New approach to the design of Schottky barrier diodes for THz mixers

Near-ideal GaAs Schottky barrier diodes especially designed for mixing applications in the THz frequency range are presented. A diode fabrication process for submicron diodes with near-ideal electrical and noise characteristics is described. This process is based on the electrolytic pulse etching of GaAs in combination with an in-situ platinum plating for the formation of the Schottky contacts. Schottky barrier diodes with a diameter of 1 micron fabricated by the process have already shown excellent results in a 650 GHz waveguide mixer at room temperature. A conversion loss of 7.5 dB and a mixer noise temperature of less than 2000 K have been obtained at an intermediate frequency of 4 GHz. The optimization of the diode structure and the technology was possible due to the development of a generalized Schottky barrier diode model which is valid also at high current densities. The common diode design and optimization is discussed on the basis of the classical theory. However, the conventional fomulas are valid only in a limited forward bias range corresponding to currents much smaller than the operating currents under submillimeter mixing conditions. The generalized new model takes into account not only the phenomena occurring at the junction such as current dependent recombination and drift/diffusion velocities, but also mobility and electron temperature variations in the undepleted epi-layer. Calculated diode I/V and noise characteristics are in excellent agreement with the measured values. Thus, the model offers the possibility of optimizing the diode structure and predicting the diode performance under mixing conditions at THz frequencies

A comparative study of uniaxial pressure effects in intraband AlGaAs/GaAs and interband InAs/AlSb/GaSb resonant tunneling diodes

We report on the effects of uniaxial pressure on (001)-oriented AlGaAs/GaAs and InAs/AlSb/GaSb double barrier resonant tunneling diodes (RTDs). The current–voltage characteristics of the AlGaAs/GaAs RTDs shift asymmetrically due to stress-induced piezoelectric fields in the barriers and well structures. Although all the materials involved are piezoelectric, the interband InAs/AlSb/GaSb resonant tunneling device surprisingly shows, in contrast to the AlGaAs/GaAs one, a symmetrical behavior for the same orientation [110] of the applied pressure. We explain the observed differences considering the different tunneling paths involved in the conduction mechanism of the two heterostructure device types as well as their pressure dependencies. ©1998 American Institute of Physics.published_or_final_versio

Condition Assessment of Bill Emerson Memorial Cable-Stayed Bridge under Postulated Design Earthquake

In this study, a three-dimensional finite element model of the Bill Emerson Memorial cable-stayed bridge was developed and validated with the acceleration data recorded during the M4.1 earthquake of May 1, 2005, in Manila, Arkansas. The model took into account the geometric nonlinear properties associated with cable sagging and soil-foundation-structure interaction. The validated model was used to evaluate the performance of a seismic protective system, the behavior of cable-stayed spans, and the accuracy of two simplified bridge models that have been extensively used by the structural control community. The calculated natural frequencies and mode shapes correlated well with the measured data. Except that the hollow columns of two H-shaped towers were near yielding immediately above their capbeams, the cable-stayed spans behaved elastically as expected under the design earthquake that was scaled up from the recorded rock motions at the bridge site. The minimum factor of safety of all cables is 2.78, which is slightly greater than the design target

Recent Advances in Understanding the Structure and Properties of Amorphous Oxide Semiconductors

Amorphous oxide semiconductors (AOSs)--ternary or quaternary oxides of post-transition metals such as In-Sn-O, Zn-Sn-O, or In-Ga-Zn-O–have been known for a decade and have attracted a great deal of attention as they possess several technological advantages, including low-temperature large-area deposition, mechanical flexibility, smooth surfaces, and high carrier mobility that is an order of magnitude larger than that of amorphous silicon (a-Si:H). Compared to their crystalline counterparts, the structure of AOSs is extremely sensitive to deposition conditions, stoichiometry, and composition, giving rise to a wide range of tunable optical and electrical properties. The large parameter space and the resulting complex deposition--structure--property relationships in AOSs make the currently available theoretical and experimental research data rather scattered and the design of new materials difficult. In this work, the key properties of several In-based AOSs are studied as a function of cooling rates, oxygen stoichiometry, cation composition, or lattice strain. Based on a thorough comparison of the results of ab initio modeling, comprehensive structural analysis, accurate property calculations, and systematic experimental measurements, a four-dimensional parameter space for AOSs is derived, serving as a solid foundation for property optimization in known AOSs and for design of next-generation transparent amorphous semiconductors

Macro- and microscopic properties of strontium doped indium oxide

Solid state synthesis and physical mechanisms of electrical conductivity variation in polycrystalline, strontium doped indium oxide In2O3:(SrO)x were investigated for materials with different doping levels at different temperatures (T=20-300 C) and ambient atmosphere content including humidity and low pressure. Gas sensing ability of these compounds as well as the sample resistance appeared to increase by 4 and 8 orders of the magnitude, respectively, with the doping level increase from zero up to x=10%. The conductance variation due to doping is explained by two mechanisms: acceptor-like electrical activity of Sr as a point defect and appearance of an additional phase of SrIn2O4. An unusual property of high level (x=10%) doped samples is a possibility of extraordinarily large and fast oxygen exchange with ambient atmosphere at not very high temperatures (100-200 C). This peculiarity is explained by friable structure of crystallite surface. Friable structure provides relatively fast transition of samples from high to low resistive state at the expense of high conductance of the near surface layer of the grains. Microscopic study of the electro-diffusion process at the surface of oxygen deficient samples allowed estimation of the diffusion coefficient of oxygen vacancies in the friable surface layer at room temperature as 3x10^(-13) cm^2/s, which is by one order of the magnitude smaller than that known for amorphous indium oxide films.Comment: 19 pages, 7 figures, 39 reference

Leisure Behavior Pattern Stability During the Transition from Adolescence to Young Adulthood

Leisure is an important context in which human development occurs. Changes in leisure behavior patterns may indicate changing developmental needs or reflect contextual changes that impact leisure behavior. The transition from adolescence to young adulthood provides an excellent opportunity for the study of the stability of leisure behavior as individuals' contexts are changed with the adoption of adult roles and the potential for disruption of leisure patterns exists. Previous studies investigating leisure and the transition from adolescence to young adulthood have tended to be cross-sectional and focus on specific leisure behaviors rather than identifying patterns of leisure behavior. The present study involved a longitudinal investigation of leisure behavior patterns over a three-year period during the transition from adolescence to young adulthood, and determined the nature of leisure pattern stability and instability during this period. In general, leisure pattern stability was the most common pathway into young adulthood. The patterns of leisure behavior and the nature of the changes that occurred with the transition from adolescence to young adulthood differed to some degree for males and females, although similarities in patterns and transitions were also found.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/45285/1/10964_2004_Article_411255.pd