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一種半導體PN結(jié)型二極管,，GaA構(gòu)造的二極管,有電流通過時會發(fā)光.當施加正向偏置時,，將發(fā)出波長取決于其材料和摻雜的光。砷化鎵(GaAs)的晶體結(jié)構(gòu)允許不可見的紅外(IR)發(fā)射,。,、在砷化鎵 中加入磷形成磷砷化鎵((GaAsP)，可使發(fā)射移向可見的紅區(qū),。磷化鎵(GaP)在紅,、黃、綠區(qū)發(fā)射,。在GaAsP上的砷化鋁鎵(A1GaAsP)則可 產(chǎn)生更明亮(高強度)的紅和黃光發(fā)射,。在GaP上的GaP可發(fā)射高強度的綠光。
（一）LED發(fā)光原理
發(fā)光二極管是由Ⅲ-Ⅳ族化合物,，如GaAs（砷化鎵）,、GaP（磷化鎵）、GaAsP（磷砷化鎵）等半導體制成的,，其核心是PN結(jié),。因此它具有一般P-N結(jié)的I-N特性，即正向?qū)ǎ聪蚪刂?、擊穿特性,。此外，在一定條件下,，它還具有發(fā)光特性。在正向電壓下,，電子由N區(qū)注入P區(qū),，空穴由P區(qū)注入N區(qū)。進入對方區(qū)域的少數(shù)載流子（少子）一部分與多數(shù)載流子（多子）復合而發(fā)光,，如圖1所示,。
 假設(shè)發(fā)光是在P區(qū)中發(fā)生的，那么注入的電子與價帶空穴直接復合而發(fā)光,，或者先被發(fā)光中心捕獲后,，再與空穴復合發(fā)光。除了這種發(fā)光復合外,，還有些電子被非發(fā)光中心（這個中心介于導帶,、介帶中間附近）捕獲，而后再與空穴復合,，每次釋放的能量不大,，不能形成可見光。發(fā)光的復合量相對于非發(fā)光復合量的比例越大,，光量子效率越高,。由于復合是在少子擴散區(qū)內(nèi)發(fā)光的，所以光僅在靠近PN結(jié)面數(shù)μm以內(nèi)產(chǎn)生,。
理論和實踐證明,，光的峰值波長λ與發(fā)光區(qū)域的半導體材料禁帶寬度Ｅg有關(guān)，即
λ≈1240/Eg（mm）
式中Eg的單位為電子伏特（eV）,。若能產(chǎn)生可見光（波長在380nm紫光～780nm紅光）,，半導體材料的Eg應(yīng)在3.26～1.63eV之間。比紅光波長長的光為紅外光?，F(xiàn)在已有紅外,、紅、黃,、綠及藍光發(fā)光二極管,，但其中藍光二極管成本、價格很高,，使用不普遍,。
（二）LED的特性
1．極限參數(shù)的意義
（1）允許功耗Pm:允許加于LED兩端正向直流電壓與流過它的電流之積的最大值。超過此值，LED發(fā)熱,、損壞,。
（2）最大正向直流電流IFm：允許加的最大的正向直流電流。超過此值可損壞二極管,。
（3）最大反向電壓VRm：所允許加的最大反向電壓,。超過此值，發(fā)光二極管可能被擊穿損壞,。
（4）工作環(huán)境topm:發(fā)光二極管可正常工作的環(huán)境溫度范圍,。低于或高于此溫度范圍，發(fā)光二極管將不能正常工作,，效率大大降低,。
2．電參數(shù)的意義
（1）光譜分布和峰值波長：某一個發(fā)光二極管所發(fā)之光并非單一波長，其波長大體按圖2所示,。
 由圖可見,，該發(fā)光管所發(fā)之光中某一波長λ0的光強最大，該波長為峰值波長,。
（2）發(fā)光強度IV：發(fā)光二極管的發(fā)光強度通常是指法線（對圓柱形發(fā)光管是指其軸線）方向上的發(fā)光強度,。若在該方向上輻射強度為（1/683）W/sr時，則發(fā)光1坎德拉（符號為cd）,。由于一般LED的發(fā)光二強度小,，所以發(fā)光強度常用坎德拉(mcd)作單位。
（3）光譜半寬度Δλ:它表示發(fā)光管的光譜純度.是指圖3中1/2峰值光強所對應(yīng)兩波長之間隔.
（4）半值角θ1/2和視角：θ1/2是指發(fā)光強度值為軸向強度值一半的方向與發(fā)光軸向（法向）的夾角,。
半值角的2倍為視角（或稱半功率角）,。
 圖3給出的二只不同型號發(fā)光二極管發(fā)光強度角分布的情況。中垂線（法線）AO的坐標為相對發(fā)光強度（即發(fā)光強度與最大發(fā)光強度的之比）,。顯然,，法線方向上南嘍苑⒐馇慷任?，離開法線方向的角度越大,，相對發(fā)光強度越小,。由此圖可以得到半值角或視角值。
（5）正向工作電流If：它是指發(fā)光二極管正常發(fā)光時的正向電流值,。在實際使用中應(yīng)根據(jù)需要選擇IF在0.6·IFm以下,。
（6）正向工作電壓VF：參數(shù)表中給出的工作電壓是在給定的正向電流下得到的。一般是在IF=20mA時測得的,。發(fā)光二極管正向工作電壓VF在1.4～3V,。在外界溫度升高時，VF將下降,。
（7）V-I特性：發(fā)光二極管的電壓與電流的關(guān)系可用圖4表示,。
 在正向電壓正小于某一值（叫閾值）時,，電流極小，不發(fā)光,。當電壓超過某一值后,，正向電流隨電壓迅速增加，發(fā)光,。由V-I曲線可以得出發(fā)光管的正向電壓,，反向電流及反向電壓等參數(shù)。正向的發(fā)光管反向漏電流IR<10μA以下,。
（三）LED的分類
1．按發(fā)光管發(fā)光顏色分
按發(fā)光管發(fā)光顏色分,，可分成紅色、橙色,、綠色（又細分黃綠、標準綠和純綠）,、藍光等,。另外，有的發(fā)光二極管中包含二種或三種顏色的芯片,。
根據(jù)發(fā)光二極管出光處摻或不摻散射劑,、有色還是無色，上述各種顏色的發(fā)光二極管還可分成有色透明,、無色透明,、有色散射和無色散射四種類型。散射型發(fā)光二極管和達于做指示燈用,。
2．按發(fā)光管出光面特征分
按發(fā)光管出光面特征分圓燈,、方燈、矩形,、面發(fā)光管,、側(cè)向管、表面安裝用微型管等,。圓形燈按直徑分為φ2mm,、φ4.4mm、φ5mm,、φ8mm,、φ10mm及φ20mm等。國外通常把φ3mm的發(fā)光二極管記作T-1,；把φ5mm的記作T-1（3/4）,；把φ4.4mm的記作T-1（1/4）。
由半值角大小可以估計圓形發(fā)光強度角分布情況,。從發(fā)光強度角分布圖來分有三類：
（1）高指向性,。一般為尖頭環(huán)氧封裝,，或是帶金屬反射腔封裝，且不加散射劑,。半值角為5°～20°或更小,，具有很高的指向性，可作局部照明光源用,，或與光檢出器聯(lián)用以組成自動檢測系統(tǒng),。
（2）標準型。通常作指示燈用,，其半值角為20°～45°,。
（3）散射型。這是視角較大的指示燈,，半值角為45°～90°或更大,，散射劑的量較大。
3．按發(fā)光二極管的結(jié)構(gòu)分
按發(fā)光二極管的結(jié)構(gòu)分有全環(huán)氧包封,、金屬底座環(huán)氧封裝,、陶瓷底座環(huán)氧封裝及玻璃封裝等結(jié)構(gòu)。
4．按發(fā)光強度和工作電流分
按發(fā)光強度和工作電流分有普通亮度的LED（發(fā)光強度<10mcd）,；超高亮度的LED（發(fā)光強度>100mcd）,；把發(fā)光強度在10～100mcd間的叫高亮度發(fā)光二極管。
一般LED的工作電流在十幾mA至幾十mA,，而低電流LED的工作電流在2mA以下（亮度與普通發(fā)光管相同）,。
除上述分類方法外，還有按芯片材料分類及按功能分類的方法,。
（四）LED的應(yīng)用
由于發(fā)光二極管的顏色,、尺寸、形狀,、發(fā)光強度及透明情況等不同,，所以使用發(fā)光二極管時應(yīng)根據(jù)實際需要進行恰當選擇。
由于發(fā)光二極管具有最大正向電流IFm,、最大反向電壓VRm的限制,，使用時，應(yīng)保證不超過此值,。為安全起見,，實際電流IF應(yīng)在0.6IFm以下；應(yīng)讓可能出現(xiàn)的反向電壓VR<0,。6VRm,。
LED被廣泛用于種電子儀器和電子設(shè)備中，可作為電源指示燈,、電平指示或微光源之用,。紅外發(fā)光管常被用于電視機,、錄像機等的遙控器中。
（1）利用高亮度或超高亮度發(fā)光二極管制作微型手電的電路如圖5所示,。圖中電阻R限流電阻,，其值應(yīng)保證電源電壓最高時應(yīng)使LED的電流小于最大允許電流IFm。
 （2）圖6(a),、(b),、(c)分別為直流電源、整流電源及交流電源指示電路,。
圖(a)中的電阻≈（E-VF）/IF,；
圖(b)中的R≈（1.4Vi-VF）/IF;
圖(c)中的R≈Vi/IF
式中，Vi——交流電壓有效值,。
 （3）單LED電平指示電路,。在放大器、振蕩器或脈沖數(shù)字電路的輸出端,，可用LED表示輸出信號是否正常,，如圖7所示。R為限流電阻,。只有當輸出電壓大于LED的閾值電壓時，LED才可能發(fā)光,。
 （4）單LED可充作低壓穩(wěn)壓管用,。由于LED正向?qū)ê螅娏麟S電壓變化非?？?，具有普通穩(wěn)壓管穩(wěn)壓特性。發(fā)光二極管的穩(wěn)定電壓在1.4～3V間,，應(yīng)根據(jù)需要進行選擇VF,，如圖8所示。
（5）電平表,。目前,，在音響設(shè)備中大量使用LED電平表。它是利用多只發(fā)光管指示輸出信號電平的,，即發(fā)光的LED數(shù)目不同,，則表示輸出電平的變化。圖9是由5只發(fā)光二極管構(gòu)成的電平表,。當輸入信號電平很低時,，全不發(fā)光。輸入信號電平增大時,，首先LED1亮,，再增大LED2亮……,。
 （五）發(fā)光二極管的檢測
1．普通發(fā)光二極管的檢測
（1）用萬用表檢測。利用具有×10kΩ擋的指針式萬用表可以大致判斷發(fā)光二極管的好壞,。正常時,，二極管正向電阻阻值為幾十至200kΩ,反向電阻的值為∝。如果正向電阻值為0或為∞,，反向電阻值很小或為0,，則易損壞。這種檢測方法,，不能實地看到發(fā)光管的發(fā)光情況,，因為×10kΩ擋不能向LED提供較大正向電流。
如果有兩塊指針萬用表（最好同型號）可以較好地檢查發(fā)光二極管的發(fā)光情況,。用一根導線將其中一塊萬用表的“+”接線柱與另一塊表的“-”接線柱連接,。余下的“-”筆接被測發(fā)光管的正極（P區(qū)），余下的“+”筆接被測發(fā)光管的負極（N區(qū)）,。兩塊萬用表均置×10Ω擋,。正常情況下，接通后就能正常發(fā)光,。若亮度很低,，甚至不發(fā)光，可將兩塊萬用表均撥至×1Ω若,，若仍很暗,，甚至不發(fā)光，則說明該發(fā)光二極管性能不良或損壞,。應(yīng)注意,，不能一開始測量就將兩塊萬用表置于×1Ω，以免電流過大,，損壞發(fā)光二極管,。
（2）外接電源測量。用3V穩(wěn)壓源或兩節(jié)串聯(lián)的干電池及萬用表（指針式或數(shù)字式皆可）可以較準確測量發(fā)光二極管的光,、電特性,。為此可按圖10所示連接電路即可。如果測得VF在1.4～3V之間,，且發(fā)光亮度正常,，可以說明發(fā)光正常。如果測得VF=0或VF≈3V,，且不發(fā)光,，說明發(fā)光管已壞。
 2．紅外發(fā)光二極管的檢測
由于紅外發(fā)光二極管,，它發(fā)射1～3μm的紅外光,，人眼看不到,。通常單只紅外發(fā)光二極管發(fā)射功率只有數(shù)mW，不同型號的紅外LED發(fā)光強度角分布也不相同,。紅外LED的正向壓降一般為1.3～2.5V,。正是由于其發(fā)射的紅外光人眼看不見，所以利用上述可見光LED的檢測法只能判定其PN結(jié)正,、反向電學特性是否正常,，而無法判定其發(fā)光情況正常否。為此,，最好準備一只光敏器件（如2CR,、2DR型硅光電池）作接收器。用萬用表測光電池兩端電壓的變化情況,。來判斷紅外LED加上適當正向電流后是否發(fā)射紅外光,。其測量電路如圖11所示
 LED顯示器結(jié)構(gòu)及分類
通過發(fā)光二極管芯片的適當連接（包括串聯(lián)和并聯(lián)）和適當?shù)墓鈱W結(jié)構(gòu)?？蓸?gòu)成發(fā)光顯示器的發(fā)光段或發(fā)光點,。由這些發(fā)光段或發(fā)光點可以組成數(shù)碼管、符號管,、米字管,、矩陣管、電平顯示器管等等,。通常把數(shù)碼管,、符號管、米字管共稱筆畫顯示器,，而把筆畫顯示器和矩陣管統(tǒng)稱為字符顯示器。
（一）LED顯示器結(jié)構(gòu)
基本的半導體數(shù)碼管是由七個條狀發(fā)光二極管芯片按圖12排列而成的,?？蓪崿F(xiàn)0～9的顯示。其具體結(jié)構(gòu)有“反射罩式”,、“條形七段式”及“單片集成式多位數(shù)字式”等,。
 （1）反射罩式數(shù)碼管一般用白色塑料做成帶反射腔的七段式外殼，將單個LED貼在與反射罩的七個反射腔互相對位的印刷電路板上,，每個反射腔底部的中心位置就是LED芯片,。在裝反射罩前，用壓焊方法在芯片和印刷電路上相應(yīng)金屬條之間連好φ30μm的硅鋁絲或金屬引線,，在反射罩內(nèi)滴入環(huán)氧樹脂,，再把帶有芯片的印刷電路板與反射罩對位粘合，然后固化,。
反射罩式數(shù)碼管的封裝方式有空封和實封兩種,。實封方式采用散射劑和染料的環(huán)氧樹脂,，較多地用于一位或雙位器件?？辗夥绞绞窃谏戏缴w上濾波片和勻光膜,，為提高器件的可靠性，必須在芯片和底板上涂以透明絕緣膠,，這還可以提高光效率,。這種方式一般用于四位以上的數(shù)字顯示（或符號顯示）。
（2）條形七段式數(shù)碼管屬于混合封裝形式,。它是把做好管芯的磷化鎵或磷化鎵圓片,，劃成內(nèi)含一只或數(shù)只LED發(fā)光條，然后把同樣的七條粘在日字形“可伐”框上,，用壓焊工藝連好內(nèi)引線,，再用環(huán)氧樹脂包封起來。
（3）單片集成式多位數(shù)字顯示器是在發(fā)光材料基片上（大圓片）,，利用集成電路工藝制作出大量七段數(shù)字顯示圖形,，通過劃片把合格芯片選出，對位貼在印刷電路板上,，用壓焊工藝引出引線,，再在上面蓋上“魚眼透鏡”外殼。它們適用于小型數(shù)字儀表中,。
（4）符號管,、米字管的制作方式與數(shù)碼管類似。
（5）矩陣管（發(fā)光二極管點陣）也可采用類似于單片集成式多位數(shù)字顯示器工藝方法制作,。
（二）LED顯示器分類
（1）按字高分：筆畫顯示器字高最小有1mm（單片集成式多位數(shù)碼管字高一般在2～3mm）,。其他類型筆畫顯示器最高可達12.7mm（0.5英寸）甚至達數(shù)百mm。
（2）按顏色分有紅,、橙,、黃、綠等數(shù)種,。
（3）按結(jié)構(gòu)分,，有反射罩式、單條七段式及單片集成式,。
（4）從各發(fā)光段電極連接方式分有共陽極和共陰極兩種,。
所謂共陽方式是指筆畫顯示器各段發(fā)光管的陽極（即P區(qū)）是公共的，而陰極互相隔離,。
所謂共陰方式是筆畫顯示器各段發(fā)光管的陰極（即N區(qū)）是公共的,，而陽極是互相隔離的。如圖13所示。
圖13
（三）LED顯示器的參數(shù)
由于LED顯示器是以LED為基礎(chǔ)的,，所以它的光,、電特性及極限參數(shù)意義大部分與發(fā)光二極管的相同。但由于LED顯示器內(nèi)含多個發(fā)光二極管,，所以需有如下特殊參數(shù)：
1．發(fā)光強度比
由于數(shù)碼管各段在同樣的驅(qū)動電壓時,，各段正向電流不相同，所以各段發(fā)光強度不同,。所有段的發(fā)光強度值中最大值與最小值之比為發(fā)光強度比,。比值可以在1.5～2.3間，最大不能超過2.5,。
2．脈沖正向電流
若筆畫顯示器每段典型正向直流工作電流為IF,，則在脈沖下，正向電流可以遠大于IF,。脈沖占空比越小,，脈沖正向電流可以越大。
（四）LED顯示器的應(yīng)用指南
1．七段數(shù)碼顯示器
（1）如果數(shù)碼宇航局為共陽極形式,，那么它的驅(qū)動級應(yīng)為集電極開路（OC）結(jié)構(gòu),，如圖14（a）所示。
如果數(shù)碼管為共陰極形式,，它的驅(qū)動級應(yīng)為射極輸出或源極輸出電路,，如圖14(b)所示。
例如國產(chǎn)TTL集成電路CT1049,、CT4049為集電極開路形式七段字形譯碼驅(qū)動電路,；而CMOS集成電路CC4511為源極輸出七段鎖存、譯碼驅(qū)動電路,。
（2）控制數(shù)碼管驅(qū)動級的控制電路（也稱驅(qū)動電路）有靜態(tài)式和動態(tài)式兩類,。
①靜態(tài)驅(qū)動：靜態(tài)驅(qū)動也稱直流驅(qū)動。靜態(tài)驅(qū)動是指每個數(shù)碼管各用一個筆畫譯碼器（如BCD碼二-十進制譯碼器）譯碼驅(qū)動,。圖15是一位數(shù)碼管的靜態(tài)驅(qū)動之例,。圖集成電路TC5002BP內(nèi)含有射極輸出驅(qū)動級，所以采用共陰極數(shù)碼管,。A、B,、C,、D端為BCD碼（二-十進制的8421碼）輸入端，BL為數(shù)碼管熄滅及顯示狀態(tài)控制端,，R為外接電阻,。
圖16為N位數(shù)字靜態(tài)驅(qū)動顯示電路。
②動態(tài)驅(qū)動：動態(tài)驅(qū)動是將所有數(shù)碼管使用一個專門的譯碼驅(qū)動器，使各位數(shù)碼管逐個輪流受控顯示,，這就是動態(tài)驅(qū)動,。由于掃描速度極快。顯示效果與靜態(tài)驅(qū)動相同,。圖17是一種四位數(shù)字動態(tài)驅(qū)動（脈搏沖驅(qū)動）方法的線路,。圖中只用了一個譯碼驅(qū)動電路TC5002BP。
圖17
TC4508BP內(nèi)含兩個鎖存器,，每個鎖存器可鎖存四位二進BCD碼,，對應(yīng)于四位十進制數(shù)的四組BCD碼分別輸入到四個鎖存器，四個鎖存器,，四組BCD碼由四個鎖存器分時輪流輸出進入譯碼器,，譯碼后進入數(shù)碼管驅(qū)動級集成電路TD62505P（輸入端I1～I7與輸出端Q1～Q7一一對應(yīng)）。Q1～Q7分別加到四個數(shù)碼管的a～g七個陽極上,。數(shù)字驅(qū)動電路TD62003P是由達林頓構(gòu)成的陣列電路,，Q1～Q4中哪一端接地，由輸入端I1～I4的四師長“使能”信號DS1～DS4控制,。由于四個鎖存器的輪換輸出也是受“使能”信號DS1～DS4控制,。所以四個數(shù)碼管輪流通電顯示。由于輪流顯示頻率較高,，故顯示的數(shù)字不呈閃爍現(xiàn)象,。
2．米字管、符號管顯示器
米字管和符號管的結(jié)構(gòu)原理相機,，所以其驅(qū)動方式也基本相同,，只是譯碼電路的譯碼過程與七段譯碼器不同。
米字管可以顯示包括英文字母在內(nèi)的多種符號,。符號管主要是用來顯示+,、-或±號等。
3．LED點陣式顯示器
LED點陣式顯示器與由單個發(fā)光二極管連成的顯示器相比,，具有焊點少,、連線少，所有亮點在同平面,、亮度均勻,、外形美觀等優(yōu)點。
點陣管根據(jù)其內(nèi)部LED尺寸的大小,、數(shù)量的多少及發(fā)光強度,、顏色等可分為多種規(guī)格。圖18所示是具有代表性的P2057A和P2157A兩種φ5高亮度橙紅色5×7點陣組件,。采用雙列直插14腳封裝,，兩種顯示器的差別是LED極性不同，如圖18所示。
該顯示器用掃描驅(qū)動方式,，選擇較大峰值電流和窄脈沖作驅(qū)動源,，每個LED的平均電流不應(yīng)超過20mA。
LED點陣管可以代替數(shù)碼管,、符號管和米字管,。不僅可以顯示數(shù)字，也可顯示所有西文字母和符號,。如果將多塊組合,，可以構(gòu)成大屏幕顯示屏，用于漢字,、圖形,、圖表等等的顯示。被廣泛用于機場,、車站,、碼頭、銀行及許多公共場所的指示,、說明,、廣告等場合。
圖19是一個LED點陣顯示器驅(qū)動電路之例,。
A LED is a 半導體 device that emits incoherent narrow-spectrum light when electrically biased in the forward direction. This effect is a form of electroluminescence. The color of the emitted light depends on the chemical composition of the semiconducting material used, and can be near-ultraviolet, visible or infrared. Nick Holonyak Jr. (born 1928) of the General Electric Company developed the first practical visible-spectrum LED in 1962.[1]
LED技術(shù)
物理功能
An LED is a special type of 半導體 二極管. Like a normal diode, it consists of a chip of semiconducting material impregnated, or doped, with impurities to create a structure called a p-n junction. As in other diodes, current flows easily from the p-side, or anode to the n-side, or cathode, but not in the reverse direction. Charge-carriers - electrons and holes flow into the junction from electrodes with different voltages. When an electron meets a hole, it falls into a lower energy level, and releases energy in the form of a photon as it does so.
The 波長 of the light emitted, and therefore its color, depends on the bandgap energy of the materials forming the pn junction. In silicon or germanium diodes, the electrons and holes recombine by a non-radiative transition which produces no optical emission, because these are indirect bandgap materials. The materials used for an LED have a direct bandgap with energies corresponding to near-infrared, visible or near-ultraviolet light.
LED development began with infrared and red devices made with gallium arsenide. Advances in materials science have made possible the production of devices with ever shorter wavelengths, producing light in a variety of colors.
Conventional LEDs are made from a variety of inorganic minerals, producing the following colors:
藍色和白色LED
Commercially viable blue LEDs based on the wide bandgap semiconductor gallium nitride and indium gallium nitride were invented by Shuji Nakamura while working in Japan at Nichia Corporation in 1993 and became widely available in the late 1990s. They can be added to existing red and green LEDs to produce white light, though white LEDs today rarely use this principle.
Most "white" LEDs in production today use a 450 nm – 470 nm blue GaN (gallium nitride) LED covered by a yellowish phosphor coating usually made of cerium-doped yttrium aluminium garnet (Ce3+:YAG) crystals which have been powdered and bound in a type of viscous adhesive. The LED chip emits blue light, part of which is efficiently converted to a broad spectrum centered at about 580 nm (yellow) by the Ce3+:YAG. The single crystal form of Ce3+:YAG is actually considered a scintillator rather than a phosphor. Since yellow light stimulates the red and green receptors of the eye, the resulting mix of blue and yellow light gives the appearance of white, the resulting shade often called "lunar white". This approach was developed by Nichia and was used by them from 1996 for manufacturing of white LEDs.
The pale yellow emission of the Ce3+:YAG can be tuned by substituting the cerium with other rare earth elements such as terbium and gadolinium and can even be further adjusted by substituting some or all of the aluminium in the YAG with gallium. Due to the spectral characteristics of the diode, the red and green colors of objects in its blue+yellow light are not as vivid as in broad-spectrum light. Manufacturing variations make the LEDs produce light with different color temperatures, from warm yellowish to cold bluish; the LEDs have to be sorted during manufacture by their actual characteristics.
White LEDs can also be made by coating near ultraviolet (NUV) emitting LEDs with a mixture of high efficiency europium based red and blue emitting phosphors plus green emitting copper and aluminium doped zinc sulfide (ZnS:Cu,Al). This is a method analogous to the way fluorescent lamps work. However the ultraviolet light causes photodegradation to the epoxy resin and many other materials used in LED packaging, causing manufacturing challenges and shorter lifetimes. This method is less efficient than the blue LED with YAG:Ce phosphor, as the Stokes shift is larger and more energy is therefore converted to heat, but yields light with better spectral characteristics, which renders colors better. Due to the higher radiative output of the ultraviolet LEDs than of the blue ones, both approaches offer comparable brightness.
The newest method used to produce white light LEDs uses no phosphors at all and is based on homoepitaxially grown zinc selenide (ZnSe) on a ZnSe substrate which simultaneously emits blue light from its active region and yellow light from the substrate.
A new technique just developed by Michael Bowers, a graduate student at Vanderbilt University in Nashville, involves coating a blue LED with quantum dots that glow white in response to the blue light from the LED. This technique produces a warm, yellowish-white light similar to that produced by incandescent bulbs.
其它顏色
Recent color developments include pink and purple. They consist of one or two phosphor layers over a blue LED chip. The first phosphor layer of a pink LED is a yellow glowing one, and the second phosphor layer is either red or orange glowing. Purple LEDs are blue LEDs with an orange glowing phosphor over the chip. Some pink LEDs have run into issues. For example, some are blue LEDs painted with fluorescent paint or fingernail polish that can wear off, and some are white LEDs with a pink phosphor or dye that unfortunately fades after a short time.
Ultraviolet, blue, pure green, white, pink and purple LEDs are relatively expensive compared to the more common reds, oranges, greens, yellows and infrareds and are thus less commonly used in commercial applications.
The semiconducting chip is encased in a solid plastic lens, which is much tougher than the glass envelope of a traditional light bulb or tube. The plastic may be colored, but this is only for cosmetic reasons or to improve the contrast ratio; the color of the packaging does not substantially affect the color of the light emitted.
有機發(fā)光二極管(OLEDs)
If the emissive layer material of an LED is an organic compound, it is known as an Organic Light Emitting Diode (OLED). To function as a semiconductor, the organic emissive material must have conjugated pi bonds. The emissive material can be a small organic molecule in a crystalline phase, or a polymer. Polymer materials can be flexible; such LEDs are known as PLEDs or FLEDs.
Compared with regular LEDs, OLEDs are lighter and polymer LEDs can have the added benefit of being flexible. Some possible future applications of OLEDs could be:
工作參數(shù)和效率
Most typical LEDs are designed to operate with no more than 30-60 milliwatts of electrical power. Around 1999, commercial LEDs capable of continuous use at one watt of input power were introduced. These LEDs used much larger semiconductor die sizes to handle the large power input. As well, the semiconductor dies were mounted to metal slugs to allow for heat removal from the LED die. In 2002, 5-watt LEDs were available with efficiencies of 18-22 lumens per watt. It is projected that by 2005, 10-watt units will be available with efficiencies of 60 lumens per watt. These devices will produce about as much light as a common 50-watt incandescent bulb, and will facilitate use of LEDs for general illumination needs.
In September 2003 a new type of blue LED was demonstrated by the company Cree, Inc. to have 35% efficiency at 20 mA. This produced a commercially packaged white light having 65 lumens per watt at 20 mA, becoming the brightest white LED commercially available at the time. In 2005 they have demonstrated a prototype with a record white LED efficiency of 70 lumens per watt at 350 mA [2].
Today, OLEDs operate at substantially lower efficiency than inorganic (crystaline) LEDs. The best efficiency of an OLED so far is about 10%. These promise to be much cheaper to fabricate than inorganic LEDs, and large arrays of them can be deposited on a screen using simple printing methods to create a color graphic display so there are compensating benefits.
使用考慮
Unlike incandescent light bulbs, which light up regardless of the electrical polarity, LEDs will only light with positive electrical polarity. When the voltage across the pn junction is in the correct direction, a significant current flows and the device is said to be forward-biased. If the voltage is of the wrong polarity, the device is said to be reverse biased, very little current flows, and no light is emitted. LEDs can be operated on an AC voltage, but they will only light with positive voltage, causing the LED to turn on and off at the frequency of the AC supply.
-
發(fā)光二極管的正確極性通常如下規(guī)定：
It should be noted that looking the inside of the LED is not an accurate way of determining polarity. While in most LEDs the large part is the "-", in some it is the + terminal. The flat tab or the short pin are more accurate ways of determining polarity.
Because the voltage versus current characteristics of an LED are much like any diode, they can be destroyed by connecting them to a voltage source higher than their turn-on voltage. Most LEDs have low reverse breakdown voltage ratings, so they will also so be damaged by an applied reverse voltage of more than a few volts. Since some manufacuters don‘t follow the indicator standards above, if possible the data sheet should be consulted before hooking up an LED, or the LED may be tested in series with a resistor.
Because of the risk of excess voltage damaging the device, a good LED driver circuit is a constant current source. If high efficiency is not required, an approximation to a current source made by connecting the LED in series with a current limiting resistor to a voltage source may be substituted. To increase efficiency, the power may be applied periodically or intermittently; so long as the flicker rate is greater than the human flicker fusion threshold, the LED will appear to be continuously lit.
Parallel operation is generally problematic. The LEDs have to be of the same type in order to have a similar forward voltage. Even then, variations in the manufacturing process can make the odds of satisfactory operation low. For more information see Nichia Application Note.
Some LED units contain two diodes, one in each direction (that is, two diodes in inverse parallel) and each a different color (typically red and green), allowing two-color operation or a range of apparent colors to be created by altering the percentage of time the voltage is in each polarity. Other LED units contain two or more diodes (of different colors) arranged in either a common anode or common cathode configuration. These can be driven to different colors without reversing the polarity.
LED units may have an integrated multivibrator circuit that makes the LED flash.
使用LED的優(yōu)點
LEDs light up very quickly. An LED will achieve full brightness in approximately 0.01 seconds, 10 times faster than an Incandescent light bulb (0.1 second), and many times faster than a compact fluorescent lamp, which starts to come on after 0.5 seconds or 1 second, but does not achieve full brightness for 30 seconds or more.
LED的應(yīng)用 LED的應(yīng)用列表
Some of these applications are further elaborated upon in the following text.
LEDs are used as informative indicators in various types of embedded systems:
LEDs may also be used to transmit digital information:
LEDs find further application in safety devices, where high brightness and reliability are critical:
LEDs are also used for illumination:
As a replacement for incandescent and fluorescent bulbs in home and office lighting, an application known as Solid State Lighting (SSL).
Finally, LEDs have additional applications not categorized above:
顯示應(yīng)用
LEDs used as a replacement for incandescent bulbs and fluorescent lamps are known as Solid State Lighting (SSL) LEDs. SSL LEDs are packaged as a cluster of white LEDs grouped together to form a light source. LEDs are moderately efficient: the average commercial LED currently outputs 32 lumens per watt (lm/W), and new technologies promise to deliver up to 80 lm/W. They are also more mechanically robust than incandescent light bulbs and fluorescent tubes. LEDs today are not sold in many places, require power source conversion in household applications, and are relatively expensive, although their costs are decreasing.
Incandescent bulbs are much less expensive but also less efficient, generating from about 16 lm/W for a domestic tungsten bulb to 22 lm/W for a halogen bulb. Fluorescent tubes are more efficient, providing 50 to 100 lm/W for domestic tubes (average 60 lm/W), but are bulky and fragile and require starter or ballast circuits that sometimes buzz audibly. Compact fluorescent light bulbs, which include a quiet integrated ballast, are relatively robust and efficient, fit in standard light bulb sockets, and are currently the best choice for efficient household lighting.
Proponents of LEDs expect that technological advances will reduce costs such that SSL can be introduced into most homes by 2020. However, they are still not commercially viable for general lighting applications, and so LEDs are found today in illumination applications where their special characteristics provide a distinct advantage.
Due to their monochromatic nature, LED lights have great power advantages over white lights when a specific color is required. Unlike traditional white lights, the LED does not need a coating or diffuser that can absorb much of the emitted light. LED lights are inherently colored, and are available in a wide range of colors. One of the most recently introduced colors is the emerald green (bluish green, about 500 nm) that meets the legal requirements for traffic signals and navigation lights.
There are applications that specifically require light without any blue component. Examples are photographic darkroom safe lights, illumination in laboratories where certain photo-sensitive chemicals are used, and situations where dark adaptation (night vision) must be preserved, such as cockpit and bridge illumination, observatories, etc. Yellow LED lights are a good choice to meet these special requirements because the human eye is more sensitive to yellow light (about 500 lm/watt emitted) than that emitted by the other LEDs.
LED顯示屏
There are two types of LED panels: conventional, using discrete LEDs, and Surface Mounted Device (SMD) panels. Most outdoor screens and some indoor screens are built around discrete LEDs, also known as individually mounted LEDs. A cluster of red, green, and blue diodes is driven together to form a full-color pixel, usually square in shape. These pixels are spaced evenly apart and are measured from center to center for absolute pixel resolution. The largest LED display in the world is 36 metres high (118 feet), at Times Square, Manhattan.
Most indoor screens on the market are built using SMD technology — a trend that is now extending to the outdoor market. An SMD pixel consists of red, green, and blue diodes mounted on a chipset, which is then mounted on the driver PC board. The individual diodes are smaller than a pin and are set very close together. The difference is that minimum viewing distance is reduced by 25% from the discrete diode screen with the same resolution.
Indoor use generally requires a screen that is based on SMD technology and has a minimum brightness of 600 candelas per square metre (unofficially called nits). This will usually be more than sufficient for corporate and retail applications, but under high ambient-brightness conditions, higher brightness may be required for visibility. Fashion and auto shows are two examples of high-brightness stage lighting that may require higher LED brightness. Conversely, when a screen may appear in a shot on a television show, the requirement will often be for lower brightness levels with lower color temperatures (common displays have a white point of 6500-9000K, which is much bluer than the common lighting on a television production set).
For outdoor use, at least 2,000 nits are required for most situations, whereas higher brightness types of up to 5,000 nits cope even better with direct sunlight on the screen. Until recently, only discrete diode screens could achieve that brightness level. (The brightness of LED panels can be reduced from the designed maximum, if required.)
Suitable locations for large display panels are identified by factors such as sight lines, local authority planning requirements (if the installation is to become semi-permanent), vehicular access (trucks carrying the screen, truck-mounted screens, or cranes), cable runs for power and video (accounting for both distance and health and safety requirements), power, suitability of the ground for the location of the screen (check to make sure there are no pipes, shallow drains, caves, or tunnels that may not be able to support heavy loads), and overhead obstructions.
參考
參見其它
LED是通過將電壓加在LED的PN結(jié)兩端,，使PN結(jié)本身形成一個能級（實際上,，是一系列的能級），然后電子在這個能級上躍變并產(chǎn)生光子來發(fā)光的,。所以,，LED需要加在PN結(jié)兩端的電壓來驅(qū)使其發(fā)光，即LED驅(qū)動,。
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