Current State of the Art in
High Brightness LEDs
M. George Craford, CTO
American Physical Society
Solid State Lighting Session
March 6, 2007
Outline
• Technology Background
and Status
• Existing and Emerging
Applications
• Challenges and
Approaches for General
Illumination
Buckingham Palace, London, England
Lit by Lumileds LEDs
Copyright Philips Lumileds
M. George Craford, March 2007, APS
2
White light source luminous efficacy:
conventional vs. LEDs
re
ssu
pre
h
hig
Na
al
met
100
r
apo
Hg v
U.S.
gen
halo
IR
halogen
e
halid
Best highpower LED
large
fluorescent
50
Best lowpower
LED
DOE
r
(LED oadmap
s)
150
Conventional Light Sources ηL(lm/W)
500W High-pressure Na
150
140W Metal Halide
122
‘TL HE’ Tube Fluorescent
105
Halogen-IR Incandescent
30
Standard Incandescent
16
LED
s
Luminous Efficacy, ηL (lm/W)
200
incandescent
0
1920
1940
1960
1980
2000
2020
Year
Copyright Philips Lumileds
M. George Craford, March 2007, APS
3
What is an LED? Why can LEDs be brighter than
any other light source?
Postively
charged carriers
Semiconductor
Bandgap
Determines Color
Negatively
charged carriers
_
Colored LEDs:
Red, Yellow - AlInGaP
Blue, Green – InGaN
White LEDs:
Red + Green + Blue, or
Blue + phosphor
+
- With applied voltage positive and negative charge carriers recombine
- Energy may be released as light or heat
- Theoretically can be a 100% efficient with unlimited life!
(compared to incandescent which is 5% efficient, 2000 hour life)
- Commercial LEDs can be expected to reach 50% efficiency and possibly
more
Copyright Philips Lumileds
M. George Craford, March 2007, APS
4
Evolution of LED Efficiency
Copyright Philips Lumileds
M. George Craford, March 2007, APS
5
State-of-Art: 350mA High-Power LEDs
External Quantum Efficiency
60%
50%
CIE eye
response
40%
AlInGaP
30%
20%
InGaN
10%
0%
350
450
550
III-P:
• Fundamental bandstructure limitations
III-N:
• IQE (~450 nm) ~55 %
• IQE (~520 nm) ~20 %
650
Peak Wavelength (nm)
(1): Nichia Chemical Co., IWN 2004
Remainder data: Philips Lumileds (70 A/cm2)
• External Quantum Efficiency: EQE = Internal QE × Extraction Efficiency
EQE = Poptical / (Eph × If)
• Power Conversion/Wallplug Efficiency: PCE or WPE = Poptical / (Vf × If)
• Luminous Efficacy (lumens/Watt): LE = PCE × V (λ)
Copyright Philips Lumileds
M. George Craford, March 2007, APS
6
Light Extraction From LEDs
Lumileds AlGaInP/GaP TruncatedInverted-Pyramid LED
Lumileds Thin Film Flip Chip
(TFFC) Structure
MQW active
region
Roughened n-GaN
n-GaN
p-GaN
n-GaP
p-GaP
Metal anode/cathode
contacts
AlGaInP
Submount/Package
OSRAM AlGaInP Micro-mirror LED
Nichia AlInGaN patterned substrate
and mesh electrode LED
AlInGaP
GaAs
Copyright Philips Lumileds
M. George Craford, March 2007, APS
7
Evolution of light extraction efficiency for
AlGaInP and InGaN-GaN LEDs
Light Extraction Efficiency, Cext (%)
100
•In the past extraction
efficiency
improvements have
been substantial
(>10x)
90
80
TFFC
VTF
70
Shaped TS
60
FC (Ag)
CC (PS)
Thick RS
50
CC (PS/ITO)
•Cext is approaching a
practical limit
FC (Al)
40
Improved TS
30
CC
10
•In the future there is
more opportunity for
improvement in IQE,
especially in the green
TS
20
Thick AS + DBR
Thick AS
Thin AS
0
1990
1995
Mike Krames, et. al., IEEE Journal of
Display Technology, March 2007
Copyright Philips Lumileds
2000
2005
2010
Year
M. George Craford, March 2007, APS
8
Making White
RYGB White
Color
Mixing
Mixing
Optics
RYGB
LEDs
Incandescence
Sunlight
Computer Monitor
RYGB White
Phosphor
DownConversion
RYG(B)
Phosphors
Blue or UV
LED
• Issues:
• Phosphor conversion
Images courtesy Jeff Tsao
(Sandia National Labs)
– Quantum deficit, optical losses, new materials issues
• Color mixing
– Optical losses, color uniformity, color control circuits
Copyright Philips Lumileds
M. George Craford, March 2007, APS
9
Trend to Higher Power Packages
“5 mm” LEDs
LUXEON® Package
•
•
•
•
Lower thermal resistance – 9 K/W
Higher operating temperature – 185°C
Higher drive currents – up to 1.5 A
Higher flux – up to 140 lm in white
• Surface mountable, Pb-free
• Superior moisture tolerance
JDEC 2a)
mm2
Chip ~0.1
Rth ~ 200 K/W
Pin <0.1 W
LUXEON® I / III
First high-power LED
on the market (1998)
• Chip ≥ 1 mm2
• Rth` ~ 15 K/W
• Pin = 1-5 W
Copyright Philips Lumileds
M. George Craford, March 2007, APS
10
White LED Performance
160
70%
140
60%
120
50%
Small LED
Lab results
100
80
Power LED
Lab results
40%
Small LED
2006
30%
60
Power LED
2006
20%
40
Power LED
~2004
20
– Lower current density
– Lower forward voltage
• Power LEDs
– More lumens/package
– Lower cost per lumen
~350 mA
0
0
• Small 5mm LEDs
10%
20
~20 mA
Pump Blue WPE (est.)
Luminous Efficacy (lm/W)
“Cool” CCT ~ 5000-7000K
40
60
80
0%
100
Current Density (A/cm2)
Copyright Philips Lumileds
M. George Craford, March 2007, APS
11
High-power LED Applications: Single-color
• Many applications benefit from
LED advantages
–
–
–
–
–
–
Low power consumption
Ruggedness
Long life
Small source size
Color purity
…
• Key applications
• Thus far, >50 % of all traffic signals in the
US have been replaced with LED versions*
• Energy and maintenance savings are key
drivers
*Strategies Unlimited
Energy savings: 89 %
Maintenance rate: 5 %
(vs incandescent lamp)
• 60 M vehicles produced annually
• ~50 % of CHMSL and ~5-10 % of RCL are LED-based today
• Market adoption driven by styling, safety, and fuel savings
– Traffic signals
– Automotive
LEDs will dominate
Copyright Philips Lumileds
Mercury Montego
M. George Craford, March 2007, APS
Honda Accord
12
LUXEON Applications
Clydes Bridge – Glasgow by Philips
Hard Rock Hotel –
by Color Kinetics
Shanghai Grand Prix –
by SpaceCannon
Shanghai Grand Prix –
by SpaceCannon
Copyright Philips Lumileds
M. George Craford, March 2007, APS
13
High-power LED Applications: RGB White
Pocket Projectors
• Flux: 12 – 100 lm
• Power: 10 – 25 W
• Weight: 1 – 1.5 lb
• Battery life: 2.5 h
• Illumination
• LCD Backlighting
• Projection
Toshiba TDP-FF1A
• LUXEON I and LUXEON III
• Replicates day-light
without harmful ultraviolet or infra-red radiation
• Exact color rendition
SONY Qualia 005
Copyright Philips Lumileds
Mitsubishi PK-10
Mona Lisa Lighting by Fraen Corporation
• TriluminosTM LED backlight
for LCD panel
• Ultra-high color gamut
(105 % NTSC)
• LEDs eliminate motion artifact
• Mercury free
• Long life
M. George Craford, March 2007, APS
Samsung SP-P300M
14
High-power LED Applications: PC White
•
•
•
•
Portable lighting
Mobile phone camera flash
Illumination
Automotive forward lighting
•
•
•
•
Surefire DEF 1
LUXEON V
2 – 80 variable lumens
1 - 40 variable hours
Non imaging optics
• Functional flash (<~3m)
• LUXEON Flash
• LUXEON Module
Casino Breda, Netherlands by Bocom
Daytime Running Light (DRL)
• Multiple LEDs per DRL
• 100°C ambient temperature
S8
Copyright Philips Lumileds
M. George Craford, March 2007, APS
S6
15
LUXEON®-Based Forward Lighting Concepts
THE WALL STREET JOURNAL MAY 4, 1971
W
ill a n y o n e liv in g , liv e to s e e o u r little lig h t
b u rn o u t?
A fte r 1 0 0 y e a rs o f c o n s ta n t u s e , it m a y lo s e
o n ly h a lf its b rig h tn e s s .
T h a t’s b e c a u s e w e d o n ’t u s e a b u lb , a fila m e n t
or a vacuum .
W e u s e a tin y c ry s ta l c h ip c a lle d a lig h t e m ittin g
d io d e . It w o rk s s o m e th in g lik e a tra n s is to r, b u t le t’s n o t
g e t in to a ll th a t.
O u r d io d e s a re a lre a d y in u s e o n c o m p u te r
p a n e ls , fre e in g th e m a n w h o u s e d to lo o k fo r b u rn e d -o u t
b u lb s a m o n g a ll th o s e h u n d re d s o f w in k in g lig h ts .
T h a t’s a g o o d m a rk e t. B u t le t’s
lo o k a t m a rk e ts to c o m e .
H o w a b o u t a fla t h e a d -lig h t a s
w id e a s y o u r c a r, to e v e n ly lig h t
th e ro a d ?
O r a n in c h -d e e p c o lo r T V s e t?
O r a w ris t w a tc h w ith o u t a d ia l,
th a t s h o w s th e tim e in n u m b e rs a t
th e in s ta n t y o u p u s h a b u tto n ?
T h a t’s p a rt o f th e fu tu re w e s e e in o u r c ry s ta l c h ip s .
A n d ju s t a s m a ll p a rt o f th e fu tu re w e
s e e in M o n s a n t o : t h e s c ie n c e c o m p a n y .
Audi Nuvolari Concept
Copyright Philips Lumileds
M. George Craford, March 2007, APS
16
InGaN Devices: Evolution of LED source brightness
unfiltered
filtered
1.E+08
TFFC
LED
spot
HID
1.E+07
Illumination
general
2 2
Luminance
Luminance(cd/m
(cd/m) )
Halogen
1.E+06
LEDs
1.E+05
Fluorescent
1.E+04
1.E+03
1960
1970
1980
1990
2000
2010
Automotive
UHP
Indicator
Displays
Projection
Projection
1.E+09
2020
• 50 M cd/m2 (“Meganits”) measured at 1 A drive current
Copyright Philips Lumileds
M. George Craford, March 2007, APS
17
LUXEON Applications – Interior Lighting
Gimla Yacht – lighting by LightGraphix - fit for life, high performance
Copyright Philips Lumileds
M. George Craford, March 2007, APS
18
LED General Illumination for Off Grid Homes
Nepal 2000*
India 2001*
Sri Lanka 2003*
* Photos Courtesy of Light Up the World and PICO Power
Copyright Philips Lumileds
M. George Craford, March 2007, APS
19
Why are LEDs Not Yet Widely Used for General
Illumination?
• Cost has been too high
• Efficiency has been too low
• White “color” needs to be warmer and better controlled
• Engineering challenges: thermal, optical, electrical
Copyright Philips Lumileds
M. George Craford, March 2007, APS
20
Efficiency Requirement for PC White
Max. White Lumens
1000
• Single-emitter Flux
1000 lm desirable
– same as 60 W light bulb
– today’s LEDs: 30 – 160 lm
LUXEON K2
100
LUXEON III
Early LUXEON I
10
5 mm lamp
1
1995
2000
2005
2010
Year
• Approach I: Cost of Ownership (COO) Analysis – 1000 lm source
Input
Power
Source
cost
Energy
cost/yr
COO
(1 yr)
COO
(5yrs)
1 x 60 W incandescent
60 W
$ <1
$ 48
$ 48
$ 240
7 x LUXEON K2 emitters
40 W
$ 18
$32
$ 50
$ 178
1 x 150 lm/W LED
6.7 W
$< 2
$5.30
$7
$ 28
at $0.10 per kWh
PC White LED: ~150 lm/W
Copyright Philips Lumileds
M. George Craford, March 2007, APS
21
Smaller high-power packaging
Reducing the cost of light
Continued efforts to develop new package technologies that enable lighting
possibilities — improved color mixing and diffusing, smaller luminaire design —
and drive the cost per lumen lower. (Coming this quarter)
Copyright Philips Lumileds
M. George Craford, March 2007, APS
22
How Achievable is 150 lm/W ?
PC White
Today*
Future
Cext (%)
~80
~90
IQE (%)
~55
~90
EQE (%)
~45
~80
Vf (V)
~3.3
~2.9
WPE (%)
~35
~75
LE (lm/W)
~70
~150
* High performance commercial
“cool white” LED.
1mm2 chip driven at 350mA.
• IQE must increase by >1.5X
• This table assumes a phosphor conversion on 200 lumens/optical Watt for “cool” white
(CCT >5000).
• For “warm” white (CCT 3000 – 4000) the conversion is significantly lower and requires
development. This is an issue for illumination.
• To achieve 1000 lumen source drive current must be ~2A which reduces luminous
efficacy (LE).
Copyright Philips Lumileds
M. George Craford, March 2007, APS
23
White Light Quality and Color Temperature
Conventional
LUXEON (Conformal Coating)
(b)
(a)
Reflector Cup
Phosphor Granules
Reflector Cup
Phosphor Granules
LED Die
Submount Die
LED Die
LED Die
Die Attach Epoxy
Die Attach Epoxy
Improved Uniformity
YAG:Ce
CRI:
CCT:
Conv. Eff.:
YAG:Ce + CaS:Eu
~75
~90
~6000
~3200K
~200
~160 lm/Wopt
YAG:Ce
YAG:Ce
+ CaS:Eu
Intensity (a.u.)
Free standing white chip
• Excellent match to blackbody radiation.
400
• Need higher performance “warm” white
with good CRI for general illumination.
Copyright Philips Lumileds
M. George Craford, March 2007, APS
500
600
700
800
Wavelength (nm)
24
Heat Removal
1000-Lumen Single Emitter (White)
40
Heat Generation (W)
35
1000 lm Single Emitter
(White)
Active
Cooling
30
25
Passive
Cooling
20
15
If PCE = 50 %,
LE = 150 lm/W
10
5
0
20
40
60
80
100
120
140
160
Luminous Efficiency (lm/W)
• LEDs pass all heat back to heat-sink and fixture.
• Today’s efficiency: Thermal management remains an issue and cost driver.
• Future efficiency: Heat management will be straightforward.
Copyright Philips Lumileds
M. George Craford, March 2007, APS
25
Red, Green, Blue Color Mixing for Warm White
Illumination
R-G540-B White LED for Illumination
RGB 540
300
White Source Efficiency (lm/W)
Blue WPE - 75%
Red
R - 615nm
Blue - 460nm
Green - 540nm
CRI – 90
CCT - 3270
250
Red WPE - 75%
200
Red WPE - 40%
0.2
0.4
0.75
150
100
Red WPE - 20%
50
0
400
450
500
550
Wavelength (nm)
600
650
700
0%
20%
40%
60%
80%
Green LED Efficiency (WPE)
•If nitride RGB all reach ≥ 75% WPE (very unlikely requiring three “miracles”) then
the source efficacy would be ~280 lm/W before color mixing losses (possibly 25% →
210 lm/W)
•If RGB all reach >40% WPE (much more reasonable to expect) then ~150 lm/W
source would be achieved which would be color tuneable
•Green is the key for enabling color tuneable white illumination to occur
Copyright Philips Lumileds
M. George Craford, March 2007, APS
26
Photonic Crystal LEDs
170 micron
250 nm
• Further increase of Cext
– Extraction of waveguided light
– Potential increase of IQE
GaN
• Directionality of emitted light
– Directed emission without lenses
– Increased surface brightness
– Important for projection applications
Flip-Chip LED
Patterning by e-beam
lithography (Sandia)
Top-view of a
Photonic Crystal
InGaN LED
Wierer et. al., Appl. Phys. Lett. 84, 3885 (2004)
Photonic Crystal LED
Conventional LED
Photonic Crystal LED
extracted
waveguided
waveguided
Light intensity vs. angle.
~1.5x total flux compared
to planar device.
QWs
reflector
Copyright Philips Lumileds
M. George Craford, March 2007, APS
27
Recent Laboratory Result
On Jan. 23, 2007 Philips Lumileds Lighting Company announced a
new achievement in white (CCT = 4685oK) from a single 1*1 mm2 chip:
Several new technologies contributed, some of them will be
incorporated in a new generation of products coming out in this
quarter …
Copyright Philips Lumileds
M. George Craford, March 2007, APS
28
Summary
• Power LEDs are improving rapidly and continued improvement is
expected. Performance of 100 lm/W will happen and ~150 lm/W is
likely.
• There are a variety of applications for which single color, RGB, and PC
white power LEDs are being utilized and should dominate.
• Key areas for future improvement are IQE (especially green) and
phosphors for warm white.
• It is now clear that LEDs also should dominate general illumination. Full
conversion at 150 lm/W would “save” over 100 nuclear reactors
worldwide.
Copyright Philips Lumileds
M. George Craford, March 2007, APS
29
How Long Will It Take??
Copyright Philips Lumileds
M. George Craford, March 2007, APS
30
Solid-State Lighting Differences
Conventional
Ballast
SSL Opportunity
Electronic Driver
Sheet Metal
Optics
Guards/screens
Heatsink
Dimmer switches
Digital Controls Solutions
for colour and white
Copyright Philips Lumileds
M. George Craford, March 2007, APS
31
Relative Efficiency (arb. scale)
High Efficiency at High Currents
Philips Lumileds
Efficiency Gain
30
25
350mA equivalent
current production
New Epi Structure
20
New Epi Structure
15
10
5
0
0
50
100 150 200
Current density (A/cm2)
250
Efficiency droop with current
Copyright Philips Lumileds
New epi structure produces
substantially more flux at 1A drive.
M. George Craford, March 2007, APS
32