23.3 Light-Emitting Diodes
653
Fig. 23.15 Schematic
band structure of cascaded
LED. The grey area
denotes an additional
heterostructure for
reduction of the series
resistance of the tunnel
junctions. Adapted
from [1954]
E C
E V
E C
E V
F n
F p
F n
F p
V F
V F
2V F
efficiency of LEDs decreases with increasing current density (droop), such stacked LED design holds
promise for increased wall-plug efficiency if the series resistance of the tunneling junctions is small.
The quantum efficiency of such device is then larger than N times the efficiency of the single stack
LED and thus way larger than 100% (similar to quantum cascade lasers, Sect. 23.4.16).
Historic Development
In Fig. 23.16, the historic development of the LED luminous efficacy (luminous flux per electrical input
power) is shown for various material systems. While the luminosity has increased by a factor of 20 per
decade in the last 40 years, the price has decreased by a factor of ten per decade (Fig. 23.17). Currently,
there is a need for the development of more efficient LEDs in the green spectral range [1956] since
their luminosity is small compared to devices for the blue and red spectral regions (Fig. 23.18).
23.3.5 White LEDs
There are different possibilities to generate white light with an LED as shown schematically in
Fig. 23.19. The highest color gamut and a tunable white point can be achieved by combining a red,
a green and a blue LED (Fig. 23.19a). Using a blue LED and a yellow phosphor (Figs. 23.19b and
23.20a,b), a white spectrum can be achieved that is, however, not very close to a blackbody spectrum
(Fig. 23.20c). A better color rendering can be obtained with the combination of two phosphors [1958].
Fig. 23.16 Historic
development of the
luminous efficacy of
semiconductor LEDs and
OLEDs. Based on [1957]
with the addition of data on
OLEDs and recent data on
LEDs. The arrows on the
right indicate efficacy of
various other light sources
653
Fig. 23.15 Schematic
band structure of cascaded
LED. The grey area
denotes an additional
heterostructure for
reduction of the series
resistance of the tunnel
junctions. Adapted
from [1954]
E C
E V
E C
E V
F n
F p
F n
F p
V F
V F
2V F
efficiency of LEDs decreases with increasing current density (droop), such stacked LED design holds
promise for increased wall-plug efficiency if the series resistance of the tunneling junctions is small.
The quantum efficiency of such device is then larger than N times the efficiency of the single stack
LED and thus way larger than 100% (similar to quantum cascade lasers, Sect. 23.4.16).
Historic Development
In Fig. 23.16, the historic development of the LED luminous efficacy (luminous flux per electrical input
power) is shown for various material systems. While the luminosity has increased by a factor of 20 per
decade in the last 40 years, the price has decreased by a factor of ten per decade (Fig. 23.17). Currently,
there is a need for the development of more efficient LEDs in the green spectral range [1956] since
their luminosity is small compared to devices for the blue and red spectral regions (Fig. 23.18).
23.3.5 White LEDs
There are different possibilities to generate white light with an LED as shown schematically in
Fig. 23.19. The highest color gamut and a tunable white point can be achieved by combining a red,
a green and a blue LED (Fig. 23.19a). Using a blue LED and a yellow phosphor (Figs. 23.19b and
23.20a,b), a white spectrum can be achieved that is, however, not very close to a blackbody spectrum
(Fig. 23.20c). A better color rendering can be obtained with the combination of two phosphors [1958].
Fig. 23.16 Historic
development of the
luminous efficacy of
semiconductor LEDs and
OLEDs. Based on [1957]
with the addition of data on
OLEDs and recent data on
LEDs. The arrows on the
right indicate efficacy of
various other light sources