Page 141 - Photonics Essentials an introduction with experiments

Light-Emitting Diodes

Light-Emitting Diodes 135

Using the following parameters:
B = 8 × 10 –11 cm /sec –1
3
n D = 5 × 10 18 cm –3
n–r = 2 × 10 –9 sec

1
8
= 9 × 10 sec –1
ac
ac bandwidth = 286 MHz
The ac modulation rate,
1 1
= Bn D +
ac n–r
is independent of the current.
A practical model for the frequency response of an electronic or an
optoelectronic device can be given by the following relationship:
R(0) R(0)
R(f) = = (6.40)

1 + 1 + 4 f
2 2
2 2 2
where R(f) is the output response of the LED.
Using this model, the bandwidth is determined at the frequency
where
R(f) 1
=
R(0) 2
This model gives a slightly different value for the bandwidth than
the standard expression that we have used in this text:
1
Bandwidth =
è
To summarize these important results:

Heavy-injection modulation rate:
1 BJ 1/2
=
ac 3qd
Light-injection modulation rate:
1 1
= Bn D +
ac n–r

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