23.3 Light-Emitting Diodes
649
100
80
60
40
20
0
Light extraction efficiency
(%)
ex
1990
1995
2000
2005
2010
Year
2015
Fig. 23.5 Historic development of maximum light extraction efficiency for AlGaInP (red circles) and (In,Ga)N (blue
squares) LEDs. Dashed lines are guides to the eye. Adapted from [1938, 1944]
junction
semiconductor
(b)
(a)
(c)
(d)
40°
30°
50°
60°
70°
80°
90°
40°
30°
50°
60°
70°
80°
90°
20°
20°
10°
10°
0°
(i)
(ii)
(iii)
Fig. 23.6 Form of various LED casings with a hemispherical, b truncated sphere and c parabolic geometry. Adapted
from [1945]. d Emission characteristics for rectangular (i), hemispheric (ii) and parabolic (iii) geometry. Adapted
from [1946]
(a)
arent
bond
cup
(b)
+
-
lens
Fig. 23.7 a Standard LED casing (schematic drawing and macrophoto), b high-power mounting (schematic drawing
and image of Luxeon LED)
Thick-Window Chip Geometry
An increase in light extraction efficiency to about 10–12% can be achieved if the top layer is fabricated
with a much larger thickness (Fig. 23.8b) of 50–70 µm instead of a few µm. However, such approach
is not scalable since larger device area would demand even larger thickness.