InGaAs Biased Detector
Chapter 4: Operation
Page 6
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The table below gives some advantages to each common type of detector
material.
Material
Dark
Current
Speed
Sensitivity
a
(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
reaffirms that larger diode areas encompass a greater junction volume with
increased charge capacity. In a reverse bias application, the depletion width of
the junction increases, 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
a
Approximate values, actual wavelength values will vary from unit to unit.
