1LIGHT EMITTING DIODESPresentation by V.S.ARJUN
2A light emitting diode (LED) is essentially a PN junctionopto-semiconductor that emits a monochromatic (single color) lightwhen operated in a forward biased direction.LEDs convert electrical energy into light energy. They arefrequently used as "pilot" lights in electronic appliances to indicatewhether the circuit is closed or not.SYMBOL
3How Does A LED Work? (1/2)When sufficient voltage is applied to thechip across the leads of the LED, electrons canmove easily in only one direction across the junctionbetween the p and n regions.In the p region there are many morepositive than negative charges.When a voltage is applied and the currentstarts to flow, electrons in the n region havesufficient energy to move across the junction intothe p region.
4How Does A LED Work? (2/2)Each time an electron recombineswith a positive charge, electric potentialenergy is converted into electromagneticenergy.For each recombination of a negativeand a positive charge, a quantum ofelectromagnetic energy is emitted in theform of a photon of light with a frequencycharacteristic of the semi-conductor material(usually a combination of the chemicalelements gallium, arsenic and phosphorus)..
5About LEDs (1/2)The most important part of a light emitting diode (LED) is thesemi-conductor chip located in the center of the bulb as shown at theright. The chip has two regions separated by a junction. The p regionis dominated by positive electric charges, and the n region isdominated by negative electric charges. The junction acts as a barrierto the flow of electrons between the p and the n regions. Only whensufficient voltage is applied to the semi-conductor chip, can thecurrent flow, and the electrons cross the junction into the p region.
7Testing LEDsNever connect an LEDdirectly to a battery or powersupply! It will be destroyed almostinstantly because too much currentwill pass through and burn it out.LEDs must have a resistorin series to limit the current to asafe value, for quick testingpurposes a 1k resistor is suitablefor most LEDs if your supplyvoltage is 12V or less.Remember to connect theLED the correct way round!
8How Much Energy Does an LED Emit?The energy (E) of the light emitted by an LED is related to theelectric charge (q) of an electron and the voltage (V) required to light theLED by the expression: E = qV Joules.This expression simply says that the voltage is proportional tothe electric energy, and is a general statement which applies to anycircuit, as well as to LEDs. The constant q is the electric charge of asingle electron, -1.6 x 10-19 Coulomb.
CBVBWhen the electron fallsdown from conductionband and fills in a holein valence band, thereis an obvious loss ofenergy.The question is;where does that energy go?
In order to achieve areasonable efficiencyfor photon emission,the semiconductormust have a directband gap.CBVBThe question is;what is the mechanismbehind photon emission in LEDs?
For example;Silicon is known as an indirect band-gapmaterial.as an electron goes from the bottom of theconduction band to the top of the valenceband;it must also undergo asignificant change inmomentum.CBVBWhat this means is thatEk
• As we all know, whenever something changesstate, one must conserve not only energy, but alsomomentum.• In the case of an electron going from conductionband to the valence band in silicon, both of thesethings can only be conserved:The transition also creates a quantizedset of lattice vibrations,called phoTons, or "heat“ .
• Photons possess both energy and momentum.• Their creation upon the recombination of an electronand hole allows for complete conservation of bothenergy and momentum.• All of the energy which the electron gives up in goingfrom the conduction band to the valence band (1.1eV) ends up in photons, which is another way ofsaying that the electron heats up the crystal.
• Thus, for a direct band gap material, the excessenergy of the electron-hole recombination can eitherbe taken away as heat, or more likely, as a photon oflight.• This radiative transition thenconserves energy and momentumby giving off light whenever anelectron and hole recombine. CBVBThis gives rise to(for us) a new typeof device;the light emitting diode(LED).
Mechanism is “injectionElectroluminescence”.Luminescencepart tells us that we are producing photons.Electro part tells us thatthe photons are being producedby an electric current.e-Injection tells us thatphoton production is bythe injection of current carriers.Mechanism behind photon emission inLEDs?e-
Producing photonElectrons recombine with holes.Energy of photon is the energy ofband gap.CBVBe-h
What is LED?• LED - Light Emitting Diode• A semiconductorcomponent similar totransistor or integratedcircuit• Electrical current throughthe semiconductor chipproduces light• Semiconductor materialsused, define the colour oflight produced
Benefits of LED• Shock and vibration proof• Small dimensions• Lightweight• Virtually no heat generation• Accurate and well controlled beam spread
Key features of LED• ·Longer lasting• ·Reduced maintenance cost• ·More energy efficient• ·Better design flexibility• ·Vivid colors• ·High reliability• ·Environmentally friendly• They are suitable at high operating speeds
LED vs. CFL vs. Incandescent LampsIncandescent Lamps• 90% of electricity used is spent producing heat, not light• Rapidly being replaced by CFLs for lesser bills and environment friendlinessCompact Fluorescent Lamps (CFL)• 75% less electricity needed to produce same amount of light as an incandescentlamp• Current estimated sale, of one brand just in Gurgaon, is around one lakh units permonthLight-emitting Diodes (LED)• 15-20% less electricity needed to produce same amount of light as a CFL• Making a slow but steady entry in the market of lighting• Opportunities exist in way too many industries• Currently hot for home decor; Corporate office lighting space hugely untapped• More environment friendly than CFLs
Why LED?•Energy Efficient, up to 90% more efficientthan traditional lighting sources•Long life span, up to 100,000 hours•Variety of color options•Low operation costs•No UV radiation•No mercury•Instant on, no start-up time•Silent operation•Reduces “Carbon Footprint”
Materials for visible wavelength LEDs• We see them almost everyday, either on calculator displays orindicator panels.• Red LED use as “ power on” indicator• Yellow, green and amber LEDs are also widely available but veryfew of you will have seen a blue LED.
34Signal Appications• Traffic• Rail• Aviation• Tower Lights• Runway Lights• Emergency/Police Vehicle LightingLEDs offer enormous benefits over traditional incandescent lampsincluding:• Energy savings (up to 85% less power than incandescent)• Reduction in maintenance costs• Increased visibility in daylight and adverse weather conditions
35Indication Household appliances VCR/ DVD/ Stereo and other audio and video devices Toys/Games Instrumentation Security Equipment Switches
36Driving LEDs• Analog LED Drive Circuits• Digital LED Drive Circuits
37Colours of LEDs (1/3)LEDs are available in red, orange, amber, yellow, green, blue andwhite. Blue and white LEDs are much more expensive than the othercolours. The colour of an LED is determined by the semiconductormaterial, not by the colouring of the package (the plastic body). LEDs ofall colours are available in uncoloured packages which may be diffused(milky) or clear (often described as water clear). The coloured packagesare also available as diffused (the standard type) or transparent.LEDs are made from gallium-basedcrystals that contain one or more additionalmaterials such as phosphorous to produce adistinct color. Different LED chiptechnologies emit light in specific regionsof the visible light spectrum and producedifferent intensity levels.
38Colours of LEDs (2/3)Tri-colour LEDsThe most popular type of tri-colour LED has a red and a greenLED combined in one package with three leads. They are called tri-colour because mixed red and green light appears to be yellow and thisis produced when both the red and green LEDs are on.The diagram shows the construction of a tri - colour LED. Notethe different lengths of the three leads. The centre lead (k) is thecommon cathode for both LEDs, the outer leads (a1 and a2) are theanodes to the LEDs allowing each one to be lit separately, or bothtogether to give the third colour.
39Colours of LEDs (3/3)Bi-colour LEDsA bi-colour LED has two LEDs wired ininverse parallel (one forwards, onebackwards) combined in one package with twoleads. Only one of the LEDs can be lit at onetime and they are less useful than the tri-colour LEDs described above.
40LED Performance (1/8)• Color• White light• Intensity• Eye safety information• Visibility• Operating Life• Voltage/Design CurrentLED performance is based on a few primary characteristics:
41LED Performance (2/8)ColourPeak wavelength is a function of the LED chip material.Although process variations are ±10 NM, the 565 to 600 NMwavelength spectral region is where the sensitivity level of thehuman eye is highest. Therefore, it is easier to perceive colorvariations in yellow and amber LEDs than other colors.
42Bargraph 7-segment Starburst Dot matrixSome Types of LEDs
Disadvantages:1.Output power gets affected due to change intemperature2.Overcurrent damages the LED3.Large power required for operation4.Luminous efficiency is low