound speed undergo small, highly laminated (pancakelike) fluctuations. Although the problem is formulated in a general context, the predictions of the theory are fully evaluated only in the special case of normal incidence and no refracting profile. However, even with these approximations, some reasonable qualitative agreement of theoretical predictions with measured data is achieved.