DET08CFC(/M)
Page 7
Rev B, April 29, 2015
The table below compares five common types of detector materials.
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 it can have
a profound impact on the photodiode’s bandwidth and response. 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 (C
J
) and the
load resistance (R
LOAD
):
=
1
(2
×
=
0.35
a
Approximate values, actual wavelength values will vary from unit to unit.