LIGHT EMITTING DIODE

LEDSubmitted To Submitted By www.pptplanet.com www.pptplanet.com

Content ObjectivesWhat is LED?4 Main IssuesBy the end of this lecture you must be able to For the LED lectures you need:Construction of Typical LED Injection Luminescence in LEDLED ConstructionReferences

3 Lectures on LEDOBJECTIVES:To learn the basic design principles of LEDTo relate properties of semiconductor material to the principle of LEDTo be able select appropriate materials for different types of LEDTo be able to apply knowledge of band gap engineering to design appropriate materials for a particular LED To acknowledge other materials that can and have been used in LED

LED are semiconductor p-n junctions that under forward bias conditions can emit radiation by electroluminescence in the UV, visible or infrared regions of the electromagnetic spectrum. The qaunta of light energy released is approximately proportional to the band gap of the semiconductor.

What is LED?Semiconductors bring quality to light!

1907 Publication report on Curious Phenomenon

H.J. Round, Electrical World, 49, 309, 1907On applying a potential to a crystal of carborundum (SiC), the material gave out a yellowish light

4 Main IssuesThe device configurationMaterials requirementsMaterials selectionMaterial issues

By the end of this lecture you must be able to Draw a typical construction of an LED. Explain your drawing. State all the issues regarding the materials selection of an LED. State all of the possible answers regarding your materials issues.Explain band gap engineeringExplain the isoelectronic doping in GaAsP systemState examples of materials that emit, UV, Vis, IR lights

For the LED lectures you need:Complete set of notes (3 lecture presentation and lecture notes)A photocopy from Kasap (p.139-150)A photocopy from Wilson (p-141-155)Some reading materials

Applications of LEDs

Your fancy telephone, i-pod, palm pilot and digital camera

Getting to know LED

Advantages of Light Emitting Diodes (LEDs)Longevity: The light emitting element in a diode is a small conductor chip rather than a filament which greatly extends the diodes life in comparison to an incandescent bulb (10 000 hours life time compared to ~1000 hours for incandescence light bulb)Efficiency: Diodes emit almost no heat and run at very low amperes.Greater Light Intensity: Since each diode emits its own lightCost:Not too badRobustness:Solid state component, not as fragile as incandescence light bulb

LED chip is the part that we shall deal with in this course

Luminescence is the process behind light emissionLuminescence is a term used to describe the emission of radiation from a solid when the solid is supplied with some form of energy. Electroluminescence excitation results from the application of an electric fieldIn a p-n junction diode injection electroluminescence occurs resulting in light emission when the junction is forward biased

How does it work?

P-n junction

Electrical Contacts

A typical LED needs a p-n junctionJunction is biased to produce even more e-h and to inject electrons from n to p for recombination to happenThere are a lot of electrons and holes at the junction due to excitationsElectrons from n need to be injected to p to promote recombinationRecombination produces light!!

Injection Luminescence in LEDUnder forward bias majority carriers from both sides of the junction can cross the depletion region and entering the material at the other side. Upon entering, the majority carriers become minority carriersFor example, electrons in n-type (majority carriers) enter the p-type to become minority carriersThe minority carriers will be larger minority carrier injectionMinority carriers will diffuse and recombine with the majority carrier. For example, the electrons as minority carriers in the p-region will recombine with the holes. Holes are the majority carrier in the p-region.The recombination causes light to be emittedSuch process is termed radiative recombination.

Recombination and Efficiency

eVo

Egpn+

h =Eg

Egpn+

(a)(b)

Electrons in CBHoles in VB

ECEVEFIdeal LED will have all injection electrons to take part in the recombination processIn real device not all electron will recombine with holes to radiate lightSometimes recombination occurs but no light is being emitted (non-radiative)Efficiency of the device therefore can be describedEfficiency is the rate of photon emission over the rate of supply electrons

Emission wavelength, g The number of radiative recombination is proportional to the carrier injection rateCarrier injection rate is related to the current flowing in the junction If the transition take place between states (conduction and valance bands) the emission wavelength, g = hc/(EC-EV)EC-EV = Egg = hc/Eg

CalculateIf GaAs has Eg = 1.43evWhat is the wavelength, g it emits?What colour corresponds to the wavelength?

Construction of Typical LED

Substrate n

AlSiO2

Electrical contacts

pLight output

LED ConstructionEfficient light emitter is also an efficient absorbers of radiation therefore, a shallow p-n junction required. Active materials (n and p) will be grown on a lattice matched substrate. The p-n junction will be forward biased with contacts made by metallisation to the upper and lower surfaces. Ought to leave the upper part clear so photon can escape. The silica provides passivation/device isolation and carrier confinement

Efficient LEDNeed a p-n junction (preferably the same semiconductor material only different dopants)Recombination must occur Radiative transmission to give out the right coloured LEDRight coloured LED hc/ = Ec-Ev = Eg so choose material with the right EgDirect band gap semiconductors to allow efficient recombinationAll photons created must be able to leave the semiconductorLittle or no reabsorption of photons

Materials RequirementsCorrect band gapDirect band gapMaterial can be made p and n-typeEfficient radiative pathways must exist

EBB424 USM SMMREDr Zainovia Lockman L222

Candidate MaterialsDirect band gap materialse.g. GaAs not Si UV-ED ~0.5-400nm Eg > 3.25eV LED - ~450-650nmEg = 3.1eV to 1.6eV IR-ED- ~750nm- 1nmEg = 1.65eVReadily doped n or p-typesMaterials with refractive index that could allow light to get out

Typical Exam QuestionDescribe the principles of operation of an LED and state the materials requirements criteria to produce an efficient LED. (50 marks)

Referencewww.google.comwww.wikipedia.comwww.studymafia.orgwww.pptplanet.comwww.pdfclass.com

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