Improvement of the efficiency droop of GaN-LEDs using an AlGaN/GaN superlattice insertion layer

With an n-AlGaN(4 nm)/GaN(4 nm) superlattice(SL) inserted between an n-GaN and an InGaN/GaN multiquantum well active layer,the efficiency droop of GaN-based LEDs has been improved.When the injection current is lower than 100 mA,the lumen efficiency of the LED with an n-AlGaN/GaN SL is relatively small compared to that without an n-AlGaN/GaN SL.However,as the injection current increases more than 100 mA,the lumen efficiency of the LED with an n-AlGaN/GaN SL surpasses that of an LED without an n-AlGaN/GaN SL. The wall plug efficiency of an LED has the same trend as lumen efficiency.The improvement of the efficiency droop of LEDs with n-AlGaN/GaN SLs can be attributed to a decrease in electron leakage due to the enhanced current spreading ability and electron blocking effect at high current densities.The reverse current of LEDs at -5 V reverse voltage decreases from 0.2568029 to 0.0070543μA,and the electro-static discharge(ESD) pass yield of an LED at human body mode(HBM)-ESD impulses of 2000 V increases from 60%to 90%.     

Two characteristic parameters for the junction temperature of white LED

This paper proposes a method for characterizing the junction temperature of light-emitting diodes (LEDs) by using two parameters, and the selected reference method is used to eliminate the self-heating effect of white LEDs. The constant current source is used to drive, which improves the practicability and reduces the measurement cost. The junction temperature of cold and warm white LED is measured with a small current of 50—400 mA as the driving current. The studied ambient temperature range is 30—80 °C. The results show that the relationship between the spectrum valley value of the calibration function, the full width at half maximum (FWHM), driving current, and junction temperature can be combined with a high degree of the fitting. Compared with the measurement results of the forward voltage method, the maximum error of the measurement of the two-parameter joint characterization junction temperature method is only 2.38 °C. It is a low-cost, practical, and effective junction temperature measurement method for white LEDs.     

使用ZnO电流扩展层提高GaN基LED提取效率

Gallium nitride （GaN） based light emitting diodes （LEDs） with a thick and high quality ZnO film as a current spreading layer grown by metal-source vapor phase epitaxy （MVPE） are fabricated successfully. Compared with GaN-based LEDs employing a Ni/Au or an indium tin oxide transparent current spreading layer, these LEDs show an enhancement of the external quantum efficiency of 93% and 35% at a forward current of 20 mA, respectively. The full width at half maximum of the ZnO （002） ω-scan rocking curve is 93 arcsec, which corresponds to a high crystal quality of the ZnO film. Optical microscopy and atomic force microscopy are used to observe the surface morphology of the ZnO film, and many regular hexagonal features are found. A spectrophotometer is used to study the different absorption properties between the ZnO film and the indium tin oxide film of the GaN LED. The mechanisms of the extraction quantum efficiency increase and the series resistance change of the GaN-based LEDs with ZnO transparent current spreading layers are analyzed.     

Correlated Color Temperature Tunable Multi-chip Light Emitting Diodes Light Source Design

One of the methods to derive white light from light emitting diodes（LEDs） is the multi-chip white LED technology, which mixes the light from red, green and blue LEDs. Introduced is an optimal algorithm for the spectrum design of the multi-chip white LEDs in this paper. It optimizes the selection of single color LEDs and drive current controlling, so that the multi-chip white LED achieves the target correlated color temperature （CCT）, as well as high luminous efficacy and good color rendering. A CCT tunable LED light source with four high-power LEDs is realized based on the above optimal design. Test results show that it maintains satisfactory color rendering and stable luminous efficacy across the whole CCT tuning range. Finally, discussed are the design improvement and the prospect of the future applications of the CCT tunable LED light source.     

Extraction efficiency enhancement of LEDs using 2-D photonic crystal

Using photonic crystals (PCs), the extraction efficiency of the light-emitting diodes (LEDs) can be greatly enhanced by the effects of photonic band gap (PBG) and grating diffraction. The two theoretical methods are also discussed. Meanwhile, we demonstrate that LEDs can achieve high extraction efficiency by employing photonic crystal as diffraction grating. PCs with square lattice of cylindrical unit cells are fabricated in GaN layer of GaN-based blue LED. We present a theoretical discussion on the extraction efficiency of PC-LED, which relies on the effective medium theory and transmission matrix method to investigate the effect of lattice constant. The results show that the extraction efficiency of the high performance LEDs can achieve 61.8% when the lattice constant is 270 nm.     

单片集成式氮化镓基发光二极管的设计与制造

We report a new monolithic structure of GaN-based light-emitting diode(LED) which can be operated under high voltage or alternative current. Differing from the conventional single LED chip, the monolithic lightemitting diode(MLED) array contains microchips which are interconnected in series or parallel. The key chip fabrication processing methods of the monolithic LED array include deep dry etching, sidewall insulated protection, and electrode interconnection. A 12 V GaN-based blue high voltage light emitting diode was designed and fabricated in our experiment. The forward current-voltage characteristics of MLEDs were consistent with those of conventional single junction light emitting diodes.     

电流模线性控制电荷泵的研究与设计

To meet the demands for a number of LEDs,a novel charge pump circuit with current mode control is proposed.Regulation is achieved by operating the current mirrors and the output current of the operational transconductance amplifier.In the steady state,the input current from power voltage retains constant,so reducing the noise induced on the input voltage source and improving the output voltage ripple.The charge pump small-signal model is used to describe the device’s dynamic behavior and stability.Analytical predictions were verified by Hspice simulation and testing.Load driving is up to 800 mA with a power voltage of 3.6 V,and the output voltage ripple is less than 45 mV.The output response time is less than 8μs,and the load current jumps from 400 to 800 mA.     

表面增透结构AlGaInP LED的研究

A kind of AlGaInP light emitting diode （LED） with surface anti-reflecting structure has been introduced to solve the problems of low light efficiency and restricted luminous intensity. The new structure can be demonstrated theoretically and experimentally, and LEDs with the new structure have higher on-axis luminous intensity and larger saturation current than conventional LEDs and LEDs with ITO film only, which is caused by higher external quantum efficiency and also higher internal quantum efficiency. The new LEDs are especially suitable for working at large injected currents.     

Electrical-efficiency analysis of GaN-based light-emitting diodes with interdigitated-mesa geometry

We propose a novel method to analyze the current-voltage (I-V) characteristics of GaN-based light-emitting diodes (LEDs) with different p-type electrodemesa geometries. The electrical efficiency is analyzed by calculating the electric field under the quasi-coplanar electrodes of GaN-based LEDs. The experimental results for GaN-based LEDs of chip sizes of 350×350 μm² and 1,000 × 1,000 μm² with interdigitated fingers are compared. A good agreement is obtained between the experimental and theoretical electrical efficiency of the GaN LEDs with a chip size of 1,000×1,000 μm². The current-crowding effect is analyzed by measuring the electroluminescence spectra of the devices. The result indicates that the current-crowding effect is largely reduced by increasing the number of interdigitated fingers. The electrical efficiency of a LED with a chip size of 1,000×1,000 μm² can be also enhanced by increasing the number of interdigitated fingers, showing the advantages of GaN LED with interdigitated-mesa geometries.     

一种快速PWM调光,双路控制,无采样电阻的升压LED驱动器

A boost LED driver featuring a high PWM dimming ratio and optimized efficiency is presented.This LED driver,which has a low dropout voltage and is able to drive 3-7 LEDs in series with constant output current and fast PWM dimming,provides an alternative technique for brightness adjustment.A dual-path control scheme with automatic switching and state maintenance is proposed.Meanwhile,a cascode current mirror structure is applied with the output transistor multiplexed as an LED PWM dimming transistor.Implemented in 0.5μm 25 V BCD process,the measurement results show that a voltage conversion range of 5 V input to 6-24 V output with constant output current is obtained.With automatically switching dual-path control and an optimized current mirror,the response time during PWM dimming is reduced to as low as 240 ns and the efficiency keeps above 89%over a wide PWM dimming ratio @ 250 mA output current.     

p-GaN与ITO欧姆接触的研究

针对GaN基发光二极管中p-GaN与透明导电薄膜ITO之间的接触进行研究,尝试找出透明导电层ITO的优化制程条件。将在不同氧流量、ITO厚度及退火温度下制备的透明电极ITO薄膜应用于GaN基发光二极管,来增加电流扩展,减小ITO与p-GaN欧姆接触电阻,降低LED工作电压及提高透过率、增强LED发光亮度。将ITO薄膜应用于218μm×363μm GaN基发光二极管LED,分析其在20 mA工作电流条件下正向电压和光输出功率的变化,在优化条件下制得的蓝光LED在直流电流20 mA下的正向电压3.23 V,光输出效率为23.25 mW。     

注入电流对GaN基LED发光特性的影响

通过调节量子阱中的In组分,制备了GaN基蓝光和绿光发光二极管（LED）。对两种LED进行变电流测试发现,注入电流由3 mA增加到900 mA过程中,波长有蓝移现象,且绿光LED的波长蓝移较明显。这是量子阱限制斯塔克效应（QCSE）造成的。由于绿光LED中In组分含量较大,QCSE较明显。并且发现,光效迅速下降,绿光L...     

GaN基蓝光micro-LED的结温和载流子温度研究

结温及载流子温度因作为影响LED发光效率的重要参数而广受关注。文章研究了GaN基蓝光集成传感微小尺寸发光二极管(micro-LED)的光致发光(PL)光谱、载流子温度等随结温的变化规律。通过内置集成传感单元芯片,设计实现了GaN基蓝光micro-LED在0.04～53.4 A/cm²电流密度下结温和PL光谱的实时精确测量,并将正向电压法测量结温的低温端范围拓展至123 K。结果表明,低温下由于载流子泄漏、串联电阻的原因,结温与正向电压的线性斜率发生变化。针对PL光谱使用高能侧斜率法计算得到不同电流密度下的载流子温度,发现载流子温度与结温在所研究的结温和电流密度范围内可以近似用二次方程拟合,并对载流子温度随结温和电流密度变化的规律进行了分析和解释。     

一体化封装LED结温测量与发光特性研究

基于一体化封装基板,制备了大功率白光LED。以低热阻的一体化封装基板为基础,设计了结温测量系统。利用光谱仪测得不同结温下LED的光电参数,并对其机理进行了分析。在工作电流为0.34A,所研究温度范围为10.8～114.9℃。实验结果表明,一体化封装的LED结温与正向电压、光通量、光效和色温有着良好的线性关系;结温的变化对主波长及色坐标影响甚微;结温的上升导致蓝光段强度下降且光谱发生红移,黄光段强度上升且光谱发生宽化,峰值波长由450nm转为550nm。     

GaN基功率LED高低温特性研究

首次对自制的GaN基大功率白光和蓝光发光二极管在-30~100°C的温度下进行了在线的光电特性测试,对两种不同LED的正向电压、相对光强、波长、色温等参数随温度变化的关系进行了数据曲线拟合,对比分析了参数变化的原因,以及这些变化对实际应用的影响。结果表明,温度对大功率LED的光电特性有很大影响,通过对比发现白光LED的部分光参数随温度的变化不仅与GaN芯片有关,同时受到荧光粉的影响。低温环境下,要考虑LED的正向电压升高和峰值波长蓝移对应用的影响;而高温条件下要考虑光功率降低和峰值波长红移对应用的影响。     

纳米柱InGaN/GaN多量子阱的干法刻蚀制备技术

纳米柱GaN基多量子阱(MQW)拥有量子尺寸效应以及应变释放等特性,对于提高GaN基发光二极管(LED)的发光效率具有重要意义.采用快速热退火(RTA)形成的自组装Ni纳米颗粒作为刻蚀掩膜,利用电感耦合等离子体反应离子刻蚀(ICP-RIE)制备纳米柱InGaN/GaN MQW.通过改变RTA温度发现在800℃以上才能有效形成Ni纳米颗粒掩膜.不同的ICP和射频(RF)功率条件下制备的纳米柱MQW光致发光强度相比于相同结构的平面MQW会发生显著变化.通过优化ICP-RIE的刻蚀条件,可以获得发光强度显著提高的纳米柱MQW结构.同时,纳米柱MQW中压电极化场的减弱会形成光致发光峰位蓝移.     

氮化镓基发光二极管的发光光谱和功率特性

用直流和脉冲电流的方法研究了氮化镓基紫色和蓝色发光二极管的发光光谱和功率特性.结果表明，紫色发光二极管的发光中心波长在直流情况下随电流的增加发生红移，在脉冲情况下随电流的增加发生蓝移；蓝色发光二极管的发光中心波长在直流和脉冲情况下都发生蓝移.两种发光二极管的功率在直流情况下会发生饱和，并随电流的进一步增加而急剧减小，以上现象可能是由于热效应和量子阱中的压电效应引起的.     

Color rendering and luminous efficacy of trichromatic and tetrachromatic LED-based white LEDs

White light-emitting diode (LED) spectra for general lighting should be designed for high luminous efficacy as well as good color rendering, which are generally in a trade-off relationship. White LEDs have uncountable metameres, they have different luminous efficacy and color rendering. Appropriate designed trichromatic and tetrachromatic LED-based white LEDs are presented that have acceptable color rendering as well as good luminous efficacy. Triachromatic white LEDs, with a wavelength combination of 460, 540, and 615 nm, offer high general color rendering index exceeding 89, and luminous efficacy 336 lm/W. The general color rendering index of tetrachromatic LED-based white LEDs combined from 460, 525, 590, and 640 nm is 95, the luminous efficacy is 306 lm/W. Further analysis shows the changing trends of the luminous efficacy, color rendering and the chromaticity coordinate of the optimized trichromatic and tetrachromatic white LEDs depending on the wavelength shift of the primary LEDs. For the optimized trichromatic white LEDs, both the luminous efficacy and color rendering change more with the wavelength shifts of the primary red LEDs than with the wavelength shifts of the blue and green LEDs. For the optimized tetrachromatic white LEDs, the changes of the luminous efficacy caused by the wavelength shifts of one red LED are smaller than the changes of trichromatic white LEDs. And the wavelength shifts of the red primary LED that have shorter wavelength affect the color rendering more than the other primary LEDs. The wavelength shifts of the blue primary LED change the chromaticity coordinate of the white LEDs more. The small changes of the chromaticity coordinate of the white LED do not mean small changes of the k and R_a.     

垂直结构GaN基LEDs电流分布计算分析

电流分布是影响大功率LEDs器件性能的重要因素,与传统结构GaN基器件比较,垂直结构器件通过采用上下电极分布,明显改善了LEDs器件内部电流分布均匀性。通过理论分析与数值计算,建立起了垂直结构GaN基LEDs电流分布模型,研究了垂直结构GaN基LEDs电流分布及I-V特性。结果表明,与传统平面结构比较,垂直结构GaN基LEDs的电流分布均匀性得到了明显改善,同时正向电压降低约7%。最后,通过晶片键合与激光剥离技术,制备了垂直结构GaN基LEDs,测试结果表明,实验结果和理论计算值相吻合。该结果对GaN基LEDs器件的优化设计具有重要指导意义。     

大功率LED的光衰机制研究

为了研究大功率LED的光衰机制,选用了一系列的大功率白光、蓝光发光二极管分别进行恒流点亮,在点亮不同时间阶段测量其光通量、发光谱及伏安特性.发现在光衰过程中,光通量有时会上升;通过LED光谱测定,发现光谱分布有明显变化;同时,pn结内阻也逐渐变大.研究表明大功率LED光衰有较为复杂的过程:其中荧光粉老化及pn结性能退化是最主要的.文章对此进行了初步的分析,为白光LED的应用及进一步研究白光LED衰减提供了参考.