Радиотехника и электроника

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Two-Color Photodetector for the Visible Spectral Range Based on ZnSe/ZnS/GaAs Bragg Reflector

Tue, 10/01/2019 - 00:00
Abstract

Effect of the ZnSe/ZnS/GaAs distributed Bragg reflector (DBR) on the parameters of the spectral response of a photodiode based on rectifying contacts in the metal–semiconductor–metal (MSM) system is studied. The calculated photoreflection spectra of the ZnSe/ZnS/GaAs heterostructure are in good agreement with the experimental data. It is shown that the MSM diode provides two-color response of the photodetector at wavelengths of 420 and 472 nm, a sharp decrease in the photosensitivity in the long-wavelength part of the response signal, high quantum efficiency (53%), and low dark current (5 × 10–10 A). It is demonstrated that the narrow-band two-color response of the detector can be tuned to the desired wavelength using appropriate selection of the parameters of the heterostructure that forms that Bragg reflector.

Nonlinear Dynamics of a Hybrid Josephson Heterostructure with Cuprate Antiferromagnetic Interlayer

Tue, 10/01/2019 - 00:00
Abstract

The nonstationary Josephson effect in a hybrid heterostructure based on the YBa2Cu3O7–δ cuprate superconductor and cuprate antiferromagnetic barrier interlayer Ca0.5Sr0.5CuO2 and with the upper superconducting electrode Au/Nb is experimentally investigated. At the frequency of external exposure f = 70 GHz, close to the plasma frequency of the Josephson junction, a giant increase in the noise signal was detected, which was explained by the appearance of stochastic oscillations on the heterostructure. Stochastic oscillations, but with less intensity, occur at the frequency f = 45 GHz. At the same time, on the current–voltage characteristics, there are intervals with negative dynamic resistance.

On the Influence of Ionic Polarization of Transistor Si-Structures on the Conductivity of p -Type Channels

Tue, 10/01/2019 - 00:00

Abstract—In the range of transverse magnetic field induction values of 0–4.5 T at temperatures of 100–200 K, the conductivity of the inversion channel of transistor Si-structures is measured after ion polarization and depolarization of samples. It was found that during polarization at a temperature of 420 K under the action of a strong electric field, around 6.5 × 1011 cm–2 ions flowed in the oxide. It was found that, up to the channel opening threshold, conductivity in the source–drain circuit is achieved due to thermal activation of charge carriers to the leakage level in the unordered potential created by the chaotic distribution of ions along the semiconductor surface. It is shown that after opening of the channel (intersection of the Fermi level of holes on the semiconductor surface with the flow level in the chaotic potential), ions appear in the conductivity as additional scattering centers; therefore, in a polarized state, the effective channel mobility is less than in the depolarized one.

Simulation of Electron-Optical Systems with a Converging Sheet Beam for Terahertz Traveling-Wave Tubes

Tue, 10/01/2019 - 00:00
Abstract

A converging sheet electron beam with a cross section of 0.05 × 2 mm2 and a current density of 200 A/cm2 formed by an electron gun is modeled using the synthesis and analysis methods with partial and complete magnetic shielding of the cathode and a linear compression of 10 and 15. The cross-sectional deformation of the beam in a focusing magnetic field is analyzed based on the three-dimensional computer model of electron-optical systems with a sheet electron beam. The possibility of generation of a low-perveance flux with small deformation in the transit channel of a comb slow-wave structure up to 30 mm in length is demonstrated.

Quartz Optical Fibers with Increased Content of Fluorine for Fluorimeter Probes

Tue, 10/01/2019 - 00:00
Abstract

A method to fabricate quartz optical fibers for probes employed in fluorescence diagnostics of tumors is proposed. It is shown that the sensitivity of fluorimeters in the near-IR spectral range can be increased using quartz–quartz optical fiber probes with the core made of the Heraeus glass and the SiO2–F cladding with an increased content of fluorine in the glass of up 7 wt %. The numerical aperture of such optical fibers amounts to 0.32. The optical fibers are used to develop a prototype of a multifiber high-aperture probe for a laser oncofluorimeter with a relatively high sensitivity in a spectral interval of 900–1100 nm.

Features of Tunneling Current in Superlattices with Electrical Domains

Tue, 10/01/2019 - 00:00

Abstract—The tunneling electron transport in GaAs/AlAs and InAs/AlSb superlattices with electric domains at room temperature is studied. At voltages above the threshold for the formation of domains, a series of maxima on the current–voltage characteristics, almost equidistant in voltage, and current hysteresis during direct and reverse voltage sweeps were found. An explanation is proposed relating the origin of these maxima to tunnel transitions between quantum wells in a triangular domain, assisted by the emission of optical phonons, and hysteresis with a transition between modes with a moving and static domain. The large asymmetry of the transition times between domain modes was found.

Analytical Performance Evaluation of a STBC Coded OFDM FSO Communication System over Turbulent Atmospheric Channel

Tue, 10/01/2019 - 00:00
Abstract

An analytical approach is presented for an optical IM/DD system with STBC coded OFDM RF subcarrier modulation over a free space optical turbulent channel. The analysis is developed to find the expressions for the output photodetector current and carrier to noise power ratio at the output of the STBC–OFDM demodulator. The average bit error rate is evaluated numerically considering log-normal distribution of the atmospheric turbulence. The numerical results show that STBC-OFDM optical link suffers power penalty due to atmospheric turbulence significantly. However, the power penalty can be reduced by increasing the number of OFDM subcarriers. For example, power penalty for (2X1) STBC with N = 16 OFDM subcarriers is 7.5 dB and the power penalty reduces to 0.55 dB when N is increased to 64. Results are evaluated for different link distance, OFDM subcarriers and various channel condition.

Formation of a Memristive Array of Crossbar-Structures Based on (Co 40 Fe 40 B 20 ) x (LiNbO 3 ) 100 Nanocomposite

Tue, 10/01/2019 - 00:00
Abstract

The possibility of scaling of recently developed memristors of a new type based on (Co40Fe40B20)x(LiNbO3)100 –x is shown. The scaling is carried out by formation of the array of elements with a surface area of 50 × 50 µm2. It is shown that the spread of resistances in the high (off-state; Roff) and low resistance states (on-state; Ron) when resistive switching (RS) does not exceed 25% and individual elements show endurance exceeding 2 × 105 times when Roff/Ron > 10. For the first time within the region of resistive switching, the possibility of quasi-continuous change of the resistive state of structures by stepwise change with an accuracy no less than 0.2% (256 steps in the range 0.5–3 MΩ) was demonstrated.

Related Plasmon Oscillations in a Cluster of Two Silver Nanocylinders of Different Diameter

Tue, 10/01/2019 - 00:00
Abstract

A two-dimensional problem of the diffraction of a plane wave of the light range on a cluster consisting of two self-similar, silver nanocylinders of different diameter is considered. The frequency characteristics of the scattering diameter, the spatial distribution of the field components, and the scatterplot are calculated by strict numerical methods. The influence of the angle of incidence, the cylinder distance, the coefficient of self-similarity and silver loss on the spectra of the scattering diameter, and the scatterplot in the light wavelength range is studied. It is shown that the maximum value of the electric field component in the quasistatic region at the output of such a system cannot be more than ten times the values of the incident field.

Efficiency Study of Kravchenko Weight Windows at Receiving Signal Structures Based on OFDM Signals in the Presence of Spectrum-Concentrated Interference

Tue, 10/01/2019 - 00:00
Abstract

In this paper, we consider signal constructions based on OFDM signals and error-correcting codes that are resistant to the effects of spectrum-concentrated interference. An algorithm for receiving these signal constructions using weight windows and a criterion for optimal weight windows are given. A number of Kravchenko weight windows based on atomic functions are shown to belong to practically optimal windows. The probabilistic characteristics for the reception of the considered signal structures formed using the convolutional code with a code rate of 1/2 are specified.

An Antenna with a Feeder for a Superconducting Terahertz Josephson Oscillator with Phase Locking

Tue, 10/01/2019 - 00:00
Abstract

A principal layout of a Josephson terahertz radiation oscillator integrated with a transmitting antenna-lens system and a harmonic mixer (HM) for phase locking of radiation has been proposed and was successfully implemented. Two antenna-feeder systems designed for the central frequencies of 0.3 and 0.6 THz and located on the same chip with the oscillator are numerically simulated and fabricated. A microstrip transmission line between the oscillator and the antenna is used as the feeder. A study was carried out on matching the oscillator power and HM for two designs; the frequency range of pumping HM was 0.25–0.45 and 0.5–0.68 THz for the designs at 0.3 and 0.6 THz, respectively. Good agreement was obtained between experimental results and numerical simulations. A study of the spectral characteristics of the radiation of the oscillator into the external space for the 0.6 THz design using a superconducting integrated spectrometer was carried out. The linewidth of an emission line in free-running regime was of the order of several megahertz; in the phase locking regime down to tens of kilohertz with a signal-to-noise ratio of more than 20 dB was obtained.

Digital Frequency-Selective Filters Based on Spectra of Atomic Functions

Tue, 10/01/2019 - 00:00
Abstract

A method for synthesis of digital low-pass filter (LPF) based on spectra of atomic functions \({{{\text{h}}}_{a}}(x)\) is first proposed. Compactly supported function \({{{\text{h}}}_{a}}(x)\) is a solution to the problem of partition of unity, which makes it possible to consider a sum of its \(S\) shifts \({{h}_{{S,a}}}(x)\) as a perfect frequency response of LPF. Truncation of the spectrum of function \({{{\text{h}}}_{{S,a}}}(x)\) is used to obtain digital filters with rapidly decaying coefficients. Formulas for estimation of deviations of frequency responses in the passband and stopband are derived. Application of new filters in multirate signal processing is considered.

Scattering of Thermal Microwave Radiation by Density Irregularities of Freshly Fallen and Fine-Grained Snow

Tue, 10/01/2019 - 00:00
Abstract

Strong variations in the coherent transmittance of thermal microwave radiation at frequencies of 22.2, 30, 37.5, 60, and 94 GHz are experimentally found in samples of freshly fallen and fine-grained snow. It is established that one of the reasons for these variations is strong forward scattering of coherent intensity by irregularities of snow density that are large compared to the wavelength.

Development of Breakdown in Air in a Strongly Nonuniform Field of a Nanosecond Pulse

Tue, 10/01/2019 - 00:00
Abstract

The results of precision measurements of the rate of breakdown development in air at atmospheric pressure in the tip–plane electrode system are presented. It is shown that breakdown begins when the threshold voltage Uthr is reached. The higher its value is, the larger is the radius of the tip electrode; a breakdown development rate of 6.2 mm/ns is practically independent of the radius of the tip electrode and is the same for pulses with peak voltages of 25–30 kV. Visualization of the discharge structure with an aluminum film 4 nm thick showed the presence of a large number of microchannels with sizes ranging from 5 to 20 μm.

Determination of the Parameters of Tunneling Barriers of Superconducting Tunnel Structures for Submillimeter Receivers

Tue, 10/01/2019 - 00:00

Abstract—The parameters of the tunneling barrier of Nb/Al–AlOx/Nb Josephson junctions were estimated in a wide range of current densities using the Simmons method. It is shown that with an increase in resistivity RnS from 60 to 2100 Ω μm2 (a reduction in the density of the tunneling current of junction J from 3.5 to 0.1 kA/cm2), the height of the tunneling barrier increases from 0.85 to 1.18 eV and its width increases from 10.7 to 13.3 Å. The experimentally observed linear dependence of the tunneling barrier parameters on RnS made it possible to estimate the capacitance of junctions in the range of RnS 10–30 Ω μm2 required to create low-noise submillimeter-range receivers.

Renormalization of the Landé Factor and Effective Mass in Small Spherical Quantum Dots

Tue, 10/01/2019 - 00:00
Abstract

Using the modified Kane theory, a model has been built for describing the formation of the effective mass and g factor of bound electrons in quantum dots several nanometers in size. It is shown that these values ​​depend critically on the dot size and are significantly different from the corresponding values ​​for the bulk semiconductor. The effective mass and g factor affect the binding energy of an electron on a quantum dot in a magnetic field and are determined by this binding energy, which ultimately forms the local band structure in the vicinity of the quantum dot. In the covariant InAs/AlSb heterostructure, the characteristics have been calculated for which a quantum dot localizes no more than one electron without localizing holes and therefore can serve as a prototype of a solid-state qubit with the controlled g factor.

Two Solution Techniques for the Problem of Diffraction of a Plane Wave by a Revolution Body Located in a Dielectric Half-Space

Tue, 10/01/2019 - 00:00
Abstract

Two techniques for calculation of the problem of diffraction of a plane wave by a dielectric revolution body located in a homogeneous dielectric half-space are suggested. The case, in which the permittivity of the medium inside the body is the coordinate function independent of the angular coordinate in the cylindrical coordinate system, is considered. When the first technique is used (in the case of a homogeneous medium of the body), the discrepancy of the boundary condition on the contour of the axial section of the body is constructed. Two methods are compared using the example of the problem of the plane wave scattering by a homogeneous sphere and circular cylinder of finite dimensions that are located in a half-space. The results of calculation of the field scattering pattern are presented for the case of diffraction by a finite circular cylinder having variable material characteristics.

Visualizing the Intensity Distribution of a Microwave Field Using a Microstrip Rectenna with a Schottky Diode

Sun, 09/01/2019 - 00:00
Abstract

A microwave radiation intensity indicator based on an antenna-coupled detector (rectenna), an amplifier, and a semiconductor light-emitting diode was constructed and tested. This device is a hybrid integrated circuit, where a microstrip log-periodic antenna on a dielectric substrate also serves as a mechanical carrier. The possibility of visual determination of the shape of the antenna radiation pattern and the shape of an object based on the reflected radiation intensity was demonstrated experimentally at 10 and 30 GHz.

Synthesis of a Three-Mirror System with Specified Shape of the Main Mirror Using Phase Front and Mapping Law

Sun, 09/01/2019 - 00:00
Abstract

Ray-optic approximation is used to derive an analytical solution to the problem of synthesis of mirrors that form an axisymmetric three-mirror system with predetermined main mirror. Such a system provides transformation a spherical front of a source into an arbitrary front with the desired mapping law. The generatrix of an auxiliary mirror is represented as a polynomial in terms of even powers of angle. The solution can be used to exactly implement the output front and obtain the desired error of mapping law. The synthesis of a three-mirror telescope in which either parabolic or spherical mirror serves as the main mirror is considered.

Isolating Capacitive Currents when Diagnosing a Heterogeneous Anisotropic Sample

Sun, 09/01/2019 - 00:00
Abstract

Formulas are determined that describe the contribution from capacitive currents to the current measured in the external circuit when diagnosing heterogeneous anisotropic samples with slowly varying parameters.

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