1 Light-emitting Diodes 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 Day 2, May 28, 2008, Pisa

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

3 How much energy is used for lighting?

4 Lighting: growth and costs Tsao

5 LED roadmap Tsao

6 Basic operation 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 Pierret Phonons

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 SRH Auger

13

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

15 What are these terms? LED efficiencies

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

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 algorithm for drawing band diagrams? What is the doping type of the active region? EFS

21 Heterojunction Band Diagrams e.g., n-Al 0.3 Ga 0.7 As (E g =1.80eV,  =3.83eV) on p-GaAs (E g =1.42eV,  =4.07eV) Separated system Joined system E 0, E l ECEFEVECEFEV  e-barrier < h-barrier

22 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? Radiative efficiency

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

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

25 Creative TIR EFS

26Reflectors Put reflector on top and use transparent substrate

27 Contact blocking

28 Current spreading layer

29 Photon "voltage" What is the photon energy? 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 V D  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 Achromatic Impression of white light via RGB White light

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

51 Colour rendering Illuminated by: (a) high-CRI source (b) low-CRI source

52 Colour rendering with LED array

53  1/ nm LI = 1cd Integrate over sphere  LF = 1 lm 50W halogen = 900 lm Illuminance is LI/area 1 lm/m 2 = 1 lux desk light  500 lux sunlight = 100,000 lux LI = luminous intensity LF = luminous flux LE = luminous efficiency (lm/W electrical ) Photometric units

54 Incandescent bulbs

55 Fluorescent bulbs

56 Challenges to white LED technologies And of course

57

58 What is wall-plug efficiency?

59

60 White-light LEDs are here!

61 Osram announces 1000 lm LED

62 The shape of things to come

63References EFS LUMILEDS Tsao, J.Y., Pierret, R.F., "Advanced Semiconductor Fundamentals", Addison-Wesley, 1987 Fonstad, C., Computer-Science/6-772Spring2003/FDFCCD B733- 6A535E375BCE/0/Lecture18v2.pdfhttp://ocw.mit.edu/NR/rdonlyres/Electrical-Engineering-and- Computer-Science/6-772Spring2003/FDFCCD B733- 6A535E375BCE/0/Lecture18v2.pdf

64Terminology