(精品)Lecture 6.ppt

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1、Semiconductor Devices for Illumination Engineering 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 quanta of light energy released is approximately proportional

2、to the band gap of the semiconductor.What is LED?Applications of LEDsYour fancy telephone,i-pod,palm pilot and digital cameraLongevity: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

3、 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 bulbAdvantages

4、 of Light Emitting Diodes(LEDs)LED chip is the part that we shall deal with in this courseLED designLuminescence 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

5、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 biasedExcitationElectron(excited by the biased forward voltage)is in the conduction bandHole is in valance bandNormally th

6、e recombination takes place between transition of electrons between the bottom of the conduction band and the top of the valance band(band exterma).The emission of light is therefore;hc/=Ec-Ev=Eg(only direct band gap allows radiative transition)EkHow does it work?P-n junctionElectrical ContactsA typ

7、ical 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 li

8、ght!Recombination and Efficiency eVoEgpn+h=EgEgpn+(a)(b)Electrons in CBHoles in VBECEVEFIdeal 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

9、emitted(non-radiative)Efficiency of the device therefore can be describedEfficiency is the rate of photon emission over the rate of supply electronsFig.1 Visible and near-visible electromagnetic spectrum.The visible portion is expanded at the top,and divided into major color bands.Also indicated is

10、relative luminosity function V()as defined by the CIE for normal photopic vision.Fig.2 Basic recombination transitions in semiconductor.ED,EA,E,are donor-type,acceptor-type,and deep-level traps respectively.Basic recombination transitions in semiconductorFig.3 The three basic optical processes betwe

11、en two energy levels.Fig.4(a)Composition dependence of the direct and indirect bandgap for GaAs1-xPx.Near the band edges,the energy of the emitted photon is governed by the relationshipwhere mr*is the reduced effective mass A joint density of states can be obtained asThe distribution of carriers is

12、governed by the Boltzmann distributionThe spontaneous emission rate is proportional to the productFig.5 Theoretical spectrum of spontaneous emission.Theoretical spectrum of spontaneous emissionFig.6(a)GaAs diode emission spectra at 300 and 77 K.(b)Dependence of emission peakand half-power width as a

13、 function of temperature.Experimental spectrum of spontaneous emissionDevice StructuresFig.7(a)Under forward bias of a p-n junction,electrons injected from n-side recombine with holes injected from p-side.(b)Higher carrier densities and improved carrier confinement in a double heterojunction.Materia

14、ls of ChoiceFig.8 Semiconductors of interest for LEDs,including the relative luminosity function of the human eye.Fig.9(a)Quantum efficiency of GaAs,PX vs.alloy composition,with and without isoelectronic impurity nitrogen.(b)Peak emission wavelength positionQuantum efficiency Definitions of Efficien

15、ciesInternal Quantum EfficiencyThe internal quantum efficiency in is the efficiency of converting carrier currentto photons,defined asFor low-level injection,the radiative recombination rate in the p-side of the junction is given bywhere Rec is the recombination coefficient and n is the excess carri

16、er density whichis much larger than the minority carrier density in equilibrium n npoRec,is 10-10 cm3/s for direct-bandgap materials,and 10-15 cm3/s for indirect-band gap materials.For low-level injection n s,)corresponding to Snells law.C:Ray outside the light escape cone(s c)has total reflection.F

17、or GaAs(ns=3.66)and GaP(ns=3.45),the critical angle is about 160-170.Three major loss mechanisms reduce the quantity of emitted photons:(1)Absorption within the LED material,(2)Fresnel loss(reflectance),(3)Critical-angle loss.The absorption loss for LEDs on GaAs substrates is large since the substra

18、te is opaque to light and it absorbs about 85%of the photons emitted at the junction.For LEDs on transparent substrates such as GaP with isoelectronic centers,photons emitted downward can be reflected back with only about 25%absorption.The solid angle of the light-escape cone can be calculated to be

19、Fig.11 LED structures for optical-efficiency consideration:(a)planar.(b)hemispherical.c)parabolic.(d)Their normalized Lambertian emission patterns.Fig.12 Historic development of maximum light extraction efficiency forAlGaInP(red circles)and InGaN(blue squares)LEDs.Power EfficiencyThe power efficienc

20、y p is simply defined as the ratio of the light power output to the electrical power inputSince the bias is approximately equal to the energy gap and light energy(qV hv),itfollows that the power efficiency is similar to the external quantum efficiency p exLuminous Efficiency The brightness of light

21、output is measured by the luminous flux(in lumens),The maximum luminous efficiency has a value of 683 lm/W.Table 1.Radiometric and photometric quantities and units.The photometric units are lumen(lm),lux(lx=lm/m2)and candela(cd=lm/sr)Fig.13 LED structures showing the direction of emitted light from(

22、a)surface emitter and(b)edge emitterTwo types of LEDFrequency ResponseThe frequency response is another important parameter to be considered in the designof LEDs for high-speed applications such as optical-fiber communication systems.It determines the maximum frequency at which the LEDs can be turne

23、d on and off,And,thus,the maximum transmission rate of data.This cutoff frequency of an LED isgiven bywhere is the overall lifetime defined asWhite LEDsFig.14 Different strategies to generate white light with LEDs.(a)Addition of R,G,and B LEDs,(b)blue LED and yellow phosphor,(c)UV LED(invisible)and

24、R,G,and B phosphors.Fig.15.(a)Scheme and(b)image of color conversion LED,(c)Spectrum(solid line)of white LED with blue LED pumping a yellow phosphor together with eye-sensitivity curve V()(dashed line).Fig.16 Historic development of the flux(in lumen)and cost(in$/lm)forsemiconductor LEDs.Quantum Wel

25、l Light-Emitting DiodeAs a practical example,we here investigate single-quantum-well(SQW)InGaN/GaN blue light-emitting diodes.These devices exhibit an output power of 4.8 mW at 20 mA injection current and 3.1 V forward voltage.The low voltage results in a relatively high power efficiency of 7.7%(rat

26、io of light power to electrical power).The external quantum efficiency is as high as 8.7%despite the large dislocation density of about 1010 cm2.Fig.17 Schematic of single quantu wellband diagram at zero bias.Electrons and holesare confined in the well and recombinePolarization EffectsFig.18 Recombi

27、nation in a quantum well in the absence of a piezoelectric field,and(B)in the presence of a transverse electric field where the overlap of wave functions decreasesFig.19 Blue InGaN/GaN MQW LED layer structure.The active region is composed of alternating InGaN quantum wells with GaN barriers.Multi Qu

28、antum Well(QW)LED Fig.20 Energy band diagram for the InGaN MQW Blue LED.Energy band diagram Quantum Dot LEDFig.21(a)Schematic cross-section of QD LED.Current is fed to a single QD via an oxide aperture.(b)Plan-view SEM image of QD LED.(c)Electroluminescence(EL)spectrum (T=10K,U=1.65 V,I=0.87 nA)of s

29、ingle In-GaAs/GaAs QD LED(diameter of oxide aperture 0.85 m,thickness 60 nm).The single line is due to(neutral)exciton recombination.The inset shows dependence of EL spectrum on injection current;at higher currents a second peak due to biexciton recombination(XXX)appears.Organic LEDFig.22 Typical OLED design for(a)bottom and(b)top emissionFig.23(a)Transparent OLED panel.(b)Flexible OLED display.(c)3mm thin,11 inch diagonal OLED TV.