Page 79 - Photonics Essentials an introduction with experiments

Electrical Response Time of Diodes

Electrical Response Time of Diodes 73

Table 4.1 Capacitance–voltage data for an InP p-n junction diode
Reverse bias (V) Capacitance (pF) Forward bias (V) Capacitance (pF)
0.00 48.64 +.8 73.84
–0.05 47.20 +.75 72.97
–0.1 45.78 +.7 71.88
–0.15 44.46 +.65 70.48
–0.2 43.21 +.6 68.90
–0.25 42.07 +.55 67.01
–0.3 41.03 +.5 64.96
–0.35 40.06 +.45 62.80
–0.4 39.15 +.4 60.71
–0.45 38.32 +.35 58.65
–0.5 37.58 +.3 56.87
–0.55 36.87 +.25 55.29
–0.6 36.18 +.2 53.89
–0.65 35.53 +.15 52.72
–0.7 34.93 +.1 51.54
–0.75 34.38 +0.05 50.25
–0.8 33.83 0.00 48.83
–0.85 33.37
–0.9 32.88
–0.95 32.43
–1.00 32.00

4.9 Summary

The speed of response of semiconductor devices such as photodiodes
or LEDs determines their usefulness in communications applications.
LEDs are now being considered for application in display screens or
as light bulbs for illumination. In these applications, the response
time of an LED is so short compared to other characteristic times
(such as the response time of the brain) that it is not a limitation on
system performance. On the other hand, the bandwidth of an optical
communication channel is several gigahertz. This is well beyond the
capability of currently known LEDs.
The response time of almost all photodiodes is determined by the
resistance–capacitance product. The dominant resistance is that of
the resistance of the following amplifier, which is usually tens to hun-
dreds of ohms depending on the bandwidth of the detection electron-
ics. Thus, the relevant resistance is external to the photodiode. The
capacitance of a photodiode that is in reverse bias is much smaller
than the capacitance of the same diode in forward bias. In this sense,
photodiodes are intrinsically “faster” than light-emitting diodes.
There is much that can be learned about a diode from its capaci-
tance–voltage characteristic: built-in voltage, doping concentration,
and, of course, its capacitance. The detection efficiency of a photodiode

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