Page 74 - Photonics Essentials an introduction with experiments

Electrical Response Time of Diodes

68 Photonic Devices

In the laboratory, you can easily measure the capacitance versus
bias voltage. The capacitance will decrease with increasing reverse
bias. The capacitance decreases because the width of the depletion re-
gion increases with increasing reverse bias. This effect can easily be
seen in Fig. 4.3. However, there is a more interesting way to plot your
results of capacitance versus reverse bias voltage. To see what this is,
we will square both sides of Eq 4.6:
=
2
C
0 qN D

2(V Bi – V)
2
A
–1 0 qN D
2
C
V Bi – V = · (4.8)
2
A
2
In most cases, the impurity concentration is constant, so that a graph
2
of (C /A ) versus the bias voltage will be a straight line. From the
2 –1
slope of this line, N D can be directly determined. If the straight line is
extrapolated to the point where (C /A ) would equal 0, then the val-
2 –1
2
ue of the corresponding voltage is the built-in voltage of the p-n junc-

Figure 4.3. The capacitance–voltage characteristic for an InP diode. Note that the ca-
pacitance decreases with increasing reverse bias voltage. By tuning the capacitance
electrically, it is possible to change the response time of the photodiode.

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