Page 120 - Photonics Essentials an introduction with experiments
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Light-Emitting Diodes
114 Photonic Devices
PHOTOCURRENT PHOTODIODE
absorption edge
EMITTED INTENSITY
LIGHT-EMITTING DIODE
E g
PHOTON ENERGY
Figure 6.7. The emission spectrum of a real LED peaks below the band gap energy,
whereas the peak in the response of a photodetector using the same material occurs at
an energy above the band gap. There is some overlap in energy between the two de-
vices. Thus, a LED is a poor detector of the radiation emitted by another LED made
from the same material.
6.4 Quantum Efficiency
The quantum efficiency, Q of an LED is
number of photons emitted n
=
number of electrons injected n e
P opt /
Q = (6.10)
I/q
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