Page 122 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

116 Photonic Devices

Figure 6.8. An interface between two materials having different indices of refraction.
Light traveling from the GaAs toward the air will be partially transmitted and partial-
ly reflected at the interface.

greater angle has no chance of transmission and is reflected back into
the semiconductor structure. If we assume that the diode is emitting
with equal intensity through 4 steradians, the fraction of the emis-
sion that can escape the diode is 4%. We can estimate the fraction of
the intensity that reaches the outside world as follows:
1 (6.13)
2
I outside = I inside · – · 0.75 · 0.04 = 0.02 · I inside
This estimate shows that the external efficiency of a semiconductor
LED is small, about 2%. Specific improvements to the geometry and
the surface of the LED can improve this figure to about 30% under op-
timum conditions. Some of these changes are discussed below. Howev-
er, before going into these modifications, it is worthwhile to recall that
the index of refraction of a typical polymer material is about 1.5. This
means that the escape cone will be much larger for light in a polymer
LED than for light in a semiconductor LED. The comparative advan-
tage of polymer LEDs is the subject of Problem 6.6.

6.5 Beating the Experts: New Thinking Creates a
Pathway to Increased Efficiency
The relatively low level of emission efficiency of LEDs that is summa-
rized in Eq. 6.13 is based on two important assumptions: a) the sur-

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