Page 130 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

124 Photonic Devices

Figure 6.13. A schematic diagram of LED modulation. The drive current determines
the operating point of the diode. The modulation current is smaller. These currents
generate excess electron–hole pairs. Some pairs recombine radiatively, while the rest
do not.

um component created by the LED drive current. We can assume that
the optical recombination is occurring in a n-type region with doping
n D . Then we can write the basic expressions for the total carrier con-
centration N and P as
N = n D + N
P = p 0 + P (6.14)
2
where n D p 0 = n i and N = P always.
First we will develop the expressions for the LED output optical
power in terms of the drive current. There are some important basic
ideas to keep in mind.

The input current of the LED creates a nonequilibrium excess den-
sity of electrons and holes ( N = P).
Some of these electrons and holes recombine by emitting photons
( int ).
Some of these photons actually escape the LED structure and are
emitted into free space, constituting the measurable output of the
LED ( ext ).
The output power of the LED in watts is proportional to the num-
ber of photons emitted. The number of photons emitted is propor-
tional to the input current. Therefore the output power of the LED
is proportional to the input current. This is a direct result of the
quantum nature of electrons and photons.

The rate equation for the change in the carrier concentration is
written as rate of change in carrier concentration = electrical pump-

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