Page 69 - Photonics Essentials an introduction with experiments

Electrical Response Time of Diodes

Electrical Response Time of Diodes 63

Figure 4.1. A schematic diagram showing the motion of minority carriers created by
photoabsorption in a diode. Minority carriers will diffuse to the junction where they are
collected and transformed into majority carriers. L e is the diffusion length for electrons
and L h is the diffusion length for holes. In the low-injection limit, the concentration of
majority carriers is constant over distance. Hence, motion of majority carriers by diffu-
sion is negligible.

these new carriers by supplying the appropriate carriers at the con-
tacts in order to maintain charge neutrality.

4.3 Diffusion Time
The time required for minority carriers to diffuse to the junction de-
pends on the minority carrier diffusion length and the diffusion coeffi-
cient:
2
L e
e =
D e
and
2
L h
h = (4.1)
D h
for electrons and holes, respectively. The diffusion coefficient can be
calculated from the carrier mobility using the Einstein relation:

q
D = (4.2)
kT
From these equations, it is easy to see that the time to diffuse a giv-
en distance will be shorter for the minority carrier with the higher mo-

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