Requirements. K. H. Loo. The Hong Kong Polytechnic University

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1 Overview of LED Lighting and Requirements K. H. Loo Faculty of Engineering The Hong Kong Polytechnic University 1

2 Content Radiative and non-radiative processes in LEDs Structure of high-brightness LEDs Origin of efficiency droop Bi-level and multi-sectional piecewise PWM driving techniques Color stability of white-leds under various driving techniques General Photo-Electro-Thermal Theory for LED Systems Conclusions 2

3 Radiative and Non-Radiative Processes in LEDs 3

4 Semiconductor s Energy Band Model 4 Energy band is formed by bringing together a large number of atoms due to energy spreading by interatomic forces The upper (conduction) band is separated from the lower (valence) band by the forbidden gap or band gap

5 Radiative Processes in LEDs p-type n-type 5 Radiative recombination occurs at high transition rate in direct bandgap semiconductors (e.g. GaAs, InP) as both momentum and energy are readily conserved during the transition Radiative recombination in indirect bandgap semiconductors (e.g. Si, Ge) must be assisted by phonon emissions (lattice vibrations) that lead to unwanted heat dissipation

6 Radiative Processes in LEDs (h) (a) (b) (c) Band-band Free-exciton Localized transition annihilation excitons Impurity-assisted radiative transition in indirect bandgap semiconductor (d) (e) (f) (g) *Figures from A. Zukauskas et al, Introduction to Solid-State Lighting, John Wiley & Sons, Radiative recombination via impurity levels

7 Radiative Processes in LEDs Mono-molecular recombination: R B n n p p Bn p r Bn p p nnp Bn p (holes in n-type) Bp n (electrons in p-type) Bi-molecular recombination: R B n p p nnp r 0 0 Bnp 7 B radiative recombination coefficient, m 3 s -1 p, n excess carrier concentration, m -3 *Figures from M. Fukuda, Optical Semiconductor Devices, John Wiley & Sons, 1999

8 Non-Radiative Processes in LEDs: Shockley-Read-Hall ll Recombination R SRH hv th N t p p 0 (holes in n-type) v N nn (electrons in p-type) e th t 0 N t trap density, m -3 e, h electron and hole capture cross section, m 2 v th carrier thermal velocity, m s -1 Non-radiative recombination via deep or trapping levels 8 *Figures from E. F. Schubert, Light-Emitting Diodes, Second edition, Cambridge University Press, 2006 Point defects on a GaAs epitaxial layer examined by cathodoluminescence

9 Non-Radiative Processes in LEDs: Auger Recombination 1 2 The energy released by electron-hole l recombination i is transferred to another electron or hole excited to a higher energy level Photons are never created and the transferred energy is finally dissipated through phonon emissions 9 Non-radiative recombination via threebody interaction 2 RAuger Cnn p n Cnp p 2 p p (in -type) (in -type) *Figure from E. F. Schubert, Light-Emitting Diodes, Second edition, Cambridge University Press, 2006

10 Non-Radiative Processes in LEDs: Surface/Edge Recombination R v N pp sr h th ts 0 S p p (holes in n-type) h 0 R v N nn sr e th ts 0 e 0 S nn (electrons in p-type) N ts surface-state density, m -2 e, h electron and hole capture cross section, m 2 v th carrier thermal velocity, m s -1 S e, S h surface recombination velocity, m s *Figures from M. Fukuda, Optical Semiconductor Devices, John Wiley & Sons, 1999

11 Total Recombination Rate and Internal Quantum Efficiencyi Single carrier injection (e.g. p-n homojunction): R R R R R total r SRH Auger sr 2 n 2 Bn p v N p p C n p v N p p (holes in n-type) 0 h th t 0 h th ts 0 Bp 0n ev thn t n n 0 C pnp ev thn ts n n 0 (electrons ect in p -type) Double carrier injection (e.g. double heterojunction): Under high excitation level ( p p p, n n n, p n), 0 0 R AnBn Cn total Bn An Bn Cn IQE *Figure from M. Fukuda, Optical Semiconductor Devices, John Wiley & Sons, 1999

12 Structure of High-Brightness LEDs: From Homojunction to Single Heterojunction p-n homojunction p-n single heterojunction 12 *Figures from M. Fukuda, Optical Semiconductor Devices, John Wiley & Sons, 1999

13 Structure of High-Brightness LEDs: From Single to Double Heterojunction p-n single heterojunction p-p-n double heterojunction 13 *Figures from M. Fukuda, Optical Semiconductor Devices, John Wiley & Sons, 1999

14 Structure of High-Brightness LEDs: From Heterojunction to Quantum Well Single quantum well (QW) 14 *Figures from E. F. Schubert, Light-Emitting Diodes, Second edition, Cambridge University Press, 2006 Multiple quantum well (MQW) - State of the art

Structure t of High-Brightness i ht LEDs Flip-chip LED Cross-sectional view 15 E.

Status Solidi (a) 188(1), pp.15-21, 2001 J.

15 Structure t of High-Brightness i ht LEDs Flip-chip LED Cross-sectional view 15 E. F. Schubert, Light-Emitting itti Diodes, Second edition, Cambridge University it Press, 2006 A. Y. Kim et al, Performance of high-power AlInGaN light emitting diodes, Physica Status Solidi (a) 188(1), pp.15-21, 2001 J. Xie et al, On the efficiency droop in InGaN multiple quantum well blue light emitting diodes and its reduction with p-doped quantum well barriers, Applied Physics Letters 93, , 2008

16 Luminous Efficacy of High-Brightness Light- Emitting i Diodes 16 R. D. Dupuis et al, History, development, and applications of high-brightness visible light-emitting diodes, Journal of LightwaveTechnology 26(9), pp , 2008

17 17 Origin of Efficiency Droop

18 What is Efficiency i Droop? max droop A max where A is the current density of interest. EQE INJ IQE EXT 18 M. F. Schubert et al, Polarization-matched GaInN/AlGaInN multi-quantum-well light-emitting diodes with reduced efficiency droop, Applied Physics Letters 93, , 2008

19 Possible Causes of Efficiency i Droop Junction heating Carrier delocalization from Indium-rich regions Auger recombination Poor hole injection Carrier leakage from active region 19

20 Origin of Efficiency Droop: Junction Heating InGaAsP/InP LED 600 Lumileds LUXEON K2 white LED Illuminance (lx) Natural convection T = o j 40 C T j = 60 o C T j = 80 o C Forward current (ma) 20 *Figure from E. F. Schubert, Light-Emitting Diodes, Second edition, Cambridge University Press, 2006 Recombination probability decreases at increased temperature due to reduced number of carriers per dk interval Junction heating does not cause efficiency droop

21 Origin of Efficiency Droop: Carrier Delocalization li i from Indium-Rich Regions 21 Inhomogeneity in indium composition induces bandgap modulation over the QW s thickness In-rich regions act as quantum confined microstructures that keep carriers away from non- radiative recombination centers associated with dislocations *Figures from E. F. Schubert, Light-Emitting Diodes, Second edition, Cambridge University Press, 2006

22 Origin of Efficiency Droop: Auger Recombination Put forward by Philips Lumileds Carrier concentration is proportional to (kt/e g ) 3/2 exp(e g /kt), Auger recombination rate

polarization fields and interface effects Carriers are generated in InGaN active layer only by photoexcitation to eliminate i the effect of poor carrier injection i Used materials

22 22 Origin of Efficiency Droop: Auger Recombination Put forward by Philips Lumileds Carrier concentration is proportional to (kt/e g ) 3/2 exp(e g /kt), Auger recombination rate becomes high as temperature increases and as bandgap energy decreases Based on photoluminescence experiments: Photoluminescence Used quasi-bulk InGaN active layer to eliminate polarization fields and interface effects Carriers are generated in InGaN active layer only by photoexcitation to eliminate i the effect of poor carrier injection i Used materials with different dislocation densities to study the effect of dislocations Used materials with different InN compositions to study the effect of carrier delocalization from In-rich regions

23 Origin of Efficiency Droop: Auger Recombination Higher electron density at QW Excessive electron density at QW Auger recombination becomes dominant 2 Bn An Bn Cn IQE Y. C. Shen et al, Auger recombination in InGaN measured by photoluminescence, Applied Physics Letters 91, , 2007

24 Origin of Efficiency Droop: Poor Hole Injection Electrons overflow Electron-hole recombination outside tid active region Poor hole injection 24 I. V. Rozhansky and D. A. Zakheim, Analysis of dependence of electroluminescence efficiency of AlInGaN LED heterostructures on pumping, Physica Status Solidi (c) 3(6), pp , 2006 Holes

25 Origin of Efficiency Droop: Poor Hole Injection Put forward by Morkoc s group at the Virginia Commonwealth University i 25 J. Xie et al, On the efficiency droop in InGaN multiple quantum well blue light emitting diodes and its reduction with p-doped quantum well barriers, Applied Physics Letters 93, , 2008

26 Origin of Efficiency Droop: Poor Hole Injection 26 X. Ni et al, Reduction of efficiency droop in InGaN light emitting diodes by coupled quantum wells, Applied Physics Letters 93, , 2008

27 Origin of Efficiency Droop: Poor Hole Injection J en ed n n n n J ep ed p p p p dn 1 J R R dt e dp 1 J R R dt e dn D Re D dt dn A RhA dt R A npn p eh n e h ed p e h ha 0 0 e P n p N A N D 0 27 I. V. Rozhansky and D. A. Zakheim, Analysis of dependence of electroluminescence efficiency of AlInGaN LED heterostructures on pumping, Physica Status Solidi (c) 3(6), pp , 2006

28 Origin of Efficiency Droop: Poor Hole Injection Without piezoelectric polarization field With piezoelectric i polarization field 28 I. V. Rozhansky and D. A. Zakheim, Analysis of processes limiting quantum efficiency of AlGaInN LEDs at high pumping, Physica Status Solidi (a) 204(1), pp , 2007

29 Origin of Efficiency Droop: Carrier Leakage from Active Region Put forward by Schubert s group at the Rensselaer Polytechnic Institute 29 M. F. Schubert et al, Effect of dislocation density on efficiency droop in GaInN/GaN light-emitting diodes, Applied Physics Letters 91, , 2007

30 Origin of Efficiency Droop: Carrier Leakage from Active Region 30 Polarization mismatch at the heterointerfaces results in the formation of large sheet charges and creates obstacles for electron transport Conduction band on n-side is higher than the conduction band on the p- side so it is energetically favorable for electrons to escape to the p-region M. H. Kim et al, Origin of efficiency droop in GaN-based light-emitting diodes, Applied Physics Letters 91, , 2007

Origin of Efficiency Droop: Carrier Leakage from Active Region Total recombination rate: 2 3 R An Bn Cn f n 2 3 4 f n n n n n... R A' n B' n C' n n 2 3 4.

31 Origin of Efficiency Droop: Carrier Leakage from Active Region Total recombination rate: 2 3 R An Bn Cn f n f n n n n n... R A' n B' n C' n n Q. Dai et al, Carrier recombination mechanisms and efficiency droop in GaInN/GaN light-emitting diodes, Applied Physics Letters 97, , 2010

32 Methods to Reduce Efficiency i Droop Use of wide double-heterostructure instead of quantum well (Gardner et al, Shen et al) equivalent to barrier width reduced to zero Use of p-doped quantum-well barriers (Xie et al) Use of GaInN (smaller bandgap) instead of GaN quantum-well barriers for improving hole transport in quantum well (Xie et al) Use of lightly doped n-type GaN confinement layer for comparable electron and hole injection (Xie et al) Use of quantum well with reduced barrier width and barrier height for more uniform hole and electron distributions coupled quantum well (Ni et al) 32 Use of polarization-matched GaInN/AlGaInN multiple quantum well and electron blocking layer (Schubert et al)

33 The Bi-Level and Multi-Sectional Piecewise PWM Driving Techniques 33

34 Implications of Efficiency Droop on the Selection of Driving i Methods Current DC 10 FWR AC PWM Current Current Time Time Time Lum minous effi iciency DC FWR AC PWM Lu uminous power DC FWR AC PWM 34 Average LED current Average LED current

35 Conceptualization of the Bi-Level Current Driving i Technique DC PWM Current am mplitude Current am mplitude t on 35 Time Lowering of peak current reduction in efficiency droop Raising of zero current limited increase in efficiency droop T Duty cycle, = t on / T Time

36 Conceptualization of the Bi-Level Current Driving i Technique v,h A v, C 1 DC Illum minance v v, 1 B 3 2 v,b C 2 PWM 3 Bi-Level 36 0 I I f I I H Average forward current

37 Illuminan nce v 37 Different Configurations of the Bi-Level A Dii Driving Technique v,h v,h v, 0 I A C 1 B 3 2 I f I = I H Average forward current Preferred Configuration v,b C Illumin nance v Illum minance v v, v, v,h v, 0 0 = I I C 1 B 3 2 I I H I f Average forward current 3 A C 1 B 2 I f I I H Average forward current K. H. Loo et al, On Driving Techniques for LEDs: Toward a Generalized Methodology, IEEE Transactions on Power Electronics, Vol. 24, No. 12, pp , 2009 v,bc v,bc

38 Measured Luminous Output under DC for Various Junction Temperatures minance (lx) Illu Natural convection T j = 40 o C T j = 60 o C T j = 80 o C LUXEON K2 white LED Forward current (ma) 38 W. K. Lun et al, Bilevel Current Driving Technique for LEDs, IEEE Transactions on Power Electronics, Vol. 24, No. 12, pp , 2009

39 Measured Luminous Output and Efficacy for Various Current Waveforms LUXEON K2 white LED Illum minance (lx x) DC Bi-level (10:1) Bi-level (10:2) PWM T j = 60 o C Illuminanc ce efficacy (l lx/w) DC Bi-level (10:1) Bi-level (10:2) PWM T j = 60 o C Average input power (W) Average input power (W) 39 W. K. Lun et al, Bilevel Current Driving Technique for LEDs, IEEE Transactions on Power Electronics, Vol. 24, No. 12, pp , 2009

40 Measured Color Shift for Various Current Waveforms Co olor shift LUXEON K2 white LED DC Bi-level (10:1) Bi-level (10:2) PWM T j = 60 o C Average input power (W) 40 W. K. Lun et al, Bilevel Current Driving Technique for LEDs, IEEE Transactions on Power Electronics, Vol. 24, No. 12, pp , 2009

41 Extension to the Multi-Sectional Piecewise i PWM Driving i Technique L 3 DC Multi-Sectional Piecewise PWM PWM minance Illum L 2 L 1 I 1 I 2 Average LED current I 3 41 S. C. Tan, General n-level Driving Approach for Improving Electrical-to-Optical Energy-Conversion Efficiency of Fast-Response Saturable Lighting Devices, IEEE Transactions on Industrial Electronics, vol. 57, no. 4, pp , April 2010

42 Sequential Driving of RGB LED with the Multi-Sectional l Piecewise i PWM 42 S. K. Ng et al, A Variable Bi-Level Phase-Shifted Driving Scheme for High Luminance RGB LED Lamps, 8th International Conference on Power Electronics ECCE Asia, Jeju, Korea, 30 May-3 June 2011

43 Sequential Driving of RGB LED with the Multi-Sectional l Piecewise i PWM Norma alized optical po ower Red LED MSP PWM PWM Average LED current (ma) Norma alized optical po ower Green LED MSP PWM PWM Average LED current (ma) 43 Nor rmalized optica al power 1.0 Blue LED MSP PWM PWM Average LED current (ma)

44 power Norma alized optical 2L R L R RGB Color Mixing with the Multi- Sectional Piecewise i PWM Red LED Norma alized optical power 2L G L GB L G Green LED m G I 2L L IGB mg G GB ' G I R I' R =2I R I G I GB I' G Average LED current Blue LED Average LED current No ormalized opt tical power 2L L B BB L B I m m B 2L L IBB mb B BB ' B 44 I B I BB I' B Average LED current

45 RGB Color Mixing with the Multi- Sectional Piecewise i PWM 45 Plot of color coordinate variations under various dimming levels in the absence of color sensing and compensation network

46 Color Shift of White LEDs Under Various Driving Methods 46

Color Shift with Increasing Current Density As carrier density increases, electrons and holes fill each band from the edge and the energy difference between transition levels increases Band-filling

47 Color Shift with Increasing Current Density As carrier density increases, electrons and holes fill each band from the edge and the energy difference between transition levels increases Band-filling effect leads to blue-shift of emitted light S. Nakamura, InGaN/AlGaN blue-light-emitting diodes, Journal of Vacuum Science & Technology A, vol. 13, no. 3, pp , J. K. Sheu et al, Luminescence of an InGaN/GaN multiple quantum well light-emitting diode, Solid-State Electronics, vol. 44, pp , 2000

Color Shift with Increasing Current Density (a) () (b) + + + + + + + + + + + + + + The large electric fields caused by ypolarization effects are screened by the high carrier concentration under a

48 Color Shift with Increasing Current Density (a) () (b) The large electric fields caused by ypolarization effects are screened by the high carrier concentration under a high injection current density Screening by high carrier concentration reduces quantum-confined Stark effect and leads to blue-shift of emitted light 48 *Figures from E. F. Schubert, Light-Emitting Diodes, Second edition, Cambridge University Press, 2006

49 Color Shift with Increasing Junction Temperature As junction temperature increases, thermal expansion increases the lattice spacing between atoms and narrows the bandgap energy Bandgap narrowing leads to red-shift of emitted light E g 2 T T Eg T 0K T Varshni formula 49 T Mukai et al, Current and temperature dependences of electroluminescence of InGaN-based uv/blue/green light-emitting diodes, Japanese Journal of Applied Physics, vol. 37, pp. L1358-L1361, 1998

50 Chrom maticity coordina ate x Color Shift Under Changes of Current Density and Junction Temperature T j = 40 o C T o j = 60 C T j = 80 o C x y z Chrom maticity coordina ate y T j = 40 o C o T j = 60 o C T j = 80 o C LUXEON K2 white LED Chrom maticity coordina ate z T j = 40 o C T j = 60 o C T = o j 80 C DC forward current (ma) DC forward current (ma) DC forward current (ma) Chromaticity coordinate x I f = 200 ma I f = 500 ma I f = 800 ma I f = 1000 ma Chromaticity coordinate y I f = 200 ma I f = 500 ma I f = 800 ma I f = 1000 ma Chromaticity coordinate z I f = 200 ma I f = 500 ma I f = 800 ma I f = 1000 ma Junction temperature ( o C) Junction temperature ( o C) Junction temperature ( o C) 50 K. H. Loo et al, On the color stability of phosphor-converted white LEDs under dc, PWM, and bi-level drive, IEEE Transactions on Power Electronics (to appear)

51 Color Shift Under Changes of Current Density and Junction Temperature: DC Drive Tj T j,max Tj T j,max T j T a Tj T j,min I f,min I f, max If I a I f I f, max Tj T a I f I f 51 K. H. Loo et al, On the color stability of phosphor-converted white LEDs under dc, PWM, and bi-level drive, IEEE Transactions on Power Electronics (to appear)

52 Color Shift Under Changes of Current Density and Junction Temperature: PWM Drive Tj T j,max Tj T j,max T j T a Tj T j,min I f,min I f, max If I a I f I f, max Tj T a I f I f 52 K. H. Loo et al, On the color stability of phosphor-converted white LEDs under dc, PWM, and bi-level drive, IEEE Transactions on Power Electronics (to appear)

53 Color Shift Under Changes of Current Density and Junction Temperature: Bi-Level Drive Tj T j,max T j T a If,min I DI (1 D) I f f, max f, min I f I f, max T T j T j,min 53 K. H. Loo et al, On the color stability of phosphor-converted white LEDs under dc, PWM, and bi-level drive, IEEE Transactions on Power Electronics (to appear)

54 Color Shift Under Changes of Current Density and Junction Temperature: All Drive Methods Ch romaticit ty coordin nate x DC (T j = 30 o C) DC (T o j = 100 C) DC PWM Bi-level Average forward current (ma) 54 K. H. Loo et al, On the color stability of phosphor-converted white LEDs under dc, PWM, and bi-level drive, IEEE Transactions on Power Electronics (to appear)

55 Minimization of Overall Color Shift Under DC Di Drive Chromaticity shift: x, y x, y dx y di dt I T, f j f j 2 Overall chromaticity shift: dx dy 2 Joule heating to LED: dt R R k V di j j c hs h f f Heat sink s thermal resistance for minimum : R jc 1 x I y I y T Rhs f f j 2 1 x T j x I f x Tj 55

56 Color Shift for Various Heat Sink s Thermal Resistance Values Under DC Drive Chr romaticit ty change e 1/ o C/W 4.7 o C/W 8.0 o C/W 13.2 o C/W DC forward current (ma) 56

57 General Photo-Electro-Thermal Theory for LED Systems 57

58 General Photo-Electro-Thermal t l Theory T R NP R P T T R P j hs jc heat a hs heat jc heat T NR R P a hs jc heat T NR R k P a hs jc h e T j = Junction Temperature ( o C) E = Luminous Efficacy (lm/w) v = Luminous Flux (lm) E E 1k T T o e j o E 1 o ke Ta To kekh NRhs Rjc P e NEP v e NE 1 k T T P k k NR R P 2 o e a o e e h hs jc e 58 S. Y. Hui and Y. X. Qin, A General Photo-Electro-Thermal Theory for Light-Emitting Diode (LED) Systems IEEE Transactions on Power Electronics, Vol. 24, No. 8, pp , 2009

59 General Photo-Electro-Thermal t l Theory The maximum luminous flux value can be obtained from d v /dp e = 0: d v NE 1 2 o ke T a To kekh NR hs Rjc P e 0 dp e P I * 1 ke Ta To e 2kk e h NRhs Rjc * 1 ke Ta To d 2kk e h NRhs Rjc Vd 59 S. Y. Hui and Y. X. Qin, A General Photo-Electro-Thermal Theory for Light-Emitting Diode (LED) Systems IEEE Transactions on Power Electronics, Vol. 24, No. 8, pp , 2009

60 General Photo-Electro-Thermal t l Theory 60 S. Y. Hui and Y. X. Qin, A General Photo-Electro-Thermal Theory for Light-Emitting Diode (LED) Systems IEEE Transactions on Power Electronics, Vol. 24, No. 8, pp , 2009

61 Conclusions 61 The explanations to the origin of efficiency droop are many and the search for the ultimate t explanation is ongoing The luminous efficacy of high-brightness LEDs is dependent on the nature of the current waveforms used to drive them Bi-level driving technique was developed to improve the luminous efficacy of PWM-driven LEDs while preserving the dimming linearity of PWM Multi-sectional piecewise PWM driving technique further improves the luminous efficacy by more closely tracing the dc output luminosity curve Color stability of white LEDs under dimming by dc, PWM, and bi-level l drive was discussed For white LEDs, dc-operation produces the highest luminous efficacy and provides the best color stability over dimming

Luminescence basics. Slide # 1

Luminescence basics. Slide # 1 Luminescence basics Types of luminescence Cathodoluminescence: Luminescence due to recombination of EHPs created by energetic electrons. Example: CL mapping system Photoluminescence: Luminescence due to