Structural study of degraded ZnMgSSe blue light emitters

Using electroluminescence (EL) topography and transmission electron microscopy (TEM), we investigated the nonluminescent regions which form while current is being injected into ZnMgSSe/ZnSSe/ZnCdSe-based blue light emitters. Small dark spots were observed just after turn-on and spread out forming rough nonluminescent triangles in the <100> directions in the EL image of the active region. TEM studies showed that the small dark spots are pre-existing stacking faults originating at the substrate/epitaxial layer interface. The nonluminescent triangles were found to be a dense region of dislocation dipoles and dislocation loops. Each dipole was aligned along two <110> directions in the {111} planes. The Burgers vectors were of the type a/2<011> inclined at 45° to the (001) junction plane.     

A modified power control scheme for remedying the effects of traffic nonuniformity in LEO satellite communications systems

It has been shown in our previous studies that the geographical traffic nonuniformity considerably affects the performance of the low earth orbit satellite communications systems. In this paper, a new scheme for improving the throughput characteristics of these systems at nonuniform traffic distribution is proposed. In this method, some parts of the users under the satellite which is flying over the area with high traffic load are assigned to its neighbor satellites with lower transmitting power levels. It is shown that the method equalizes the traffic loads of the satellites to some degree and, hence, can improve the throughput characteristics of the system.     

Irregular sampling theorems for wavelet subspaces

From the Paley-Wiener 1/4-theorem, the finite energy signal f(t) can be reconstructed from its irregularly sampled values f(k+δ_κ) if f(t) is band-limited and sup_κ|δ_κ|<1/4. We consider the signals in wavelet subspaces and wish to recover the signals from its irregular samples by using scaling functions. Then the method of estimating the upper bound of sup_κ|δ_κ | such that the irregularly sampled signals can be recovered is very important. Following the work done by Liu and Walter (see J. Fourier Anal. Appl., vol.2, no.2, p.181-9, 1995), we present an algorithm which can estimate a proper upper bound of sup_κ |δ_κ|. Compared to Paley-Wiener 1/4-theorem, this theorem can relax the upper bound for sampling in some wavelet subspaces     

TM surface-wave power combining by a planar active-lens amplifier

Power combining of TM surface waves by a planar active-lens amplifier is the subject of this paper. An amplifier gain of 11 dB at 8.25 GHz with a 3-dB bandwidth of 0.65 GHz has been demonstrated. Gain is measured from input to output connector to facilitate comparisons with more conventional amplifiers. Measurements of output power versus input power are also presented. The amplifier behaved in a linear manner and no problems with spurious oscillations were encountered. Construction of the amplifier is compatible with planar fabrication technologies. A key component of the combiner is a microstrip-fed Yagi-Uda slot-array antenna for TM surface-wave excitation of a thick dielectric slab. Design and optimization guidelines for the antenna are presented as well as detailed spectral-domain and finite-difference time-domain (FDTD) analysis results. Measured and simulation results show an input return loss and front-to-back ratio better than 10 dB over a 5% bandwidth. Calculated and measured results for the fields radiated by the antenna confirm forward radiation of the dominant TM mode in the thick dielectric slab. Integration of the computed radiated fields shows the antenna has a surface-wave launching efficiency of 85%     

Quasi-optical waveguide-mode converters using a pair of phase correction mirrors

This paper describes a quasi-optical method for the conversion of modes transmitted through highly oversized circular waveguides. A waveguide-mode is radiated once from a waveguide cut in the form of a radiation beam, which is then properly shaped by two curved mirrors and directed back into the waveguide. The curved mirror shapes are iteratively and automatically determined for given propagation distances using the design technique for phase correction mirrors. The proposed method gives favorable results in designing a waveguide expander/reducer, a TE₀₁-TE₀₂ mode converter, and a TE₀₁-HE₁₁ mode converter.     

Experimental studies of bistatic scattering from two-dimensionalconducting random rough surfaces

Despite the recent development of analytical and numerical techniques for problems of scattering from two-dimensional rough surfaces, very few experimental studies were available for verification. The authors present the results of millimeter-wave experiments on scattering from two-dimensional conducting random rough surfaces with Gaussian surface roughness statistics. Machine-fabricated rough surfaces with controlled roughness statistics were examined. Special attention was paid to surfaces with large rms slopes (ranging from 0.35 to 1.00) for which enhanced backscattering is expected to take place. Experimentally, such enhancement was indeed observed in both the copolarized and cross-polarized returns. In addition, it was noticed that at moderate angles of incidence, the scattering profile as a function of observation angle is fairly independent of the incident polarization and operating frequency. This independence justifies the use of the geometric optics approximation embodied in the Kirchhoff formulation for surfaces with large surface radius of curvature. When compared with the experimental data, this analytical technique demonstrates good agreement with the experimental data     

Fabrication and characterization of lateral field-emitter triodes

The author fabricated a field-emitter triode with tungsten electrodes arranged laterally on a quartz glass substrate by using the photolithography and dry etching techniques. The device consists of an array of 170 field-emitter tips with a 10-μm pitch, a columnar gate, and an anode. The emission characteristics followed the Fowler-Nordheim tunneling theory. The mutual conductance was about 0.02 μS at an anode voltage of 300 V. The authors improved the fabrication process to obtain an emitter with an operating voltage of about 100 V     

An experimental 1.5-V 64-Mb DRAM

Low-voltage circuit technologies for higher-density dynamic RAMs (DRAMs) and their application to an experimental 64-Mb DRAM with a 1.5-V internal operating voltage are presented. A complementary current sensing scheme is proposed to reduce data transmission delay. A speed improvement of 20 ns was achieved when utilizing a 1.5-V power supply. An accurate and speed-enhanced half-V_CC voltage generator with a current-mirror amplifier and tri-state buffer is proposed. With it, a response time reduction of about 1.5 decades was realized. A word-line driver with a charge-pump circuit was developed to achieve a high boost ratio. A ratio of about 1.8 was obtained from a power supply voltage as low as 1.0 V. A 1.28 μm² crown-shaped stacked-capacitor (CROWN) cell was also made to ensure a sufficient storage charge and to minimize data-line interference noise. An experimental 1.5 V 64 Mb DRAM was designed and fabricated with these technologies and 0.3 μm electron-beam lithography. A typical access time of 70 ns was obtained, and a further reduction of 50 ns is expected based on simulation results. Thus, a high-speed performance, comparable to that of 16-Mb DRAMs, can be achieved with a typical power dissipation of 44 mW, one tenth that of 16-Mb DRAMs. This indicates that a low-voltage battery operation is a promising target for future DRAMs     

A new circuit topology for continuous group delay synthesis

A new circuit topology is realized in order to synthesize group delay continuously. The circuit is based upon a new variable coupling coefficient coupled line section and a microstrip feedback loop. The new variable coupler is designed by periodically loading an edge coupled microstrip coupler. With this new design, coupling variation of more than 10 dB may be achieved. To the best of the authors knowledge, this is the first report of this type of microstrip coupler. The complete circuit consumes no do power and exhibits more than 500 ps of continuous variable true time delay. Insertion loss is better than 3 dB and return loss is better than 15 dB     

Current status of field-emission displays

Field-emission displays (FEDs) promise to provide high-resolution emissive flat-panel displays. Currently, the development of Spindt-type FEDs is at an advanced stage with monocolor FEDs being supplied to the marketplace. Full-color FEDs are getting ready for commercial production. A number of other field-emission electron sources, including carbon nanotube and ballistic electron tunneling, are now being actively developed. FEDs are expected to be low-power displays, as little or no power is consumed in generating electrons. FEDs will be developed and used for the applications from small- and medium-size panels to large-size televisions