Xiaoyu Sun

 Intro – what is led  How does it works  Where it uses  Advantages and disadvantages  Green city

3 Photo from Photo Credit - sandstep via Flickr Photo from

5

6 When current flows across a diode, negative electrons move one way and positive holes move the other way.

The holes are at lower energy level than the free electrons, so when a electron falls it loses energy.

The energy is emitted in the form of a light photon. The wavelength depends on the band gap energy of the materials forming the p-n junction.

Direct band gap Indirect band gap

 Indicators and signs  Lighting  Smart lighting  Non-visual applications

Left: A supermarket in Switzerland Right: Parking garage in Raleigh, NC

 Smart lighting  Non-visual applications

 Dimming  Cool light  Shock resistance  Focus  Toxicity Efficiency Color Cost Lifetime On/Off time

 High price  Temperature dependence  Voltage sensitivity  Area light source

 Raleigh, NC  Toronto, Canada  Ann Arbor, MI

 Roughly estimation: Assume φ = 45 lm I = 350 mA V = 3.42 V luminous efficacy 45 lm/(0.35 A x 3.42 V) = 38 lm/W (From the website of U.S. department of energy)

CategoryTypeOverall luminous efficacy Overall luminous efficiency Incandescent5 W tungsten incandescent (120 V) 50.7% 40 W tungsten incandescent (120 V) % 100 W tungsten incandescent (220 V) % 100 W tungsten glass halogen (220 V) % 100 W tungsten incandescent (120 V) % 2.6 W tungsten glass halogen (5.2 V) % Light-emitting diodewhite LED % - 13% white OLED10215% Prototype LEDsUp to 161Up to 25%

ColorWavelength (nm)Voltage (V) Semi-conductor Material Infraredλ > 760ΔV < 1.9GaAs, AlGaAs Red610 < λ < < ΔV < 2.03 AlGaAs, GaAsP, AlGaInP, GaP Orange590 < λ < < ΔV < 2.10 GaAsP, AlGaInP, GaP Yellow570 < λ < < ΔV < 2.18 GaAsP, AlGaInP, GaP Green500 < λ < < ΔV < 4.0 InGaN / GaN, GaP, AlGaInP, AlGaP

ColorWavelength (nm)Voltage (V) Semi-conductor Material Blue450 < λ < < ΔV < 3.7 ZnSe, InGaN SiC/ Si as substrate Purplemultiple types2.48 < ΔV < 3.7 Blue w/red, white w/ purple Violet400 < λ < < ΔV < 4.0InGaN Ultravioletλ < < ΔV < 4.4 diamond (C), AlN, AlGaN, AlGaInN WhiteBroad spectrumΔV = 3.5 Blue/UV diode with yellow phosphor

Reference: [1] S. S. Iyer 1, Y. -H. Xie. Science 2 April 1993: Vol no. 5104, pp. 40 – 46. Light Emission from Silicon [2] Nobuyoshi Koshida, Hideki Koyama. Efficient Visible Photoluminescence from Porous Silicon. [3] [4]