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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 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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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.
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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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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.
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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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