Light Emitting Diode Contents 1.History 2.Application 3.Epitaxy 4.Fabrication 5.Packaging 6.OLED 7.Future The history of lighting and LED In ancient times , there were no lamps, so people could only use fire for lighting. People stepped into the world of light . The first generation of lighting kerosene lamp candle Incandescent lamps were widely used because of the extensive application of electricity. The second generation of lighting incandescent lamps Now, fluorescent lamps are the most widely used. The third generation of lighting fluorescent lamps compact fluorescent lamp LED is the fourth generation of lighting. The development of LEDs The application of LED LED signal lights Street Lights A—LED Street Lights B—Metal Halide Street Lights C—Sodium Street Lamps Solar Street Lamps Airport runway lighting and LED tunnel lamps Automotive Application with LEDs LED interior lighting Bar Landscape Lighting Mobile Application With LEDs LED Screen Display The painted scroll is made up of 44,000 LEDs. The Olympic rings are made up of 2,000 blue and 44,794 white LEDs. Epitaxy The first generation materials: Si, Ge ; The second generation materials: GaAs, InP, GaP, InAs, AlAs; The third generation materials: SiC, ZnAs, GaN, diamond. Epitaxial growth technology (GaN-based materials ) MOCVD (the most widely used) (metal organic chemical vapor deposition ) MBE (molecular beam epitaxy) HVPE (hydride vapor phase epitaxy) The choice of substrate materials Sapphire common used, fabrication maturity, low price; large lattice dismatch, poor electric conductivity, poor thermal conductivity. SiC small lattice dismatch, good electric conductivity; high price. Si low price, big area, high quality, good electric conductivity; large lattice dismatch. The epitaxial structure of GaN-based LED p-type Mg-doping GaN Multi-quantum wells n-type Si-doping GaN AlN suffer layer Substrate (silicon or sapphire) vertical structure of LED epitaxial wafer Fabrication Hundreds of thousands of LED chips are produced on one epitaxial wafer. silicon epitaxial wafer sapphire epitaxial wafer LED chips 1 mm2 Multi-quantum wells (MQW) Blue light comes from MQW, which is also called active region. vertical structure of LED epitaxial wafer Can we make LED electrodes with vertical structure? 1.The answer depends on the buffer and substrate. 2.The vertical structure will cause high resistance. 3.It is not easy for fabrication when the two electrodes are at different sides. Photoetching and ICP etching ICP : Inductively Coupled Plasma P-electrode and n-electrode are made at the same side to form lateral structure. Transparent electrode evaporation Since p-GaN layer is in high resistance, we need to evaporate transparent electrode to improve current spreading. The electrode is as thin as about 12 nm, so it is transparent. LED chip VS lateral structure LED chip vertical structure LED chip Different p-electrode structures To improve current spreading and luminous efficiency LED chips in our lab (Semiconductor Lighting Research Center) The mechanism of LED balanced state The mechanism of LED Forward voltage is applied. Electrons and holes recombine to emit light. LED Packaging The development of LED packaging Purpose for packaging To get the input signal To protect the LED chips To get the output of visible light Two main methods of LED packaging 1.Pin type packaging 2. Planar packaging How to make white LED? LED is single color, such as red, yellow, green, blue and so on. However, white is mixed color. We need to mix colors to make white LED. Two main methods of making white LED 1. blue LED + yellow phosphor = white LED 2. red + green + blue = white OLED OLED = Organic Light Emitting Diode Its light-emitting mechanism is similar to LED. the structure of OLED Classification According to light-emitting materials : Small molecular OLED:Alq3 Polymer OLED (PLED)：PPV,MEH-PPV According to display models : Passive Matrix(PM-OLED) : Acitive Matrix (AM-OLED):TFT FOLED:Flexible OLED Comparison Advantages Thickness can be less than 1 mm. Simple process; Low cost(30~40%of LCD). Self-luminous; Highly efficient and kind to environment. Low drive voltage and low power consumption. Microsecond response time(1000 times)and wide view angle(>170°). High brightness(100~14000 cd/m2 ), wide temperature range(-30~80 ℃) Disadvantages Life is usually only 5,000 hours, less than that of LCD at least 1 million hours. Color purity is not enough to show bright and rich color. Large-size OLED panel production has very low yield(<30%), compared with LCD’s(>99%). High patent fees. Application Compared with IPhone First OLED wireless digital photo frame of KODAK View from side(LG‘s first OLED television) First OLED notebook in the world(SUMSUNG) Application 3D-movie head-mounted display Bracket lights TV and display Military display Folding E-book; Rollable E-newspaper The future of LED I’M CUTE! My name is LED . Firstly, I am smaller . Secondly，I need less power. Thirdly，I enjoy longer life. At the same time，you can call me energy saving lamp . It’s last but not the least, I’m a faddish girl . My weakness When it comes to my weakness, my price is too high. And then, my luminous flux is low. Stroboscopic phenomenon. Strong stimulation. My future is bright! With the development of technology and the reduction of cost, I will be the most popular in the world!