Si Biased Detector
Page 7 Rev F, December 8, 2014
The table below gives some advantages to each common type of detector
material.
Material
Dark
Current Speed
Sensitivity
1
(nm) Cost
Silicon (Si) Low High 400 – 1000 Low
Germanium (Ge) High Low 900 – 1600 Low
Gallium Phosphide (GaP) Low High 150 – 550 Med
Indium Gallium Arsenide
(InGaAs)
Low High 800 – 1800 Med
Extended Range: Indium
Gallium Arsenide (InGaAs)
High High 1200 – 2600 High
4.5. Junction Capacitance
Junction capacitance (C
J
) is an important property of a photodiode as this can
have a profound impact on the bandwidth and the response of a photodiode. It
should be noted that larger diode areas encompass a greater junction volume
with increased charge capacity. In a reverse bias application, the depletion width
of the junction is increased, thus effectively reducing the junction capacitance
and increasing the response speed.
4.6. Bandwidth and Response
A load resistor will react with the photodetector junction capacitance to limit the
bandwidth. For best frequency response, a 50 Ω terminator should be used in
conjunction with a 50 Ω coaxial cable. The bandwidth (f
BW
) and the rise time
response (t
r
) can be approximated using the junction capacitance and the load
resistance (R
LOAD
):
=
1
(2
×
)
=
0.35
1
Approximate values, actual wavelength values will vary from unit to unit.
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