结温与热阻制约大功率LED发展

LED结温高低直接影响到LED出光效率、器件寿命、可靠性、发射波长等。保持LED结温往允许的范围内,是大功率LED芯片制备、器件封装和器件应用等每个环节都必须重点研究的关键因素,尤其是LED器件封装和器件应用设计必须着重解决的核心问题。首先介绍pn结结温对LED器什性能的影响,接着分析大功率LED结温与器件热阻的关系．基于对器件热阻的分析,得出了结温与热阻已经制约大功率LED进一步向更大功率发展的结论,并提出了如下两个观点：1．要在保持低成本和自然散热方式下提高LED器件的功率,根本的出路是提高光转换效率;2．在日前没有提高光转换效率的情况下,发展超过5W的大功率器件对工程应肘没有实质意义。     

利用温变电容特性测量发光二极管结温的研究

结区的温度, 简称结温, 是发光二极管(LED) 的重要参数之一, 它对LED 器件的出光效率、光色、器件可靠性和寿命均有很大影响, 准确测量LED 器件的结温对制备LED 芯片、器件封装和应用有着重要的意义. 本文利用反向偏压下的LED的势垒电容随温度变化的特性, 提出了一种LED结温测量的新方法. 论文首先测量和分析了LED在室温下反向偏压时的电容-电压(C-V)曲线和不同反向偏压下的电容-温度(C-T)曲线, 结果表明, 在合适的偏压下, LED的电容随温度的增大而显著增加, 并呈现良好的线性关系. 在LED工作中监测其电容的变化, 并与C-T曲线进行对比, 实现了LED结温的测量, 其测量结果和传统的正向电压法的结果相对比, 两者符合较好. 最后, 利用上述方法测量了LED 在恒流和恒压条件下的结温的实时变化过程. 较传统的结温测量方法, 本方法的优点在于只须要一次定标测量, 且可实现LED在任意电压和电流下的结温测量.     

基于热场分析的LED日间行车灯优化设计

针对LED日间行车灯的设计需求,提出通过热场仿真分析实现其热性能优化设计的方法。首先,根据LED日间行车灯的结构建立其有限元仿真模型,通过实验观测LED日间行车灯工作中的温度分布情况,并以实验观测结果验证仿真模型的有效性;然后,基于以上模型仿真分析日间行车灯的灯珠间距、铝基板厚度与LED灯珠结温之间的关系;以上述关系为约束条件,结合生产成本,得出在LED灯珠一定的情况下,灯珠间距为37 mm、铝基板厚度为1 mm的最优化设计方案,使LED日间行车灯能够可靠工作且成本更低。     

Analysis of junction temperatures for groups III–V semiconductor materials of light-emitting diodes

This study utilizes the Shockly equation and Fourier’s law with the optical, electrical and thermal properties of LEDs to predict the variation of the junction temperature with the injection current. It is shown that the relationship of the junction temperature with the injection current can be determined by the effective thermal conductivity, temperature coefficient of junction voltage, series resistance, integral constant (forward voltage at the initial forward current and junction temperature), ambient temperature and external quantum efficiency. The effective thermal conductivity, temperature coefficient of junction voltage, and series resistance are calculated from the material properties based on the structures of LEDs instead of measurements in this study. The junction temperature of AlGaInP/GaInP MQW red LED predicted from this study agrees with the available experimental data and the junction temperatures of GaInN UV LED and AlGaN UV LED calculated by this work are also consistent with these obtained from the emission peak shift method. The elevated temperatures of AlGaN and GaInN are larger than that of AlGaInP/GaInP MQW red LED due to the difference of the thermal conductivity for the LED substrate. This study also presents the effects of the parameters on the variation of the junction temperature with the injection current. The effective thermal conductivity and ambient temperature significantly affect the junction temperature of LEDs.     

Enhanced quadratic photocurrent in commercial light emitting diodes for autocorrelation measurement of ultrashort laser pulses

The quadratic photocurrent in commercial light emitting diodes (LEDs) has been studied in reverse bias mode for autocorrelation measurement of ultrashort laser pulses. It is found that the photocurrent can be greatly enhanced by operating the LED biased just below the reverse bias breakdown threshold. The effect of aging of LEDs on laser exposure in this mode of operation is found to be similar to that for the photovoltaic mode. The large internal gain in LED junction has enabled the recording of the second order autocorrelation signal of ∼200 fs laser pulses from 100 MHz laser oscillator with two orders of magnitude smaller average and peak power product compared to the case of the photovoltaic mode. PACS 42.65.Re; 42.50.Hz; 85.60.Jb     

Determining junction temperature based on material properties and geometric structures of LEDs

Based on the material properties and geometric structures of LEDs, this paper proposes an analytical model for predicting the junction temperature. The parameters relevant to the junction temperature are calculated using the material properties and geometric structures of LEDs. The junction temperature of AlGaInP LED predicted from this work agrees with the available experimental data. Effects of LED working parameters and material properties on the junction temperatures are discussed in this study.     

白光LED智能调光数学模型

LED灯的光通量不仅受电流的影响,还受到结温变化的影响。针对传统LED调光存在只改变电流大小而忽略结温变化对光通量影响所存在的不足,结合LED结温和管脚温度存在特定的关系,利用实验装置采集HL001WY型Ga N基白光LED在不同管脚温度和正向电流下的光通量实验数据并进行二次项趋势回归,利用定标和归一化方法建立光通量、电流以及管脚温度三者变化关系的数学模型。计算结果表明,利用该方法建立的模型得到的计算数值与实验实际测量值的相对误差小于4.5%。     

LED结温与光谱特性关系的测量

采用恒定驱动电流改变环境温度和恒定环境温度改变驱动电流两种方法分别对直径5 mm封装的AlGaInP型红光和黄光LED,InGaN型绿光和蓝光LED,以及InGaN蓝光+荧光粉的白光LED的结温与其光谱特性进行了测量,得到了不同条件下LED结温与光谱特性的关系.结果表明;AlGaInP LED的峰值波长与结温有良好线性关系,InGaN LED的峰值波长则与结温没有明显对应关系;但白光LED发射光谱的白、蓝功率比与结温有良好线性关系;对AlGaInP LED及蓝光激发的白光LED,通过光谱特性测量可快速、准确地确定光源系统中各LED的结温继而预测光源系统的有效寿命.     

Si衬底GaN基LED的结温特性

结温是发光二极管的重要参数之一,它对器件的内量子效率、输出功率、可靠性及LED的其他一些性能有很大的影响。首次报道Si衬底GaN基LED的结温特性。利用正向压降法测量Si衬底上GaN基LED的结温,通过与蓝宝石衬底上GaNLED的结温比较,发现Si衬底GaNLED有更低的结温,原因归结为Si有更好的导热性。同时也表明:用Si作GaNLED的衬底在大功率LED方面具有更大的应用潜力。     

ZnO nanorods–polymer hybrid white light emitting diode grown on a disposable paper substrate

We demonstrate intrinsic white light emission from hybrid light emitting diodes fabricated using an inorganic–organic hybrid junction grown at 50 °C on a paper substrate. Cyclotene was first spin coated on the entire substrate to act as a surface barrier layer for water and other nutrient solutions. The active area of the fabricated light emitting diode (LED) consists of zinc oxide nanorods (ZnO NRs) and a poly(9,9‐dioctylfluorene) (PFO) conducting polymer layer. The fabricated LED shows clear rectifying behavior and a broad band electroluminescence (EL) peak covering the whole visible spectrum range from 420 nm to 780 nm. The color rendering index (CRI) was calculated to be 94 and the correlated color temperature (CCT) of the LED was 3660 K. The low process temperature and procedure in this work enables the use of paper substrate for the fabrication of low cost ZnO–polymer white LEDs for applications requiring flexible/disposable electronic devices. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Study of the Light Emitting Diode as a photoreceptor: Spectral and electrical characterization as function of temperature and lighting source

In this study, the temperature influence on the spectral responsivity of a Light Emitting Diode (LED) used as a photoreceptor, combined to light source spectrum is correlated to electrical characteristics in order to propose an alternative method to estimate LED junction temperature, regardless of the absolute illumination intensity and based on the direct correlation between the integral of the product of two optical spectra and the photo-generated currents. A laboratory test bench for experimental optical measurements has been set in order to enable any characterizing of photoelectric devices in terms of spectral behaviour, in a wavelength range placed between 400–1000?nm, and of current-voltage characteristics as function of temperature by using two different illumination sources. The temperature is analysed in a range from 5?°C up to 85?°C, so as to evaluate thermal variation effects on the sensor performance. The photo-generated current of two LEDs with different peak wavelengths has been studied. Research has observed and mathematically analysed what follows: since the photo-generated current strictly depends on the combination between the spectral response of the photoreceptor and the lighting source response, it becomes possible to estimate indirectly the junction temperature of the LEDs by considering the ratio between the photogenerated currents obtained by using two different illumination sources. Such results may for one thing increase knowledge in the fields where LEDs are used as photo-detectors for many applications and for another, they could be extended to generic photodetectors, thus providing useful information in photovoltaic field, for instance.     

LED感应局部加热封装试验研究

采用感应局部加热技术,对大功率发光二极管(LED)封装进行了试验研究。结果表明,由于感应加热对材料和结构具有选择性,封装过程中仅Cu-Sn合金焊料层加热,实现了芯片和覆铜陶瓷基板间的热键合。封装后的LED性能测试表明,该封装技术不仅降低了热阻,使LED在高电流下(4倍电流)仍能保持较低的工作温度,而且降低了热应力和整体高温对芯片结构的损坏,提高了器件性能和可靠性。     

一种大功率LED散热系统模糊控制器设计

基于当前的COB封装LED芯片,分析了芯片的热阻模型,推导出发光结在理想温度下工作时的基板温度。针对大功率LED存在的散热问题,基于课题组双进双出射流冲击水冷散热系统,设计了一种模糊控制器,选取温度变化和温度变化率为控制输入量,并对各控制输入量的范围设定进行了说明。根据设计的控制器进行程序编写,下载到控制芯片中进行实际验证,在20℃环境温度下,芯片基板温度最终维持在35.5~36.5℃之间,保证了灯具正常、稳定工作,为大功率LED散热系统提供了一种控制器设计方案,具有一定的实际意义。     

High-brightness gallium nitride nanowire UV–blue light emitting diodes

We report on high-brightness GaN nanowire UV–blue light emitting diodes (LEDs), which are fabricated by coupling of n-GaN nanowires and p-GaN substrates using two assembly methods, random dispersion (RD) and dielectrophoresis assisted assembly deposition (DAAD). These GaN nanowire LEDs have bright UV–blue emission (411–437?nm) from the n-GaN nanowire/p-GaN substrate junction and the light emission is strong enough to be observed with the naked eye even for a single GaN nanowire LED. The results reported here should have significant implications for the fabrication of highly efficient, low-cost UV–blue LEDs with low power consumption, as compared to conventional thin-film based GaN LEDs.     

传统白光LED与远程荧光粉白光LED的发光性能比较

研究了传统白光LED与蓝光激发的球冠形远程荧光粉白光LED在不同电流、不同热沉温度下的发光性能,并对其机理差异展开了探讨。实验结果表明：随热沉温度和驱动电流的上升,传统白光LED的量子效率和电光转换效率急剧下降,并导致其Y/B比（Yellow/Blue Ratio）下降,相关色温上升。而在远程荧光粉白光LED中,其量子效率、光转换效率和相关色温在相同实验条件下变化幅度都较小。由光强空间分布和Y/B比空间分布可知,远程荧光粉白光LED的光强分布呈类似蝠翼分布,且Y/B比空间均匀性远大于传统白光LED。     

GaN基白光LED的结温测量

用正向电压法、管脚法和蓝白比法等三种方法测量GaN基白光LED的结温,获得了较为准确的结温,误差可以控制在4℃以内.正向电压法在恒定电流的条件下,得到了正向电压与结温的线性关系;蓝白比法在不同环境温度和不同注入电流两种情况下,都得到了蓝白比与结温较好的线性关系.提出了蓝白比法可能的物理机制,提高环境温度和增大注入电流都会使结温升高,蓝光峰值波长也会改变,这两个因素都会影响荧光粉的激发和发光效率.降低结温需要考虑的主要因素有白光LED的接触电阻、串联电阻和外量子效率,封装材料的热导率,反射杯和管脚的设计,以及空气散热部分的散热面积等.     

A model for LED spectra at different drive currents

A mathematical model for the spectra of monocolor light-emitting diodes （LEDs） and phosphor-coated white LEDs at different drive currents is established.The simulation program of the color rendering of a white light LED cluster is developed based on this model.The program can predict not only the spectral power distribution and color rendering index （CRI）,but also the number of LEDs,drive currents,input power,and luminous efficacy of a white light LED cluster at a given color temperature according to the requirement of the luminous flux.The experimental results show that the relative spectral power distributions （SPDs） and chromaticity coordinates of the model LED are very close to that of the real LED at different drive currents.Moreover,the correlated color temperature （CCT）,CRI,special color rendering index （R9） luminous flux,input power,and luminous efficacy of the white light LED cluster predicted by simulation are also very close to the measured values.Furthermore,a white/red cluster with high rendering （CCT=2903 K,CRI=91.3,R9=85） and a color temperature tunable warm-white/red/green/blule LED cluster with high rendering （CCT=2700 6500 K,CRI 〉 90,R9 〉 96） are created.     

温度和电流对白光LED发光效率的影响

对大功率白光LED发光效率进行了研究,得出温度和电流对LED发光效率的影响:随着温度的升高,势阱中辐射复合几率降低,从而降低了发光效率;电流的升高,使更多的非平衡载流子穿过势垒,降低了发光效率。LED工作时,过高的工作温度或者过大的工作电流都会产生明显的光衰:如果LED工作温度超过芯片的承载温度,这将会使LED的发光效率快速降低,产生明显的光衰,并且对LED造成永久性破坏;如果LED的工作电流超过芯片的饱和电流,也会使LED发光效率快速降低,产生明显的光衰。并且LED所能承载的温度与饱和电流有一定关系,散热良好的装置可以使LED工作温度相对降低些,饱和电流也可以更大,LED也就可以在相对较大的电流下工作。     

白光LED光斑均匀性的改进

用于照明领域的白光LED,其出射光斑的色度均匀性对于产品性能有着更加重要的意义.介绍了目前工业上制作白光LED主要采用的荧光粉灌封点胶工艺.并在目前主流灌封点胶工艺的基础上通过改善荧光粉层结构形状,以提高白光LED器件的出射光斑均匀性.并通过九点法对不同工艺结构下LED出射光斑的空间色度分布进行了测量和分析.通过分析,虽然采用不同的粉层结构,能够一定程度上改善白光光斑的色度均匀性.但总体上,采用这种传统的点胶工艺制作的器件的白光光斑性能不好,现有的荧光粉层灌封点胶工艺存在很大的弊端.荧光粉层的可控性是影响色度均匀性(即光斑均匀性)的主要因素,包括单个器件内的光斑和器件之间的颜色一致性都不理想.     

Large-scale SiO2 photonic crystal for high efficiency GaN LEDs by nanospherical-lens lithography

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 expo- sure 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.