Physics LED

Vishay Semiconductors
Physics of Optoelectronic Devices
Light-Emitting Diodes
This section deals with the principles and characteristics of the technically most important types of visible
emitters which are formed, without special lateral
structures, as whole-area emitters.
Materials for LEDs
The materials for light-emitting diodes in the visible
spectrum (400 - 700 nm) are semiconductors with
bandgaps between 1.8 and 3.1 eV, with Eg
(eV) = hν = 1240 /λ (nm). In order to permit effective
recombination, the materials should have direct bandto-band transitions or permit other recombination
paths with high efficiency. Neither of these requirements are fulfilled by the well-known semiconductors
silicon and germanium, as their bandgaps are too
small and they are indirect by nature.
As a further requirement, the materials have to enable
manufacturing in the form of monocrystals (volume
crystals or at least epitaxial layers) and a sufficiently
developed technology must be available for processing.
Element combination from groups III and V of the periodic element system results in semiconducting compounds with bandgaps between 0.18 eV (InSb) and
approximately 6 eV (AlN); see figure 1.
I
II
III
IV
V
VI
VII
Li
Be
B
C
N
O
F
Ne
Na
Mg
Al
Si
P
S
Cl
Ar
K
Ca
Zn
Ga
Ge
As
Se
Br
Kr
Rb
Sr
Cd
In
Sn
Sb
Te
J
Xe
H
VIII
He
III
V
N
P
As
Sb
Al
AlN
6.0
AlP
2.45
AlAs
2.15
AlSb
1.65
Ga
GaN
3.4 *
GaP
2.26
GaAs
1.42 *
GaSb
0.73 *
In
InN
1.95 *
InP
1.34 *
InAs
0.36 *
InSb
0.18 *
Figure 1. Periodic system of elements and III-V compounds
For every composition the bandgap is given in eV.
Direct semiconductors are denoted by an asterisk.
The largest direct bandgaps are found in the group III
– nitrides GaN and InN. These nitrides are supercede
silicon carbide as semiconducting material for blue
light emitting diodes.
Document Number: 80097
Rev. 1.2, 05-Jul-04
Gallium arsenide has a direct bandgap of 1.42 eV and
is not only important for light emitting diodes, but is
also a significant substrate material.
III–V compounds can form mixed crystals with properties between those of the binary compounds. The
most important mixed crystal systems pertaining to
LEDs are GaAs–AlAs, GaAs – GaP, InP – GaP – AlP.
The bandgap and the lattice constant of Gal-xAlxAs
and GaAsl-xPx are shown in figure 2. The direct region
is shown as a solid line and the indirect region as a
dashed line. Up to x = 0.44 (band gap 1.96 eV),
Gal-xAlxAs is direct, and the lattice constant changes
only slightly over the whole mixture range. The highest direct bandgap for GaAsl-xPx is 1.99 eV (x = 0.45).
In the indirect region of GaAs1-xPx, the efficiency of
radiating recombination can be increased considerably by doping with nitrogen (isoelectronic center).
This reduces the effective bandgap by approximately
0.06 eV (second dashed line).
The bandgaps required for common LED colors are
marked at the top of figure 2. Whereas red emission
can be achieved with Ga0.6Al0.4As or GaAs0.6P0.4
(both direct), the indirect GaAsl-xPx:N is used for the
other colors.
In figure 3, the bandgaps and lattice constants of
InGaAlP are presented. In this diagram, binary compounds are shown as points, ternary compounds as
lines, and the quaternary mixed crystal InGaAlP as a
shaded area. The binary crystals InP, GaP and AlP
form the apecies. Their characteristic form is a result
of the complex nature of the band structure and the
transition from the direct to the indirect region and
vice versa. The diagram shows which InGaAlP compositions are direct and which mixed crystals are lattice matched to a given substrate material. Both are
necessary prerequisites for the manufacture of particularly bright light emitting diodes. The vertical dashed
line drawn through the direct bandgap region shows
the compositions that are exactly lattice matched to,
and can be manufactured on a GaAs substrate. The
corresponding notation In0.5(GaxAl1–x)0.5P gives the
ratio of the amounts of atoms in the crystal lattice. The
bandgap is direct if the aluminium concentration is
below x = 0.7. The spectral region from red via orange
and yellow to green can be covered by variation of x
in InGaAlP. Extremely high luminescent effiencies for
orange and yellow are obtained in double heterostructures of InGaAlP situated on a Bragg reflector on
a GaAs substrate.
www.vishay.com
49
Vishay Semiconductors
p-Electrode(Al)
p-Diffusion
Si3N4
yellow 590 nm
orange 625 nm
green 565 nm
red 660 nm
95 11428
x = 0.4
0.570
AlAs
Lattice Constant ( nm )
GaAs
0.565
n-GaAs1–x Px
Ga0.56 Al 0.44As
0.560
n-GaAs Substrate
GaAs 0.55P0.45
0.555
96 11509
GaAs1–xPx:N
0.550
n-Electrode (Au)
GaP
0.545
1.4
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
Figure 4. Standard red
Bandgap ( eV )
The DH-red element consists of three epitaxial layers
on p-GaAs substrate. The active layer with 35 % Al
content is manufactured between two layers of
Gal-xAlxAs of > 60 % Al content.
Figure 2. Bandgaps and lattice constants
95 11463
3.0
450
Indirect Region
AlP
500
AlAs
550
GaP
Direct Region
600
650
700
2.0
AlInGaP Compositions
Lattice Matched to GaAs
800
1.5
n-Electrode (Au)
Wavelength ( nm )
Bandgap ( eV)
2.5
n-Ga1–x Al xAs (x > 0.6)
p-Ga 0.65 Al 0.35 As
p-Ga 1–x Al x As (x > 0.6)
p-GaAs Substrate
GaAs
InP
96 11510
1.0
5.4
5.5
5.6
5.7
5.8
Lattice Constant ( °A )
5.9
Figure 3. Bandgaps and lattice constants for InGaAlP
Technologies
The figures below show two structures on a GaAs
substrate: figure 4, standard red and figure 5, DH-red.
Standard red consists of GaAs0.6P0.4 which was
deposited with a GaAs1–xPx buffer layer on a n-GaAs
substrate by vaporphase epitaxy. The element is
manufactured in a planar process with masked Zn diffusion. The substrate is opaque in this case. This
means that the rear contact can be provided over the
complete area.
www.vishay.com
50
p-Electrode (Au)
Figure 5. DH-red
Other standard LEDs are shown in figures 6 and 7.
The chips for orange and yellow are similar to the
standard red type. Due to the large amount of P, 0.6
or 0.85, the material is deposited with suitable buffer
layers on the transparent GaP substrate, and a reflective rear contact is provided.
The green element consists entirely of GaP and can
therefore be manufactured with liquid-phase epitaxy.
The grown PN junction is divided by mesa etching in
this example to enable measurement of individual
chips on the wafer.
A new type of LED for applications where a particularly high brightness is required is shown in figure 8.
The structure is similar to that of a DH-red element. A
double heterostructure is arranged on a light-absorbing GaAs substrate. A combination of a Bragg reflec-
Document Number: 80097
Rev. 1.2, 05-Jul-04
Vishay Semiconductors
tor between the substrate and DH-structure and
window layer on the DH-structure effect an extremely
high luminescent efficiency in this type of LED.
Al
p-Diffusion
Anode
GaN
Si3N4
n–Ga(AsP):N
SiC Substrate
x = 0.6/0.85
16 026
n-GaAs1–x Px
Backside Metallization
Cathode
n-GaP Substrate
Figure 9. GaN / InGaN / straight wall
Anode (+)
96 11511
InGaN
AuGe
Figure 6. Orange and yellow
AuZn
SiC Substrate
p-GaP:N
n-GaP:N
Cathode (-)
Backside Metallization
InGaN
n-GaP
19007
Figure 10. Mushroom shaped
n-GaP Substrate
n
ITO
96 11513
AuGe
p
Epi Layer
ITO
Organic Binder
Figure 7. Green
Al
p-Electrode(Al)
Si Substrate
p-Type Window Layer
AlInGaP-DH-Structure
Reflector
n-GaAs Substrate
96 11512
n-Electrode (Au)
Figure 8. AlInGaP technology
Document Number: 80097
Rev. 1.2, 05-Jul-04
18990
Figure 11. OMA-Chip
LED Characteristics
The most important characteristics of the LEDs dealt
with here are summarized in table 1. The emission of
LEDs is almost monochromatic and can be characterized by a peak wavelength (column 4) and a spectral
half bandwidth (column 6). The lowest spectral half
bandwidth is generated by LEDs with direct band-toband recombination, while other mechanisms and
material inhomogeneities lead to wider emission.
Here, it is evident that the efficiency drops as the
wavelength is reduced.
Liquid-phase elements generally have a higher efficiency than comparable vapor-phase elements. For
applications in which a Si detector is to be used as a
www.vishay.com
51
Vishay Semiconductors
receiver, for example, the emitted radiated power φe
is important. If, however, the receiver is the human
eye, the light flux φv is decisive.
Figure 12 shows the sensitivity of the human eye in
accordance with DIN 5031. The maximum value is
683 lm/W at 555 nm. Green LEDs are close to this
maximum value, while the curve for red emission
drops rapidly. An orange LED (630 nm), for example,
appears approximately 5 times brighter than a red
LED (660 nm) at the same efficiency.
White LED
In general white light is a mixture of the 3 basic colors
red, green and blue. A cost effective solution is white
light out of only one chip by using the physical principle of luminescence conversion. A high brightness
InGaN-based LED chip, mounted in the reflector cup
is covered by a luminescence converter consisting of
an inorganic phosphor material dissolved in Silicon,
while the whole system is embedded in transparent
epoxy resin. Part of the blue light is absorbed by a
TAG–Phosphor converter and emitted at a longer
wavelength. The complementary colors blue and yellow mix to form white.
Due to the rapid current rise in the forward direction,
LEDs are always connected in series with a current
limiting element.
Figure 13 shows how the working point is set with a
series resistor. If the supply voltage varies greatly, a
constant current source is used. Digital displays and
similar devices are often operated in multiplex mode.
The seven data lines carry the information for the digits in a time-staggered manner. Each LED is operated
for one-quarter of the time with four times the current
in order to achieve the same intensity as in continuous operation. In displays with larger numbers of
LEDs, it is advisable to reduce the number of driver
stages and connecting leads. Due to the diode characteristics of LEDs, a large number of LEDs can be
operated with only a few leads if the circuit is
designed accordingly.
1000
100
K m * Vλ
(lm/W)
acc.DIN 5031
part 3
96 11579
10
565nm
668 lm/W
green
650nm
73 lm/W
red
1
400 440 480 520 560 600 640 680 720 760 800
λ– Wavelength ( nm )
Figure 12. Human eye sensitivity diagram
Electrically, LEDs are ordinary PN diodes (except for
GaN). The most important parameter for normal operation is the forward voltage VF. (Table 1, third column
from the right) where the numerical value in V corresponds approximately to the bandgap of the semiconductor used. The speed of LEDs is characterized by
their switching times (last columns); red LEDs made
of direct materials are fastest, while indirect types are
considerably slower due to the special recombination
mechanism. Generally, LEDs are robust and their lifetimes (more than 105 hours) are more than sufficient
for practically all applications.
www.vishay.com
52
Document Number: 80097
Rev. 1.2, 05-Jul-04
Vishay Semiconductors
40
LED
30
U = US – (R x IF)
US = 5 V
R = 140 Ω
20
10
I
F–
Forward Current ( mA )
50
0
0
96 11580
1
2
3
4
5
VF – Forward Voltage ( V )
Figure 13. LED operating point
Document Number: 80097
Rev. 1.2, 05-Jul-04
www.vishay.com
53
Vishay Semiconductors
Color
Technology
Red
Red
Red
Red
Red (OMA)
Softorange
Softorange
Yellow
Yellow
Yellow Green
Green
Pure Green
Pure Green
True Green
Blue Green
Blue
Blue
White
GaAlAs on GaAs
GaAsP on GaP
AlInGaP on GaAs
AlInGaP on GaAs
AlInGaP on Si
AlInGaP on GaAs
GaAsP on GaP
AlInGaP on GaAs
GaAsP on GaP
AlInGaP on GaAs
GaP on GaP
GaP on GaP
AlInGaP on GaAs
InGaN on SiC
InGaN on SiC
InGaN on SiC
GaN on SiC
InGaN/TAG on SiC
lp
ld
Dl
fv
fe
VF
tr
tf
Efficiency*
nm
650
635
643
620
622
610
610
590
585
574
565
555
561
518
503
463
428
nm
648
620
630
618
615
605
605
588
590
572
570
560
562
523
505
470
466
nm
20
38
15
20
18
17
36
20
38
20
38
22
20
35
30
25
65
not
defined
mlm
60
30
200
400
600
400
25
200
30
80
35
12
30
250
200
75
25
220
mW
0.82
0.20
1.22
1.5
2.5
1.2
0.06
0.78
0.05
1.12
0.05
0.02
0.05
0.55
0.79
1.21
0.96
1.21
V
1.80
2.00
1.85
1.85
2.8
1.90
2.00
1.90
2.00
2.0
2.00
2.00
2.00
3.10
3.20
3.60
3.70
3.60
ns
100
300
45
45
45
45
300
45
300
45
450
450
45
30
30
30
30
30
ns
100
150
30
30
30
30
150
30
150
30
200
200
30
30
30
30
30
30
lm/W
3.3
1.5
10.8
21.6
28
21.1
1.3
21.1
1.5
4
1.8
0.6
1.5
8.1
6.3
2.1
0.7
6.1
5500K
x =0.33 / y =0.33
Table 1: Typical technology characteristics @ IF = 10 mA
*This table gives an overview comparision for all major Vishay technologies. Some LED datasheets might state minor differences.
Conversion Tables
Radiometry
Definition
Symbol
Φe
Unit
Watt, W
W/m2
W/sr
Power
Radiant flux
(radiant power)
Output power per unit area
Output power per unit solid angle
Output power per unit solid angle
and unit emitting area
Radiant emittance/exitance
Radiant intensity
Radiance
Me
e
W/m2*sr
Input power per unit area
Energy
Irradiance
Radiant energy
Ee
W/m2
Qe
Ws
Energy per unit area
Radiant exposure (irradiation)
He
(W*s)/m2
Unit
Ie
L
Photometry
Symbol
Luminous flux
Φv
(luminous power)
Luminous exitance
Luminous intensity
Luminance
Mv
v
cd/m2
Illuminance
Luminous energy
(quantity of light)
Ev
lx = lm/m2
Lux, lx
Qv
lm *s
Light exposure
Hv
(lm*s)/m2
Table 2: Corresponding radiometric and photometric, symbols and units
asb
sb
L
fL
cd * m
cd * ft–2
-4
-2
1
p
π*10-4
0.2919
10
9.29*10
-4
-4
-2
1/p
1
0.0929
1/π*10
10
2.957*10
4
4
1
p
929
2919
10
π*10
4
4
2
1/π
1
929
1/π 10
10
2.957*10
10.764
33.82
1
p
1.076*10-3
3.382*10-3
3.426
10.764
1/π
1
3.426*10-4
1.0764*10-3
1550
4869
0.155
0.4869
144
452.4
–2
-2
1 cd*m
1 asb (Apostilb)
1 sb (stilb)
1 L (Lambert)
1 cd*ft-2
1 fl (Footlambert)
1 cd*in -2
Unit
1 lx
1 lm*cm–2
1 fc (footcandle)
lx
1
104
10.764
Table 3: Luminance Conversion Units (DIN 5031 part 3)
fc
lm * cm–2
10–4
1
–4
10.764*10
0.0929
929
1
Unit
Lumen,lm
lm/m2
candela, cd
Iv
L
Notes
cd * in–2
-4
6.45*10
2.054*10-4
6.452
2.054
6.94*10-3
2.211*10-3
1
ft = foot
in = inch
Notes
instead of lm*cm–2, formerly Phot (ph)
Table 4: Illuminance Conversion Units
www.vishay.com
54
Document Number: 80097
Rev. 1.2, 05-Jul-04
Vishay Semiconductors
Special Notes and Conversion Diagrams
a)
At standard illuminant A:
1 klx ≈ 6.1 mW/cm2 or 1 mW/cm2 ≈ 164 lx
At 555 nm it is valid: 683 lm ª 1 W
634.5 lm/ft2 ≈ 1 mW/cm2
1 lumen/ft2 = 1 footcandle
4 π candlepower = 1 lumen (lm)
b)
c)
1.0
520
540
0.8
560
0.6
580
500
d
y
250
0.4
S(x,y)
490
E v/Ee ( lx/mW/cm2 )
200
600
W
0.2
700
400
150
0.0
0.0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8
x
96 11583
100
Figure 16. Chromaticity diagram acc. to CIE 1931
50
0
2000
2200
2400
2600
2800
3000
Tf – Color Temperature ( K )
96 11581
Figure 14. EV/Ee (Tf)
1.8
Z
1.6
1.4
X, Y, Z
1.2
Y=V(λ)
1.0
X
0.8
0.6
0.4
X
0.2
0.0
350 400 450 500 550 600 650 700 750 800
λ– Wavelength ( nm )
96 11582
Figure 15. Color matching functions acc. to CIE 1931
Document Number: 80097
Rev. 1.2, 05-Jul-04
www.vishay.com
55
Vishay Semiconductors
www.vishay.com
56
Document Number: 80097
Rev. 1.2, 05-Jul-04