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Light-emitting Diodes for general lighting applications

D.L. Pulfrey. Department of Electrical and Computer Engineering University of British Columbia Vancouver, B.C. V6T1Z4, Canada. pulfrey@ece.ubc.ca. http://nano.ece.ubc.ca. Light-emitting Diodes for general lighting applications. Day 2, May 28, 2008, Pisa. Examples of colour lighting.

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Light-emitting Diodes for general lighting applications

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  1. D.L. Pulfrey Department of Electrical and Computer Engineering University of British Columbia Vancouver, B.C. V6T1Z4, Canada pulfrey@ece.ubc.ca http://nano.ece.ubc.ca Light-emitting Diodes for general lighting applications Day 2, May 28, 2008, Pisa

  2. Examples of colour lighting EFS: 18 million LEDs in New York city EFS: Regensburg bridge

  3. How much energy is used for lighting?

  4. Lighting: growth and costs Tsao

  5. LED roadmap Tsao

  6. Basic operation Radiative recombination

  7. Recombination in direct- and indirect- bandgap materials

  8. GaP is indirect ! How can this work? EFS

  9. Competing NON-radiative recombination processes in direct-bandgap materials Phonons Pierret

  10. Competing NON-radiative recombination processes in direct-bandgap materials Which of these 2 mechanisms is more likely to occur?

  11. Minority carrier recombination lifetime

  12. Minority carrier recombination lifetime due to non-radiative processes Auger SRH

  13. http://www.eng.yale.edu/posters150/pdf/woodall.pdf

  14. Optical Output Power How do we relate this to current ?

  15. LED efficiencies What are these terms?

  16. Current efficiency Fraction of LED current due to electrons recombining in the semiconductor (as opposed to at the contacts) EFS

  17. Improving the current efficiency EFS

  18. Carrier capture, recombination, and escape EFS

  19. Choice of material for heterostructure

  20. AlGaAs/GaAs DH LED What is the doping type of the active region? What is the algorithm for drawing band diagrams? EFS

  21. Heterojunction Band Diagrams e.g., n-Al0.3Ga0.7As (Eg=1.80eV, =3.83eV) on p-GaAs (Eg=1.42eV, =4.07eV) Separated system Joined system E0, El  EC EF EV • e-barrier < h-barrier

  22. Radiative efficiency • Need short radiative lifetime - choose material What are the B values for Si and GaAs ? • Need long non-radiative lifetime How do we get this?

  23. Extraction efficiency Solve #2 with wide bandgap "cladding"

  24. TIR EFS nr is about 3.5 for GaAs-family materials What is the critical angle? What is a typical value for ext in a cheap LED?

  25. Creative TIR EFS

  26. Reflectors Put reflector on top and use transparent substrate

  27. Contact blocking

  28. Current spreading layer

  29. What is the photon energy? Photon "voltage" EFS

  30. Intensity spectrum EFS

  31. The range of AlGaInP LEDs

  32. The range of AlInGaN LEDs

  33. Operating voltage How can the LED survive being operated at VD  Eg/q ?

  34. Current control Why is it important to minimize T ? EFS

  35. Effect of T on rad EFS What about the effect of T on non-rad ?

  36. Thermal resistance

  37. Brighter and brighter EFS

  38. White light • Achromatic • Impression of white light via RGB http://astro-canada.ca/_en/a3300.html

  39. Generating LED white light

  40. Perception of light

  41. Eye sensitivity function What is luminous efficacy?

  42. Colour matching functions Cone stimulation Chromaticity coordinates

  43. Chromaticity diagram

  44. Additive colour mixing What is the colour gamut?

  45. Chromaticity and LEDs

  46. Additive LEDs

  47. Additive possibilities

  48. Dichromatic LED Not quite complementary, but broadening (35nm for blue and 50nm for green) give possibility of white light. Li et al.,JAP, 94, 2167, 2003

  49. White light using phosphors

  50. Blue/yellow phosphor LEDs

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