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  • 1985-1989  (5)
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  • 1
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A threshold voltage sensitivity analysis of high electron mobility transistors (HEMTs) based on a self-consistent model for the electron distribution is presented. The calculated sensitivity values are related to experimentally obtained uniformity data on 2-in. wafers grown by molecular-beam epitaxy (MBE). The effect of doping concentration, compensation, and thickness of the doped and undoped (Al,Ga)As layers and the mole fraction are considered. Enhancement mode transistors are generally found to be less sensitive than transistors of depletion type. The analysis brings out that the most important design parameters to control are the thickness of the doped (Al,Ga)As layer and its doping concentration (compensation). Changes in the Al mole fraction are found to have a negligible influence. The effects of varying growth conditions are estimated and possible ways of improving the threshold uniformity are discussed.
    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: Results of measurements made using the multipole wiggler at beamline 16 of Photon Factory are reported. Double crystal rocking curves, Si(111) and Si(333) topographs, and thermal maps of the cooled-crystal surface were measured at total incident power levels up to 1600 W and power densities up to 0.7 W/mm2 , for two different crystal designs. Significant crystal deformation occurred in the form of surface curvature for both designs. Finite element calculations provided a preview of the experimental results and proved very valuable in interpretation of the measurements.
    Type of Medium: Electronic Resource
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  • 3
    ISSN: 1089-7623
    Source: AIP Digital Archive
    Topics: Physics , Electrical Engineering, Measurement and Control Technology
    Notes: The high-brilliance, insertion-device-based photon beams of the next generation of synchrotron sources (Argonne's APS and Grenoble's ESRF) will deliver large thermal loads (1–10 kW) to the first optical elements. Considering the problems that present synchrotron users are experiencing with beams from recently installed insertion devices, new and improved methods of cooling these first optical elements, particularly when they are diffraction crystals, are clearly needed. A series of finite element calculations were performed to test the efficiency of new cooling geometries and various cooling fluids. The best results were obtained with liquid Ga metal flowing in channels just below the surface of the crystal. Ga was selected because of its good thermal conductivity and thermal capacity, low melting point, high boiling point, low kinetic viscosity, and very low vapor pressure. Its very low vapor pressure, even at elevated temperatures, makes it especially attractive in UHV conditions. A series of experiments were conducted at CHESS in February of 1988 that compared liquid gallium-cooled silicon diffraction crystals with water-cooled crystals. A six-pole wiggler beam was used to perform these tests on three different Si crystals, two with new cooling geometries and the one presently in use. A special high-pressure electromagnetic induction pump, recently developed at Argonne, was used to circulate the liquid gallium through the silicon crystals. In all experiments, the specially cooled crystal was used as the first crystal in a two crystal monochromator. An infrared camera was used to monitor the thermal profiles and correlated them with rocking curve measurements. A second set of cooling experiments were conducted in June of 1988 that used the intense, highly collimated beam from the newly installed ANL/CHESS undulator. Tests were performed on two new Ga-cooled Si crystals and compared with the standard water-cooled Si crystal. One of the crystals had cooling channels at two levels in the crystal that allowed one to actively control the shape of the crystal surface. The second one had rectangular cooling channels located just beneath the diffraction surface. Both crystals showed major improvements over the water-cooled crystal.
    Type of Medium: Electronic Resource
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  • 4
    Electronic Resource
    Electronic Resource
    [S.l.] : American Institute of Physics (AIP)
    Journal of Applied Physics 59 (1986), S. 2870-2878 
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: A precise model for calculation of the spatial distribution of electrons in n-(Al,Ga)As/GaAs heterostructures is described. The model has been used to investigate the spatial distribution of electrons in a n-Al0.28Ga0.72 As/GaAs structure as a function of temperature. The results were compared with Hall-effect experiments on heterostructures as well as thick Al0.28Ga0.72 As films grown by molecular-beam epitaxy (MBE). For both types of samples it was found that it was necessary to use either a conduction-band density of states which is much higher than expected from literature data or to use a temperature-dependent activation energy. The slight increase in concentration of the two-dimensional electron gas observed for lower temperatures could be attributed to a sharpening of the transition region between high- and low-ionization degrees in the depletion layer close to the interface. It thus indicates that the Hall factor changes very little as expected for a degenerate electron system.
    Type of Medium: Electronic Resource
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  • 5
    ISSN: 1089-7550
    Source: AIP Digital Archive
    Topics: Physics
    Notes: In this article we analyze the combined effects of the deterministic nonuniform flux distribution in a molecular-beam epitaxy system, and the random variations in device parameters induced by processing. As a case study, the distribution of the saturation current (Idss) before gate metallization of a high-electron mobility transistor is investigated. The magnitude of the observed statistical variations has been compared to calculated values given by a theoretical expression for the saturation current. Using a semiempirical method to describe the radial dependence of a device parameter and its variation, simple expressions describing the shape of the wafer distribution, mean value, standard deviation, yield, and radial probability of finding devices within set limits, have been derived. The expressions can be used to relate the layer properties in the wafer center to the mean value of the same properties as obtained from a map of the whole wafer. The expressions for radial probability of finding a working device can be easily added to the software of an automatic wafer tester in order to speed up the screening procedure of wafers.
    Type of Medium: Electronic Resource
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