Light Emitting Diodes .


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Layout. Nuts and bolts of Light Emitting Diodes (Electrical)Basics of Light Emitting Diodes (Optical)High inside proficiency designsHigh extraction effectiveness structuresVisible Spectrum LED\'sWhite-Light LED\'s The guarantee of strong state lighting. Driven Electrical Properties-PN intersections. From Light-Emitting Diodes, Fred Schubert..
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Light Emitting Diodes EE 698A Kameshwar Yadavalli

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Outline Basics of Light Emitting Diodes (Electrical) Basics of Light Emitting Diodes (Optical) High inward proficiency plans High extraction productivity structures Visible Spectrum LED\'s White-Light LED\'s The guarantee of strong state lighting

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From Light-Emitting Diodes, Fred Schubert. PN intersection diode in forward inclination, the electron-gap recombination prompts to photon discharge I = I s (e eV/kT - 1) Threshold voltage V th = E g/e I = I s e eV/η kT where η is the ideality figure LED-Electrical Properties-PN intersections Double Heterostructure is utilized to restrict the transporters, enhancing the radiative recombination rate

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LED-Electrical Properties-Hetero intersections Grading of the heterojunction is done to diminish the resistance seen via bearers From Light-Emitting Diodes, Fred Schubert.

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LED-Electrical Properties-Hetero intersections From Light-Emitting Diodes, Fred Schubert.

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LED-Electrical Properties-Carrier misfortune The constrainment boundaries are commonly a few hundred meV (>>kT) Due to Fermi-Dirac appropriation of transporters in the dynamic district, a few bearers will have vitality higher than that of the hindrances In AlGaAs/GaAs and AlGaN/GaN the obstructions are high In AlGaInP/GaInP the hindrances are lower bringing about higher spillage streams From Light-Emitting Diodes, Fred Schubert.

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LED-Electrical Properties-Blocking layers Electron Blocking Layers are required to anticipate electron escape at high infusion current densities From Light-Emitting Diodes, Fred Schubert.

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LED-Optical Properties-Efficiency η int = # of photons radiated from dynamic area every second # of electrons infused into LED every second = P int/(h ν ) I/e η extr = # of photons transmitted into free space every second # of photons discharged from dynamic district every second = P/(h ν ) P int/(h ν ) From Light-Emitting Diodes, Fred Schubert.

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LED-Optical Properties-Emission Spectrum The linewidth of a LED transmitting in the noticeable range is generally slender contrasted and the whole obvious range (saw as monochromatic by the eye) Optical strands are dispersive, constraining the bit rate X remove item achievable with LED\'s Modulation speeds accomplished with LED\'s are 1Gbit/s, as the unconstrained lifetime of transporters in LED\'s is 1-100 ns From Light-Emitting Diodes, Fred Schubert.

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LED-Optical Properties-Light Escape Cone Total interior reflection at the semiconductor air interface decreases the outside quantum effectiveness. The edge of aggregate inside reflection characterizes the light escape cone. sin θ c = n air/n s Area of the escape cone = 2 π r 2 (1-cos θ c ) P escape/P source = (1-cos θ c )/2 = θ c 2/4 = (n air 2/n s 2 )/4 From Light-Emitting Diodes, Fred Schubert.

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Light force in air (Lambertian emanation example) is given by I air = (P source/4 π r 2 ) X (n air 2/n s 2 ) cos Φ Index differentiate between the light radiating material and the encompassing locale prompts to non-isotropic outflow design LED-Optical Properties-Emission Spectrum From Light-Emitting Diodes, Fred Schubert.

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LED-Optical Properties-Epoxy encapsulants Light extraction proficiency can be expanded by utilizing arch molded encapsulants with a substantial refractive list. Effectiveness of a commonplace LED increments by an element of 2-3 upon exemplification with an epoxy of n = 1.5. The vault state of the epoxy infers that light is occurrence at an edge of 90 o at the epoxy-air interface. Henceforth no aggregate inside reflection. From Light-Emitting Diodes, Fred Schubert.

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Temperature reliance of discharge power Emission force diminishes with expanding temperature. Causes incorporate non-radiative recombination by means of profound levels, surface recombination, and bearer misfortune over heterostucture obstructions. From Light-Emitting Diodes, Fred Schubert.

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High inner proficiency LED outlines Radiative recombination likelihood should be expanded and non-radiative recombination likelihood should be diminished. High transporter focus in the dynamic district, accomplished through twofold heterostructure (DH) plan, enhances radiative recombination. R=Bnp DH configuration is utilized as a part of all high productivity plans today. From Light-Emitting Diodes, Fred Schubert.

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High inside productivity outlines Doping of the dynamic locales and that of the cladding districts unequivocally influences inner proficiency. Dynamic area ought not be intensely doped, as it causes transporter overflow into the constrainment areas diminishing the radiative proficiency Doping levels of 10 16 - low 10 17 are utilized, or none by any means. P-sort doping of the dynamic district is regularly done because of the bigger electron dispersion length. Bearer lifetime relies on upon the grouping of larger part transporters. In low excitation administration , the radiative transporter lifetime diminishes with expanding free bearer fixation. Henceforth productivity increments with doping. At high focus, dopants initiate abandons going about as recombination focuses. From Light-Emitting Diodes, Fred Schubert.

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P-N intersection relocation Displacement of the P-N intersection causes critical change in the inside quantum effectiveness in DH LED structures. Dopants can redistribute because of dispersion, isolation or float. From Light-Emitting Diodes, Fred Schubert.

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Doping of the constrainment locales Resistivity of the imprisonment areas ought to be low with the goal that warming is negligible. High p-sort conc. in the cladding area keeps electrons in the dynamic locale and keeps them from diffusing into the constrainment district. Electron spillage out of the dynamic area is more extreme than opening spillage. From Light-Emitting Diodes, Fred Schubert.

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Non radiative recombination The centralization of imperfections which cause profound levels in the dynamic area ought to be least. Additionally surface recombination ought to be limited, by keeping all surfaces a few dissemination lengths far from the dynamic area. Plateau carved LEDs and lasers where the plateau draw uncovered the dynamic locale to air, have low inside productivity because of recombination at the surface. Surface recombination additionally decreases lifetime of LEDs. From Light-Emitting Diodes, Fred Schubert.

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Lattice coordinating Carriers recombine non-radiatively at nonconformist disengagements. Thickness of nonconformist disengagement lines per unit length is corresponding to cross section crisscross. Subsequently the effectiveness of LED\'s is relied upon to drop as the jumble increments. From Light-Emitting Diodes, Fred Schubert.

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High extraction effectiveness structures Shaping of the LED bite the dust is basic to enhance their productivity. LEDs of different shapes; hemispherical arch, reversed cone, truncated cones and so forth have been exhibited to have better extraction productivity over ordinary outlines. However cost increments with many-sided quality. From Light-Emitting Diodes, Fred Schubert.

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The TIP LED utilizes propelled LED bite the dust molding to limit inside misfortune instruments. The shape is limited catching of light. TIP LED is a powerful LED, and the glowing productivity surpasses 100 lm/W. TIP gadgets are sawn utilizing slanted dicing cutting edge to acquire chip sidewall edges of 35 o to vertical. High extraction proficiency structures Krames et. al, Appl. Phys. Lett., Vol. 75, No. 16, 18 October 1999

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Visible range LEDs The plot diagrams the increases made in brilliant proficiency till date. From Light-Emitting Diodes, Fred Schubert.

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Visible range LEDs The discharge range of the blue, green and red LEDs show that the green LED has a more extensive range. Compound widening prompts to ghastly expanding that is more noteworthy than 1.8 kT linewidth. From Light-Emitting Diodes, Fred Schubert.

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White-light LEDs White light can be created in a few diverse ways. One route is to blend to reciprocal hues at a specific power proportion. Another path is by the outflow of three hues at specific wavelengths and power proportion. Most white light emitters utilize a LED emanating at short wavelength and a wavelength converter. The converter material ingests a few or all the light radiated by the LED and re-transmits at a more extended wavelength. Two parameters that are essential in the era of white light are brilliant proficiency and shading rendering list. It is demonstrated that white light sources utilizing two monochromatic integral hues result in most noteworthy conceivable radiant productivity. From Light-Emitting Diodes, Fred Schubert.

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White-light LEDs Wavelength converter materials incorporate phosphors, semiconductors and colors. The parameters of intrigue are ingestion wavelength, outflow wavelength and quantum productivity. The general vitality proficiency is given by η = η ext ( λ 1/λ 2 ) Even if the outside quantum effectiveness is 1, there is dependably a vitality misfortune related with change. Basic wavelength converters are phosphors, which comprise of an inorganic host material doped with an optically dynamic component. A typical host is Y 3 Al 5 O 12 . The optically dynamic dopant is an uncommon earth component, oxide or another compound. Regular uncommon earth components utilized are Ce, Nd, Er and Th. From Light-Emitting Diodes, Fred Schubert.

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White-light LEDs Phosphors are steady materials and can have quantum efficiencies of near 100%. Colors additionally can have quantum efficiencies of near 100%. Colors can be typified in epoxy or in optically straightforward polymers. Nonetheless, natural colors have limited lifetime. They turn out to be optically latent after 10 4 - 10 6 optical moves. From Light-Emitting Diodes, Fred Schubert.

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White LEDs in view of phosphor converters A blue GaInN/GaN LED and a phosphor wavelength converter suspended in a

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