《led原理》PPT课件.ppt

1、发光二极管led,led,2.1 引言,1、定义：发光二极管（LED）是一种固态发光，是利用半导体或类似结构把电能转换成光能的元件，属于低场下的注入式电致发光。 2、特点： B高，室温下，全色LED大屏幕，500010000cd/m2 工作电压低，15V，可与Si逻辑电路匹配 响应速度快，107 1 09s 彩色丰富，已研制出红绿蓝和黄橙的LED 尺寸小，寿命长（十万小时） 视角宽，96年，达80度；97年，达140度,3、缺点： 电流较大，对七段式LED数码管，10mA/段 功耗大，装配大型矩阵屏时散热问题突出 4、简史 1923年，由Losev在产生p-n结的SiC中发现注入式电致发光 6

2、0年代末，LED得到迅速发展 1964年，Gvimmeiss和Scholz以GaP间接带材料隙得到橙、黄、绿的LED 80年代，蓝光的LED研制出来，用宽带隙n型或半绝缘的GaN、ZnS或ZnSe上形成肖特基势垒形成,2.2 LED的发光机理,PN结断面示意图,What happens when p-type 间接带隙除 -族化合物半导体LED,Rs=10欧姆.Rs上的压降不影响量子效率,但减小了功率效率. 功率效率的可以表示为:,提高功率效率,则Rs下降,掺杂浓度升高.单不能太高(因为要影响LED的结晶完整性),一般取1018cm-3为宜. 提高LED效率的途径 a.选择适当的掺杂浓度 在p

3、-n结加上正向偏压以后，注入结区的载流子有一部分被晶格缺陷和有害杂质俘获，形成空间非辐射复合。这种复合尽量避免，因俘获中心有限，可加大注入电流使其饱和，扩散电流jd开始起主要作用,在LED中，只有一种扩散电流对发光贡献大。 在红光GaP中，主要是jdn在p区的复合，要增加注入效率，就要提高jdn在全部扩散电流中的比例，因为施主杂质浓度（ND）大于受主杂质浓度（NA），要适当提高NA。 但是NA和 ND不能太高，否则因缺陷过多造成过多的光子吸收，减少电子迁移率和增加空穴向N区注入，这些都会降低注入效率，所以NA， ND 都有一个最佳值，一般都在1018cm-3这个数量级上。,b.选择适当的结深

4、辐射复合发出的光从p-n结到达晶体表面之前会受到较大的吸收，为了减小吸收，把结做得薄一些，但是太浅了又会使注入的载流子在体内来不及充分复合就到达电极流走了。因此结深也有一个最佳值，其计算公式：,c.改进LED的结构 由于 -族材料的折射率大（n34），即使垂直入射到空气界面的光也有50的发射，与界面的法线大于16度（全反射临界角）的光完全发射回器件内部。,只有 c立体角内的光才能出射出器件之外，将c16.2度代入，得外量子效率约0.03,即到达表面的光只有3%左右可以出射出来。但是由于晶体本身在光的传输过程中还有很强的吸收作用，因而实际的外量子效率比计算值低。,用球面发射表面结构 这种结构减小

5、了界面发射，但使材料内部光程增大，在增加了吸收,为了提高外量子效率，可采取下列措施：,用折射率较大的介质做成圆顶光窗，以增大半导体内的全发射临界角。,LED-Optical Properties-Light Escape Cone,Total internal reflection at the semiconductor air interface reduces the external quantum efficiency. The angle of total internal reflection defines the light escape cone. sinc = nair/

6、ns Area of the escape cone = 2r2(1-cosc) Pescape / Psource = (1-cosc)/2 = c2/4 = (nair2/ns2)/4,LED-Optical Properties-Emission Spectrum,Light intensity in air (Lambertian emission pattern) is given by Iair = (Psource/4r2) X (nair2/ns2) cos Index contrast between the light emitting material and the s

7、urrounding region leads to non-isotropic emission pattern,LED-Optical Properties-Epoxy encapsulants,Light extraction efficiency can be increased by using dome shaped encapsulants with a large refractive index. Efficiency of a typical LED increases by a factor of 2-3 upon encapsulation with an epoxy

8、of n = 1.5. The dome shape of the epoxy implies that light is incident at an angle of 90o at the epoxy-air interface. Hence no total internal reflection.,在p-n结背面设置合适的反射面，可以利用正面发出的光，也可以使后面的光得到有效的利用,反射面,电极与p-n结的欧姆接触处具有高的吸收系数，应减小接触面积，但会增加串联电阻Rs和降低热导，因而可采用折中的办法，用绝缘层（SiO2）覆盖在二极管表面，同时在其上开一小窗口作欧姆接触。采用此法的红光

9、GaAsP的LED，其吸收系数由700/cm下降到230/cm,相应亮度提高了23倍。,对于GaAs红外LED，可采用掺硅将其辐射波长向长波长方向偏移以减小吸收损耗 选择适当的p-n结半导体材料，使发射光谱与视觉曲线有最大的重叠。,即使矩形结构的LED，会可采取折射率大，吸收小的透明材料封装LED，可增加出射光，入对于GaAsP的LED用环氧树脂封装（nr 1.55), c=25.5度，出射光增加了1.45倍，用低熔点玻璃封装（nr 2.42.6)，则外发光效率可提高47倍。,High extraction efficiency structures,Shaping of the LED di

10、e is critical to improve their efficiency. LEDs of various shapes; hemispherical dome, inverted cone, truncated cones etc have been demonstrated to have better extraction efficiency over conventional designs. However cost increases with complexity.,High extraction efficiency structures,The TIP LED e

11、mploys advanced LED die shaping to minimize internal loss mechanisms. The shape is chosen to minimize trapping of light. TIP LED is a high power LED, and the luminous efficiency exceeds 100 lm/W. TIP devices are sawn using beveled dicing blade to obtain chip sidewall angles of 35o to vertical.,High

12、internal efficiency LED designs,Radiative recombination probability needs to be increased and non-radiative recombination probability needs to be decreased. High carrier concentration in the active region, achieved through double heterostructure (DH) design, improves radiative recombination. R=Bnp D

13、H design is used in all high efficiency designs today.,Doping of the active regions and that of the cladding regions strongly affects internal efficiency. Active region should not be heavily doped, as it causes carrier spill-over in to the confinement regions decreasing the radiative efficiency Dopi

14、ng levels of 1016-low 1017 are used, or none at all. P-type doping of the active region is normally done due to the larger electron diffusion length. Carrier lifetime depends on the concentration of majority carriers. In low excitation regime , the radiative carrier lifetime decreases with increasin

15、g free carrier concentration. Hence efficiency increases with doping. At high concentration, dopants induce defects acting as recombination centers.,High internal efficiency LED designs,P-N junction displacement,Displacement of the P-N junction causes significant change in the internal quantum effic

16、iency in DH LED structures. Dopants can redistribute due to diffusion, segregation or drift.,Doping of the confinement regions,Resistivity of the confinement regions should be low so that heating is minimal. High p-type conc. in the cladding region keeps electrons in the active region and prevents t

17、hem from diffusing in to the confinement region. Electron leakage out of the active region is more severe than hole leakage.,Non radiative recombination,The concentration of defects which cause deep levels in the active region should be minimum. Also surface recombination should be minimized, by kee

18、ping all surfaces several diffusion lengths away from the active region. Mesa etched LEDs and lasers where the mesa etch exposes the active region to air, have low internal efficiency due to recombination at the surface. Surface recombination also reduces lifetime of LEDs.,Lattice matching,Carriers

19、recombine non-radiatively at misfit dislocations. Density of misfit dislocation lines per unit length is proportional to lattice mismatch. Hence the efficiency of LEDs is expected to drop as the mismatch increases.,4、温度特性 温度上升，亮度下降；温度增加1度，发光效率减小1，当LED消耗功率大，则结温上升，输出亮度下降，所以减小功耗，改良散热条件很重要。,Causes inclu

20、de non-radiative recombination via deep levels, surface recombination, and carrier loss over heterostucture barriers.,5、发射光谱 LED的发射光谱由半导体禁带宽度以及杂质浓度决定。描述光谱分布的两个主要参量是峰值波长max与半高宽 。 对GaAs1-xPx和Ga1-xAlxAs由于x不同， max620680nm， 2030nm； 对GaP（红）: max700nm， 10nm； 对GaP（绿）: max570nm， 25nm。 器件工作时的温度 会影响发射光谱，随着温度升高

21、， 变大， max也会发生漂移0.3o.4nm/度， 在光通信中是一个很重要的参量。,6、响应时间 在快速显示，快速调制时，器件对信息的反应速度，即对启亮和熄灭时间有一定的要求，LED响应时间由以下因素决定： LED的上升（启亮）与I有关。 随着I上升，启亮时间近似指数增长，这与发光中心和其它陷阱俘获载流子的情况有关，而LED的衰减与I无关。因为LED是少子注入的正向偏置的p-n结的自发辐射。,响应时间与少子的寿命、结电容、寄生电容有关，但是主要由寿命决定 LED的响应时间很短，主要是由于载流子的直接跃迁复合时间较短。 对GaP为100ns，GaAsP只有几个ns。,7、寿命 LED的寿命一般

22、很长，j1A/cm2,寿命为106h，与j有关，近似表示为,影响LED寿命的因素有： 表面漏电流的增加 象铜之类的沾污物的内扩散 在p-n结附近形成非辐射复合中心。 对于前面两个因素，可采用合适的钝化、封装以及清洗技术予以消除，对于后一个原因可以在制作LED时尽量保证晶格的完整性，降低其缺陷密度，来达到缓解非辐射复合中心产生的速度，但不能完全消除。,2.3.4 LED显示器件常用材料性能,1、GaAs：Eg1.43ev,直接带隙，max在红外线范围，不能直接用于显示 GaAs：Si max940nm； GaAs：Zn max900nm 近年来研制成功的将稀土荧光粉LaF3：Y：Er涂于芯片上，

23、可将红外转移到绿光 这是利用多次连续机理原理实现的。 选择适当的荧光粉可以得到气体颜色的光。 其缺点是：发光不均匀，响应速度慢,2、GaP Eg2.25ev,间接带隙，这类器件有：,优点：透光性能好，颜色丰富，掺入不同的杂质，可发红、黄、绿光； 缺点：外量子效率低 现已采用合成熔融扩散技术和液相外延技术制成高效的绿色GaP的LED.,3、GaAs1-xPx 由GaAs和GaP混晶制成，x：混晶比 Eg随x变化 x0.45 器件为间接跃迁型 掺N的GaAs1-xPx会使发光效率提高,AlGaAs,AlGaAs was the first material for which very high

24、brightness LEDs were demonstrated. The AlGaAs system is nearly lattice-matched to the GaAs substrates for all compositions. When the Al content increases, the bandgap becomes large and indirect.With increasing Al composition, the wavelength will also decrease.,AlGaInP,The AlGaInP system was identifi

25、ed early as one of the most promising for high-performance LEDs. AlGaInP have high external quantum efficiency, like 20% at 630 nm, 10% at 590 nm, and 2% at 570 nm.,AlGaInN,AlGaInN are very differently from the conventional III-V semiconductors.Due to they have large bond strengths, so they require

26、high growth tempertures. They can be grown on SiC and sapphire, but the lattice match between GaN and SiC is much better than for sapphire.,Tungsten(60W),Red Filtered,Edisons First Light Bulb,Halogen (30W),Fluorescent (40W),Low-Pressure Sodium (18W),Yellow Filtered,GREEN,GaAs P,0.6 0.4,GaP:Zn,O,GaP:

27、Zn,O,GaP:N,GREEN,AlGaAs/GaAs,AlGaAs/AlGaAs,AlGaInP/GaAs,AlGaInP/GaP,RED - ORANGE - YELLOW,BLUE,SiC BLUE,PERFORMANCE (LUMENS/WATT),100,0.1,10,1,GaAsP,GaAsP:N,RED - ORANGE - YELLOW,RED - ORANGE - YELLOW,InGaN,RED,RED,RED,RED,1960 1970 1975 1980 1985 1990 1995 2000 2005,RED,Evolution of the Visible-Spe

28、ctrumLight-Emitting Diode,Courtesy of:,Green Filtered,InGaN,InGaN,YELLOW,Shaped AlGaInP/GaP,RED - ORANGE - YELLOW,2.4 LED的应用,LED具有工作电压低（13V），电流小（几几十mA），响应速度快（10-6 10-9s),寿命长（ 105h），B高，可与半导体集成电路匹配，工艺简单，能实现多路驱动等特点，因此可以在以下方面得到很好的应用：,一、指示灯 普通的钨丝灯耐振动性差、易破碎等问题，LED指示灯不断更新换代，其寿命在数十万小时以上，而且功耗小，发光响应速度快，亮度高，小型

29、耐振动等特点，在各种应用中占有明显的优势。 常见的应用：电话、音响制品、家电制品、各种计测仪表以及集中控制盘等许多领域中，有着广泛的应用。 LED显示屏的应用领域：证券交易、金融信息显示；机场航班等的动态显示等。,White-light LEDs,White light can be generated in several different ways. One way is to mix to complementary colors at a certain power ratio. Another way is by the emission of three colors at ce

30、rtain wavelengths and power ratio. Most white light emitters use an LED emitting at short wavelength and a wavelength converter. The converter material absorbs some or all the light emitted by the LED and re-emits at a longer wavelength. Two parameters that are important in the generation of white l

31、ight are luminous efficiency and color rendering index. It is shown that white light sources employing two monochromatic complementary colors result in highest possible luminous efficiency.,White-light LEDs,Wavelength converter materials include phosphors, semiconductors and dyes. The parameters of

32、interest are absorption wavelength, emission wavelength and quantum efficiency. The overall energy efficiency is given by = ext(1/ 2) Even if the external quantum efficiency is 1, there is always an energy loss associated with conversion. Common wavelength converters are phosphors, which consist of

33、an inorganic host material doped with an optically active element. A common host is Y3Al5O12. The optically active dopant is a rare earth element, oxide or another compound. Common rare earth elements used are Ce, Nd, Er and Th.,White LEDs based on phosphor converters,A blue GaInN/GaN LED and a phos

34、phor wavelength converter suspended in a epoxy resin make a white Light LED. The thickness of the phosphor containing epoxy and the concentration of the phosphor determine the relative strengths of the two emission bands,二、数字、文字显示 实现数字、文字显示的方法有两种： 1、单片集成法 用集成电路工艺在一片基片上制成若干相互绝缘的LED管，可按8字或矩阵排列，采用平面或台面

35、工艺实现单片集成。此法的不足是：面积小；基片利用率低。 2、混成法 在绝缘片上装配相互分开的LED管或芯片，按吕字形或矩形排列，每一笔画可用单个或多个LED或芯片做成。此法的优点是：结构简单；可自行排列；成本较低。,A light emitting diode driver circuit,三、LEDTV显示 用作TV显示的GaP LED器件，采用混成器件结构。首先用液相外延技术制作高效率的GaP芯片，装配在陶瓷板上，芯片间距0.8mm，n区直接连接到金属带上，构成Y电极，p区的电极采用直径为0.04mm的金丝连接起来，构成X电极，每一个芯片有一镀金反射罩以提高亮度。,四、LED存储显示 有两种方案“ 1、采用p-n p-n发光二极管实现存储显示,I,2、具有光电导特性的LED存储器,五、LED的多色显示,LED-Based Lighting Treatment for Wound Healing,六、LED Lighting,LED-Based L