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  • 1
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 85 (1999), S. 2667-2680 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Potential barrier profiles for large applied fields and/or high temperature are developed for the study of field and thermionic emission electron sources intended for radio frequency power tube applications. The numerical implementation provides a fast and flexible method to obtain the barriers which govern current density, and yet allows for complications such as nanoprotrusions, adsorbates, "internal" field emission, the sputtering of low work function emission sites, and so on. The model consists of (i) a modified form of the Wigner Lattice expansion of the electron ground state energy to evaluate the exchange and correlation potential, (ii) a simplified form of the ionic core potential to correct the "Jellium" model, (iii) a triangular representation of the barrier with a single adjustable parameter which enables both the solution of Schrödinger's equation in terms of Airy functions and thus an exact evaluation of the electron density near the barrier, and (iv) a numerical integration of Poisson's equation to evaluate the dipole potential and positive background boundary. An iterative calculation is performed such that the barrier used in the solution of Schrödinger's equation becomes equivalent to the barrier predicted from the exchange-correlation and dipole potentials. As a test of the method, evaluations of the work function of various metals are made. A good correspondence is found between the potential profiles and an "analytic" image charge potential (which contains modifications to the standard image charge model). Modifications to the Richardson–Laue–Dushman and Fowler Nordheim equations, so as to obtain current density estimates, are described. The (only) adjustable parameter used to correlate theory and experimental work functions is the magnitude of the ionic core "radius," which is often close to the actual radius of the metal ions in the test cases considered. The temperature and field dependence of the work function, which is dependent upon electron penetration of the barrier and its effect on the dipole potential, are investigated. The method is suggested to be suitable for the analysis of more complex potential barrier profiles that are encountered in actual (realistic) thermionic and field emission electron sources. The limitations of the model are discussed and methods to circumvent them are proposed. © 1999 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 2
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: We have developed a high-resolution ac susceptometer that uses a rf superconducting quantum interference device to directly measure the flux coupled into a superconducting detection coil from a sample's changing magnetic moment in an applied ac field. The system operates in a frequency range from 0.01 to 1500 Hz and an applied ac field range of 0.1–400 μT with a sensitivity of about 5×10−12 A m2 for magnetic moment measurement, and at a reduced sensitivity down to 0.001 Hz. The instrument is based on an existing dc magnetometer system and uses that system's temperature control and dc superconducting magnet to allow operation over a temperature range from 2 to 400 K and in applied dc fields of ±5.0 T. During a measurement all operations are controlled automatically by computer from a menu-driven software system, with user input required only on initiation of a measurement sequence. Both real and imaginary components of the ac susceptibility can be determined.
    Type of Medium: Electronic Resource
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  • 3
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 77 (2000), S. 585-587 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A field emitter array held at the threshold of emission by a dc gate potential from which current pulses are triggered by the application of a laser pulse on the backside of the semiconductor may produce electron bunches ("density modulation") at gigahertz frequencies. We develop an analytical model of such optically controlled emission from a silicon tip using a modified Wentzel–Kramers–Brillouin and Airy function approach to solving Schrödinger's equation. Band bending and an approximation to the exchange-correlation effects on the image charge potential are included for an array of hyperbolic emitters with a distribution in tip radii and work function. For a simple relationship between the incident photon flux and the resultant electron density at the emission site, an estimation of the tunneling current is made. An example of the operation and design of such a photon-assisted field emission device is given. © 2000 American Institute of Physics.
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  • 4
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 55 (1989), S. 669-671 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The Wigner particle trajectories in phase space for double-barrier semiconductor quantum well structures are investigated by numerically calculating the Wigner distribution function. The determination of the particle dynamics of the phase points allows for an estimation of tunneling times, particle currents, and particle energies associated with the statistics of bound and open trajectories across the double-barrier structures.
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  • 5
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 83 (1998), S. 7982-7992 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: An analytical model of a unit cell and of an array of field emitters with a distribution of tip radii is used to estimate the total current and current characteristics on the basis of geometry and materials. Based on the unit cell modeling, analytical estimates of ring cathode inductance, resistance, and capacitance are made to estimate the drive power required to sinusoidally modulate the array in order to produce a bunched electron beam. For a configuration of parameters indicative of a next generation field emitter array (FEA) ring cathode, the characteristics of the array are used to estimate the gain, efficiency, power output, and optimized length of an emission-gated Twystrode (TWT) using a simple model of the beam-wave interaction. The integrated analytical approach and its numerical implementation ("Cassandra") are validated by comparison to a 1D TWT code (CHRISTINE) which estimates the output power and the optimized length of a TWT. Using "Cassandra," it is shown that with next generation FEA parameters, interesting and significant levels of performance may be anticipated for a compact, broadband rf inductive output amplifier; in particular, an electronic efficiency of 32% with 15 dB gain may be possible from an array producing 260 mA peak and 71 mA average current at 10 GHz modulation using a helix 1.51 cm long.
    Type of Medium: Electronic Resource
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  • 6
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 67 (1990), S. 2153-2155 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The development of a more accurate numerical scheme for simulating double-barrier semiconductor structures has highlighted sensitivities of the computational results to numerical parameters for the different approximation schemes. In numerically evaluating the time evolution of the Wigner function, a second-order differencing scheme (SDS) was used instead of a simple up/down wind differencing scheme (UDS). In our investigations of the numerical aspects of these schemes, we have found: (a) the proximity of the "computational box'' boundaries to the double-barrier region affects the peak-to-valley ratio of the I-V curve and the value of the bias at peak current; (b) the peak-to-valley ratio is larger for the SDS than it is for the UDS; (c) the current at the resonant bias for SDS is larger than that calculated using UDS; (d) the rise in the current in the nonresonant regions for both SDS and UDS is dependent on how the bias is applied; and (e) the presence of an accumulation of electrons in the first heterojunction of the first barrier provides a closer correspondence between simulation and experimentally observed I-V.
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  • 7
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: The relaxation-time approximation is used in the numerical simulation of the Wigner distribution function to incorporate scattering. The effects within the constant relaxation-time approximation are (a) a decrease in the peak-to-valley ratio of the current-voltage curve; (b) a reduction in the oscillations of the Wigner distribution function, especially at resonance bias; (c) a suppression of the decay time of current oscillations after a sudden bias shift, indicating a smaller switching time than for no scattering; (d) a degradation in the resonant tunneling trajectories towards the characteristics of nonresonant trajectories; (e) a decrease in the spatial range of the quantum influences near resonance; and (f) ballistic transport sets in [i.e., the mean free path of the electrons is greater than the barrier region (110 A(ring))] for temperatures less than 74 K.
    Type of Medium: Electronic Resource
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  • 8
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 68 (1996), S. 400-402 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: n-type doping of AlxGa1−xSb epilayers (0≤x≤1) grown by organometallic vapor phase epitaxy has been achieved by using tritertiarybutylaluminum, triethylgallium, and trimethylantimony as the organometallic precursors and diethyltellurium as the doping source. Electron concentrations exceed 1017 cm−3 for layers with x〈0.3, and decrease to ∼1016 cm−3 for x=1 as a result of higher residual acceptor concentration. Lattice-mismatched double-heterostructure diode lasers with AlGaSb cladding layers and GaSb active layer are demonstrated, and indicate the potential of OMVPE for growth of GaSb-based materials for electronic and optoelectronic devices. © 1996 American Institute of Physics.
    Type of Medium: Electronic Resource
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  • 9
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 63 (1993), S. 702-704 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Field emission from tips, in comparison to planar surfaces, is complicated by current enhancement effects and changes in the image charge. We show that the approximations typically used for the calculation of the transmission coefficient are inappropriate for small spherical surfaces but adequate for elliptical surfaces. A simple analytical formula for the average emitting area may be derived based on the Fowler–Nordheim equation under conditions of field emission. By use of the elliptical approximation, a simple scale factor is sufficient to deal with spherical and pointed tips.
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  • 10
    Electronic Resource
    Electronic Resource
    Woodbury, NY : American Institute of Physics (AIP)
    Applied Physics Letters 69 (1996), S. 3941-3943 
    ISSN: 1077-3118
    Source: AIP Digital Archive
    Topics: Physics
    Notes: Dark, nonemissive defects form on the metal cathode in most molecular organic and polymer-based light emitting devices and eventually lead to the failure of the device. These defects have been characterized in situ using optical microscopy and ex situ using atomic force microscopy and Auger electron spectroscopy. On the basis of these observations, an electromigration mechanism for the formation of dark spot defects is proposed. The high current density required to operate polymer-based light emitting devices leads to electron-induced diffusion of the Al cathode when a short circuit forms in the emissive polymer layer. This process results in a "pileup'' of metal at the short circuit (anode) and a surrounding circular region where the Al is depleted, appearing as a dark spot on the cathode. © 1996 American Institute of Physics.
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