Data are presented on high-power AlGaInN flip-chip light-emitting diodes (FCLEDs). The FCLED is “flipped-over” or inverted compared to conventional AlGaInN light-emitting diodes (LEDs), and light is extracted through the transparent sapphire substrate. This avoids light absorption from the semitransparent metal contact in conventional epitaxial-up designs. The power FCLED has a large emitting area (∼0.70 mm2) and an optimized contacting scheme allowing high current (200–1000 mA, J∼30–143 A/cm2) operation with low forward voltages (∼2.8 V at 200 mA), and therefore higher power conversion (“wall-plug”) efficiencies. The improved extraction efficiency of the FCLED provides 1.6 times more light compared to top-emitting power LEDs and ten times more light than conventional small-area (∼0.07 mm2) LEDs. FCLEDs in the blue wavelength regime (∼435 nm peak) exhibit ∼21% external quantum efficiency and ∼20% wall-plug efficiency at 200 mA and with record light output powers of 400 mW at 1.0 A.
Ballistic electron emission microscopy (BEEM) has been used to study transport in a double barrier resonant tunneling structure. Unlike conventional transport techniques, BEEM allows the injected electron energy to be varied independent of the band profile. We report the observation of quasi-bound states and band-structure effects as deduced from the temperature evolution of the BEEM spectra. The BEEM thresholds are found to be in good agreement with the calculated energetically favorable levels. Our results show that BEEM is a powerful spectroscopic tool for studying quantum structures.
We report an extensive investigation of semiconductor band-structure effects in single-barrier Al x Ga 1Ϫx As/GaAs heterostructures using ballistic-electron-emission spectroscopy ͑BEES͒. The transport mechanisms in these single-barrier structures were studied systematically as a function of temperature and Al composition over the full compositional range (0рxр1). The initial ͑⌫͒ BEES thresholds for Al x Ga 1Ϫx As single barriers with 0рxр0.42 were extracted using a model which includes the complete transmission probability of the metal-semiconductor interface and the semiconductor heterostructure. Band offsets measured by BEES are in good agreement with previous measurements by other techniques which demonstrates the accuracy of this technique. BEES measurements at 77 K give the same band-offset values as at room temperature. When a reverse bias is applied to the heterostructures, the BEES thresholds shift to lower voltages in good agreement with the expected bias-induced band-bending. In the indirect band-gap regime ͑xϾ0.45͒, spectra show a weak ballistic-electron-emission microscopy current contribution due to intervalley scattering through Al x Ga 1Ϫx As X valley states. Low-temperature spectra show a marked reduction in this intervalley current component, indicating that intervalley phonon scattering at the GaAs/Al x Ga 1Ϫx As interface produces a significant fraction of this X valley current. A comparison of the BEES thresholds with the expected composition dependence of the Al x Ga 1Ϫx As ⌫, L, and X points yields good agreement over the entire composition range. ͓S0163-1829͑97͒04827-3͔
The floating frame of reference (FFR) formulation is widely used in multibody system (MBS) simulations for the deformation analysis. Nonetheless, the use of elastic degrees-of-freedom (DOF) in the deformation analysis can increase significantly the problem dimension. For this reason, modal reduction techniques have been proposed in order to define a proper set of assumed body deformation modes. Crucial to the proper definition of these modes when the finite element (FE) FFR formulation is used is the concept of the reference conditions, which define the nature of the deformable body coordinate system. Substructuring techniques, such as the Craig–Bampton (CB) method, on the other hand, have been proposed for developing efficient models using an assembly of their lower order substructure models. In this study, the appropriateness and generality of using the CB method in MBS algorithms are discussed. It is shown that, when a set of reference conditions are not applied, the CB transformation leads to the free–free deformation modes. Because a square CB transformation is equivalent to a similarity transformation that does not alter the problem to be solved, the motivation of using the CB method in MBS codes to improve the solution is examined. This paper demonstrates that free–free deformation modes cannot be used in all applications, shedding light on the importance of the concept of the FE/FFR reference conditions. It is demonstrated numerically that a unique model resonance frequency is achieved using different modes associated with different reference conditions if the shapes are similar.
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