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Introduction of Light-Emitting Diode

Xiaoyu Sun24012083

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Intro – what is ledHow does it worksWhere it usesAdvantages and disadvantages

Green city

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Light-emitting Diode

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What is LED

Photo from www.mulelighting.com

Photo Credit - sandstep via Flickr

Photo from www.gizmowatch.com

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Structure

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Inner workings

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How it works

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

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How it works (cont’d)

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

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How it works (cont’d)

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.

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Direct bandgap vs. Indirect bandgap

Direct band gap

Indirect band gap

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Indicators and signs

Lighting

Smart lighting

Non-visual applications

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Where it works

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Indicators and signs

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Lighting

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

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Smart lighting

Non-visual applications

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Other applications

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Dimming Cool light Shock resistance Focus Toxicity

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Advantages

• Efficiency• Color• Cost• Lifetime• On/Off time

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High price

Temperature dependence

Voltage sensitivity

Area light source

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Why bulbs haven’t been replaced

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Raleigh, NC

Toronto, Canada

Ann Arbor, MI

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Green cities

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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)

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Efficiency

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Category Type Overall luminous efficacy

Overallluminous efficiency

Incandescent 5 W tungsten incandescent (120 V)

5 0.7%

40 W tungsten incandescent (120 V)

12.6 1.9%

100 W tungsten incandescent (220 V)

13.8 2.0%

100 W tungsten glass halogen (220 V)

16.7 2.4%

100 W tungsten incandescent (120 V)

16.8 2.5%

2.6 W tungsten glass halogen (5.2 V)

19.2 2.8%

Light-emitting diode white LED 10-90 1.5% - 13%

white OLED 102 15%

Prototype LEDs Up to 161 Up to 25%

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Efficiency (cont’d)

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Color Wavelength (nm) Voltage (V)

Semi-conductor Material

Infrared λ > 760 ΔV < 1.9 GaAs, AlGaAs

Red 610 < λ < 760 1.63 < ΔV < 2.03

AlGaAs, GaAsP, AlGaInP, GaP

Orange 590 < λ < 610 2.03 < ΔV < 2.10

GaAsP, AlGaInP,GaP

Yellow 570 < λ < 590 2.10 < ΔV < 2.18

GaAsP, AlGaInP,GaP

Green 500 < λ < 570 2.18 < ΔV < 4.0InGaN / GaN, GaP, AlGaInP, AlGaP

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Colors vs. Materials

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Color Wavelength (nm) Voltage (V)

Semi-conductor Material

Blue 450 < λ < 500 2.48 < ΔV < 3.7ZnSe, InGaNSiC/ Si as substrate

Purple multiple types 2.48 < ΔV < 3.7 Blue w/red,white w/ purple

Violet 400 < λ < 450 2.76 < ΔV < 4.0 InGaN

Ultraviolet λ < 400 3.1 < ΔV < 4.4diamond (C), AlN, AlGaN, AlGaInN

White Broad spectrum ΔV = 3.5Blue/UV diode with yellow phosphor  

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Colors vs. Materials (cont’d)

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Cost Saving

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Lifetime

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Reference: [1] S. S. Iyer 1, Y. -H. Xie. Science 2 April 1993: Vol. 260. no. 5104, pp. 40 –

46. Light Emission from Silicon[2] Nobuyoshi Koshida, Hideki Koyama. Efficient Visible Photoluminescence

from Porous Silicon. [3] http://en.wikipedia.org[4] http://electronics.howstuffworks.com

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