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Typical structure of LED in which panel (a) is for the schematic side view of LED and panel (b) for the top view of SEM micrograph of LED.
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In this paper, we have discussed the effect of electrical stress on GaN light emitting diode (LED). With the lapse of time, the LED with an applied large current stress can reduce its current more than without such a stress under a large forward-voltage drop. Its scanning electron microscopy (SEM) image shows that there exist several pits on the su...
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Context 1
... was per- formed in a planar-type ICP system, in which the ICP source operated at 13.56 MHz. Figure 1(a) shows the typical structure of the LED, which describes the sam- ples that we used in the experiment. The top view of LED with a planar p electrode and a planar n elec- trode is shown in Fig.1(b). ...
Context 2
... was per- formed in a planar-type ICP system, in which the ICP source operated at 13.56 MHz. Figure 1(a) shows the typical structure of the LED, which describes the sam- ples that we used in the experiment. The top view of LED with a planar p electrode and a planar n elec- trode is shown in Fig.1(b). ...
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Citations
... GaN-based light-emitting diodes (LEDs) have aroused an innovation for general illumination/decoration, traffic lights, and LCD backlight units. [1][2][3][4] Future demands for more extensive applications require higher brightness and lower power consumption. The LED quantum efficiency is determined by both internal quantum efficiency (IQE) and light extraction efficiency (LEE). ...
Wafer-scale SiO2 photonic crystal (PhC) patterns (SiO2 air-hole PhC, SiO2-pillar PhC) on indium tin oxide (ITO) layer of GaN-based light-emitting diode (LED) are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for exposure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO2-hole PhC and SiO2-pillar PhC LEDs, respectively. No degradation to LED electrical properties is found due to the fact that SiO2 PhC structures are fabricated on ITO current spreading electrode. For SiO2-pillar PhC LEDs, which have the largest light output power in all LEDs, no dry etching, which would introduce etching damage, was involved. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.
... GaN-based light-emitting diodes (LEDs) have aroused an innovation for general illumination/decoration, traffic lights, and LCD backlight units. [1][2][3][4] Future demands for more extensive applications require higher brightness and lower power consumption. The LED quantum efficiency is determined by both internal quantum efficiency (IQE) and light extraction efficiency (LEE). ...
Semiconductor Lighting Wafer-scale SiO 2 photonic crystal (PhC) patterns (SiO 2 air-hole PhC, SiO 2 -pillar PhC) on indium tin oxide (ITO) layer of GaN-based light-emitting diode (LED) are fabricated via novel nanospherical-lens lithography. Nanoscale polystyrene spheres (PSs) are self-assembled into a hexagonal closed-packed monolayer array acting as convex lens for exposure using conventional lithography instrument. The light output power is enhanced by as great as 40.5% and 61% over those of as-grown LEDs, for SiO 2 -hole PhC and SiO 2 -pillar PhC LEDs, respectively. No degradation to LED elec-trical properties is found due to the fact that SiO 2 PhC structures are fabricated on ITO current spreading electrode. For SiO 2 -pillar PhC LEDs, which have the largest light output power in all LEDs, there was not involved any dry etching at all that would introduce etching damage. Our method is demonstrated to be a simple, low cost, and high-yield technique for fabricating the PhC LEDs. Furthermore, the finite difference time domain (FDTD) simulation is also performed to further reveal the emission characteristics of LEDs with PhC structures.
Based on multivariate random analysis and statistical physics, the reliability model of a system with stress-strength interference and initial failure is developed, the system has multi-state and dependent multi-subsystem. According to the definition of conditionally ordered order statistics, the probability density function of random strength vector is studied. Considering the coherence of subsystem items, the system reliability evaluation for different structures is given, and the reliability of any coherent structure is represented as a linear combination of the reliabilities of (n-i+1)-out-of-n system (1≤i≤n). Finally, an example is given to demonstrate the effectiveness of the model, where the stress-strength variable has a bivariate Pareto distribution.
Accelerated aging and life-time tests at ambient temperatures of 40°C, 70°C and room temperature were carried out on GaN-based white light-emitting diodes (LEDs). The electrical, optical and thermal characteristics of the device were compared before and after different aging times in order to investigate the failure mechanism of the device. Here, we mainly analyzed the failure mechanism related with the chip and the phosphor of the LED. The current-voltage characteristics demonstrated that both the series resistance and the tunnel current increase under lower forward voltages after aging, which were due to the degradation of p-type Ohmic contact and the increase of defect density. The thermal characteristics confirmed that the thermal resistance increased rapidly under high aging temperature. This was mainly attributed to the fracture of different materials inside the devices caused by the difference in thermal expansion coefficients. Optical measurements indicated that high aging temperature could accelerate the degradation of output power and reduce the conversion efficiency of the phosphor as well. Finally, the life time of the device was calculated using Arrhenius-equation, and the failure mechanism was analyzed.
We increase the hole concentration of the p-GaN contact layer of the epitaxial wafer of conventional GaN-based devices by laser-induced Zn doping. Improvement of the photoelectric property of light-emitting diodes (LED) is confirmed. Compared with LED with no laser-induced doping, the forward voltage under 20 mA current is decreased from 3.33 V to 3.13 V, the series resistance is decreased from 30.27 Ω to 20.27 Ω, and the degradation coefficient at room temperature is reduced from 1.68 × 10-4 to 1.34 × 10-4. In addition, the reverse leakage current of the LED is reduced from over 0.2 μA to less than 0.025 μA after an aging time of 1600 h by accelerated lifetime testing, and the lifetime is increased about 41%. These results are attributed to the improvement of p-type ohmic contact and the decrease of the thermal resistance due to laser-induced doping of Zn to the p-GaN contact layer.
Based on Monte-Carlo simulation, the lifetime distribution on array interconnection of high-power LED module is studied. It's assumed that the forward voltage of power white LEDs follows normal distribution after bins, the mean lifetime at 350 mA follows Lognormal distribution, and the relationships between work current, temperature and lifetime are consistent with Eying model. For n × n LED array, the reliability lifetime does not decrease but increases slightly while the total number of LEDs increases. The results indicate that the array interconnect circuit has obvious advantage over the conventional series-parallel connection circuit, which can improve the reliability of LED array significantly. For different current derating values, the lifetime increases monotonically as the derating increases, and the larger the derating, the more increase of the lifetime. Especially, applying large derating to the array with a large number of LEDs can improve the reliability of LED array greatly.
The degradation of a device can be described by the degradation model in the accelerated tests. Because the degradation is closely related to the degradation mechanism, which reflects the intrinsic physical or chemical reactions, the degradation model can be established based on the temperature effect on the reaction rate and the change of reaction volume concentration in the physical/chemical reactions. Different degradation processes can be studied using the degradation model, including both the monotonic and nonmonotonic degradation processes. Moreover, the accelerated test is carried out for the GaN LED, figuring out the parameters for the degradation model, the ratio of different degradation processes, and the time constant.
The characteristics of a blue light-emitting diode (LED) with an AlInN/GaN superlattice (SL) electron-blocking layer (EBL) are analyzed numerically. The carrier concentrations in the quantum wells, energy band diagrams, electrostatic fields, and internal quantum efficiency are investigated. The results suggest that the LED with an AlInN/GaN SL EBL has better hole injection efficiency, lower electron leakage, and smaller electrostatic fields in the active region than the LED with a conventional rectangular AlGaN EBL or a AlGaN/ GaN SL EBL. The results also indicate that the efficiency droop is markedly improved when an AlInN/GaN SL EBL is used.
The effects of a polarization field on the current transport mechanisms in ultraviolet light emitting diodes (UV-LEDs) are studied by analyzing forward current-voltage (I—V) characteristics based on the experimental data and theoretical simulation. The results indicate that polarization electric field suppresses the diffusion current and meanwhile enhances the tunneling current in the metal-face UV LEDs under forward bias. The presence of a large polarization field in the deep UV-LEDs is responsible for the current transport mechanism dominated by the tunneling process at a moderate forward bias.
Thermal property is one of the most important properties of light-emitting diode (LED). Thermal property of LED packaging material determines the heat dissipations of the phosphor and the chip surface, accordingly having an influence on the light-emitting efficiency and the life-span of the device. In this paper, photoacoustic piezoelectric (PAPE) technique has been employed to investigate the thermal properties of polyvinyl alcohol (PVA) and silicon dioxide, which are the new and the traditional packaging materials in white LED, respectively. Firstly, the theory of PAPE technique has been developed for two-layer model in order to investigate soft materials; secondly, the experimental system has been set up and adjusted by measuring the reference sample; thirdly, the thermal diffusivities of PVA and silicon dioxide are measured and analysed. The experimental results show that PVA has a higher thermal diffusivity than silicon dioxide and is a better packaging material in the sense of thermal diffusivity for white LED.
