Persons

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The article describes a hardware-software complex for measurement of characteristics of electric and optical noise of light emitting diodes (LED) in a range of frequencies from 1 kHz to 40 kHz by a method of discrete samples. Electric noise of several types of heterojunction LEDs is investigated: red with quantum holes (QH) on the basis of AlInGaP/GaAs, green and blue without QH on the basis of AlInGaN/SiC. Spectra of all investigated LEDs in a range of frequencies from 1 kHz to 10 kHz look like 1/f γ;, meanwhile at red LEDs value of factor is much lower than 1, and at green and blue - is close to 1, and the noise correlation time of red LEDs several times exceeds correlation time of blue and green LEDs. It is demonstrated that the resulted functions of noise distribution at all LEDs types are close to a Gaussian with approximately identical dispersion.

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The power of the HEMT-transistors on the substrate from silicon achieves 100 W, on the substrate from silicon carbide - 1.5 kW. This presents high requirements to the quality of heat removal from the crystal active region to case and further to the environment. In the paper the thermal resistance “junction-to-case” the measurement results for GaN HEMT-transistors have been presented. The measurements have been performed using the apparatus, which includes two methods of thermal resistance measurements. In the first method - according to the standard OCT 11 0944-96, a sequence of heating current pulses is passing through the transistor channel and the junction temperature is measured. In the second method the modulated heating power has been used and the response - variable component of the junction temperature has been measured. To exclude the influence of the heating pulses duration, that is typical for the standard method, preliminary measurements of the transient response of the thermal impedance have been performed. To reduce the influence of the delay time, caused by transient electrical processes in the transistor when it switches from the heating mode to the measurement of the thermal sensitive parameter (TSP) mode, an extrapolation of the TSP signal of this parameter has been performed by the end of the heating pulse. The comparative analysis has shown that the measurements results, obtained by the standard and modulation methods differ by less than 2%. The influence of the amplitude of pulses of the heating current heating pulses has been shown. It has been determined that with increasing the heating current the thermal resistance measured values increases, which indicates to the nonlinear nature of the dependence of the temperature in the transistor channel on the power dissipated in it.

• Counter: 1075 | Comments : 0

The reasons and mechanisms of Change of the quantum efficiency and other characteristics of InGaN/GaN in various operation conditions are actively being investigated. The results of experimental research of changes of the external quantum efficiency of low-power InGaN/GaN green LEDS with the quantum well in the space charge region (SCR) of heterostructures and without quantum well in the process of accelerated tests have been presented. It has been found that after 8 hrs of testing at the temperature of 300 K in the pulsed mode with the current pulse amplitude of 0.5 A, the pulse duration of 100 microseconds and the relative pulse duration of 100 the external quantum efficiency of all LEDs without quantum wells has increased, while the external quantum efficiency of LEDs with quantum well has decreased in the range of operating currents. It has been shown that at low injection levels the radiation power of LEDs without the quantum well is determined by the recombination processes by the mechanism of Shockley-Reed-Hall and with the quantum well - by the tunnel-recombination processes. The current training in the accelerated pulse mode of green light diodes based on the InGaN/GaN during 4 hours can be used as a technological operation to stabilize light-technical characteristics of these diodes and to reveal potentially unreliable items in large scale production conditions.

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For homogeneity of parameters of dielectrics more and more rigid requirements are claimed, especially while using them in the micro- and radioelectronic items. The purpose of the work is to develop an automated method for monitoring the electrical parameters of planar dielectrics with increased requirements for accuracy and resolution. A short comparative analysis with the available methods for controlling the parameters of solid dielectrics has been carried out. A method for microwave monitoring of the nonhomogeneities of planar dielectrics by the wave characteristics of a scanning microstrip line (MSL) based on the application of the Radon transform has been described. A mathematical model for processing the MSL wave characteristics and for constructing a map of distributing the nonhomogeneities has been presented. The estimates of realizability of the parameters and the limits of the mathematical model applicability have been given. The technical realization of the method has been considered with an example of the installation for monitoring the uniformity of the parameters of planar dielectrics and the results of simulation of the measuring part in the AWR Microwave Office. The estimation of methodical errors, which in case of the dielectric permittivity measurement is on average ±1·10 and of the resolving power, which reaches 0.2 mm according to preliminary calculations, has been carried out. The described method is applicable in the radio-measuring equipment, used in the field of the microwave technology development, microelectronics, production of composite dielectric materials with high repeatability of parameters.

• Counter: 1902 | Comments : 0

Powerful bipolar microwave transistors (PBT) operate, as a rule, in conditions close to the limiting ones, which requires controlling the release of heat from the active region of the crystal to the transistor body and further to the environment. One of the most efficient is the control of PBT heat characteristics, including the dependence of the heat resistance - the device body on the electrical mode parameters. However, the quantitative evaluations of connection of defects with measuring the heat parameters of devices in literature are absent. In the work the results of modeling using the COMSOL Multiphysics software the temperature distribution in the structures of a power bipolar microwave transistor (PBT), with defects of electrophysical and thermal nature have been presented. The dependences of the maximum overheating of the crystal working surface on the size and location of the defects have been obtained. It has been shown that the temperature dependence of power density released in the structure of PBT leads to a nonlinear dependence of the maximum and average temperature of the crystal surface on the total power dissipated in PBT. The developed thermal models can serve as the basis for creating the methods for diagnosing the PBT of thermoelectric characteristics and identifying the defective products. The comparative measurements of thermal characteristics of the serial high-power microwave transistors of the KT920B type with no defects and with an artificially introduced electrophysical-type defect in a diode switch-on using a T3Ster meter have shown that the thermal resistance of the body-to-case PBT with the defects has increased by 25-40% compared to the thermal resistance of the defect-free device. In this case, the thermal characteristics of PBT in the diode switching on of the «base-collector» transition are more sensitive to structural defects than in the diode switching on of the emitter-base transition, and more preferable for the purposes of diagnosing the PBT quality.

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To explain the current dependencies of the mean square of the low-frequency noise of green InGaN light emitting diodes, the double stage low-frequency noise equivalent circuit of LED has been offered. It has been shown that the non-monotonic dependence of the low-frequency noise of the LED’s injection current can be explained by two low-frequency noise generators: noise current generator, localized near the heterojunction and determined by tunnel-recombination processes at the interface, and the generator being determined by the recombination processes in the active region of the structure.

• Counter: 1707 | Comments : 0

Effective diagnostic method of quality control of the digital integrated circuits (DIC) is measurement of their thermal parameters. Values of thermal parameters of real products are defined by quality of their production and can significantly differ from calculated values. A method for measuring the transient thermal characteristics (TTC) of the digital integrated circuits (DIC) with the temperature dependence of the frequency of the DIC ring oscillator has been described. An iterative algorithm for calculating parameters of the linear one-dimensional Foster thermal circuit uses the values of TTC, which correspond to the zeros of the TTC second derivative, has been shown. The algorithm was tested in determining the zeros of the second derivative of TTC by direct numerical differentiation method and by differentiating the 9-th order polynomial approximating function. It has been shown that TTC of the tested DIC corresponds to the three-stage thermal circuit. Both methods of calculation give almost identical values of thermal parameters of the thermal scheme which well corresponds to different technological layers of the DIC structure.

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The important factor, determining the functional properties, the limiting modes of operation and reliability of hetero-transient LEDs with the high-Ohm substrate is an account of heat release in it. A nonlinear thermoelectric model of InGaN/LED, taking into account the Joule heat release in the semiconductor substrate has been presented. The distribution of the potential on the elements of the structure and the current dependence of the thermal resistance of the semiconductor device structure have been obtained. The model adequacy has been confirmed by comparison of the calculated and experimental dependences of the thermal resistance of the powerful LED structure on the total current. The presented heat-electric model of LED diode accounting the Joule heat release permits to calculate its thermal resistance. The adequacy of the developed model has been experimentally verified.

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Thermal methods of quality control of the plated-through hole (PTH) of printed circuit board (PCB) are based on thermal models. However, known thermal models of PTH take no account of heat transfer to PCB material thus not allowing for PTH heat characteristic tying up with adhesion quality. In this work, an axisymmetric thermal model of a single-layer PCB PTH under one-sided heating conditions is considered. It was shown that the ratio of the temperature increments of the upper (heated) and lower end of the PTH in the considered range of heating power does not depend on the power level. A linear thermal equivalent scheme of the PTH has been proposed, which includes the longitudinal thermal resistance of the PTH metallization, determined by the parameters and quality of the metallization layer, the thermal resistance, which determines the convection heat exchange between the ends of the PTH with the adjacent PCB surface and the environment, and the thermal resistance of the area of the PCB material adjacent to the PTH, depending on the quality of the metallization adhesion and the PCB dielectric. Thermal equivalent circuit parameters determined by the ratio of the temperature increment of the upper and lower ends of the PTH and their difference can serve as the basis for the development of a nondestructive inspection procedure for PTH quality control by way of its unilateral heating, for example, by a laser beam.

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The results of experimental approbation of the method and device for determining the parameters of the thermal circuits of semiconductor products with p-n junctions using the wide-pulse modulation of heating current by the harmonic law on an example of measuring thermal impedance of the power light-emitting diodes have been presented.

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In many radiotechnical systems the microwave power amplifiers based on the monolithic integrated circuits, especially in the output power amplifiers (OPA) of transceiving modules AFAR, have become commonly used. It has been shown that selective distributions of the submodules of the output power amplifiers (OPA) of the APAL transceiving modules of the AFAR X-range on minimum and maximum output power and also on the maximum standing wave ratio (SWR) have a two-modal character, that demonstrates the existence of the prevailing factors of the assembly quality, leading to an appearance of the second mode. The distribution of the OPA submodules on the standing wave ratio by the form is close to the exponential distribution law, and on the consumed current - to the normal distribution law with a small relative average quadratic deviation. The absence of significant correlation between the consumed current and the level of the minimum output power means that the dispersion is determined by the level of the minimum output power but not by the quality of the OPA assembly, and not by the quality of monolithic integrated circuits.

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The reliability of technical devices using the InGaN/GaN- light emitting diodes depends on the methods and tools of their quality non-failure control. For diagnostics of non-homogeneities in the characteristics of light-emitting heterostructures by an area, such parameters as the low-frequency noise level and the threshold current value of luminescence start are used. The sampling distributions of commercial green LEDs based on InGaN/GaN heterostructures on the parameters of photocurrent, the threshold current and the level of low-frequency noise have been presented. It has been determined that between the average value of the photocurrent arising upon irradiation of the LED chip by laser radiation with a wavelength of 405 nm, the level of the low-frequency noise, measured in the range of the low current densities and the threshold current there are correlations, which indicate to a relationship between the photocurrent level and the density of defects in the heterostructure. It has been shown that in InGaN/GaN heterostructures of green luminescence the photocurrent level is predominantly higher in the defect structures.

• Counter: 1771 | Comments : 0

The parameters of low-frequency (LF) noise with a look spectrum G ( f ) ~1/ f are widely used for diagnostics of quality and reliability of semiconductor devices. For metrological support of the gages for the LF-noise parameters the problem of development of the LF-noise generator with an adjustable spectrum form indicator γ arises. The block diagram of the LF-noise generator with a source of white noise and the multilink RC-filter with variable and additional resistors in RC-links has been proposed. An application in the RC-links of the filter of dual variable resistors permits to change the spectrum form indicator γ ranging from 0 to 4 in the frequency range up to 6 decades. The experimental check of the generator with the 6-unit filter has shown an opportunity to synthesize a difficult range of LF-noise with the noise spectrum form indicator close to the value established by an operator.

• Counter: 1835 | Comments : 0

Some options of approximating the light-emitting diodes (LED) spectrums of radiation by various mathematical functions have been considered. On the example of red luminescence LED it has been shown that at the approximation of the LED spectrums by symmetric functions - the Gaussian function and parabola - there is a considerable error of shift of radiation wave central length, caused by asymmetry of the LED real spectrums. The general mean square error of approximation and the error in determining the radiation wave central length and the width of the LED radiation spectrum can be several times decreased at the approximation of the LED spectrum by the sum of two Gaussian functions.

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It has been analytically demonstrated that in the symmetric bipolar transistor structures with small defects with the preset full emitter current the difference between the currents, that flow through the symmetrical parts of the structure, is proportional to the value of the defect, and as a result of the structure self-heating, practically is linearly increasing with the collector voltage increase. The results of the theoretical analysis have been confirmed by computer modeling based on the program package «Workbanclm, supplemented by the block of computation of the structure parts temperatures.

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The results of measuring the temperature coefficient of the logic unit of logic elements in the CMOS digital microcircuits under various loading current have been presented. The model explaining the obtained dependences based on the temperature dependence of the mobility of charge carriers in the channel target MOSFET transistors has been offered.

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The comparative analysis of the accuracy of the approximation for the experimental temperature dependencies of the radiation intensity of the high-power light emitting diodes (LED) has been presented by the exponential and sedate functions. It has been shown that at strong temperature dependence of the radiation intensity of LED, used in the LED modules, the limiting density of the LED installation on a heat sink radiator exists, at which the module light intensity is maximum.

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The model of the thermal defeat of the semiconductor microwave diode by powerful microwave impulse with the temperature dependent density of power has been presented. The distribution of the maximum temperature on semiconductor structure depending on duration of the impulse radiation has been found.

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The possibility of evaluating the quality of heterojunction LEDs by the level of the threshold current (glow current start) has been considered. The block diagram of the setup operation of measuring the LEDs threshold current has been presented. The principle of the setup operation is to specify the linearly increasing current through the LED by the precision source, to measure the signal proportional to the LED optical radiation power by the highly sensitive photodetector based on the high-speed photodiode and low-noise transimpedance amplifier and to compute the value of the threshold current in the Mathcad program. The results of the sample measurements of the threshold current of various types of LEDs, illustrating a wide variation of parameters of the sampling distribution of LEDs by the level of the threshold current, have been presented.

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