InGaAs Biased Detector
Chapter 4: Operation
Rev G, April 17, 2017
Page 7
4.7. Terminating Resistance
We use a load resistance convert the generated photocurrent into a voltage
(V
OUT
) for viewing on an oscilloscope:
Depending on the type of the photodiode, load resistance can affect the
response speed. For maximum bandwidth, we recommend using a 50
Ω
coaxial
cable with a 50
Ω
terminating resistor at the opposite end of the cable. This will
minimize ringing by matching the cable with its characteristic impedance. If
bandwidth is not important, you may increase the amount of voltage for a given
light level by increasing R
LOAD
. In an unmatched termination, the length of the
coaxial cable can have a profound impact on the response, thus is
recommendable to keep the cable as short as possible.
4.8. Shunt Resistance
Shunt resistance represents the resistance of the zero-biased photodiode
junction. An ideal photodiode will have an infinite shunt resistance, but actual
values may range from the order of ten
Ω
to thousands of M
Ω
and is dependent
on the photodiode material. For example, and InGaAs detector has a shunt
resistance in the order of 10 M
Ω
while a Ge detector is in the k
Ω
range. This can
significantly affect the noise current on the photodiode. For most applications,
however, the high resistance produces little effect and most of the time ignored.
4.9. Series Resistance
Series resistance models the resistance of the semiconductor material, and we
ignore this low resistance. The series resistance arises from the contacts and the
wire bonds of the photodiode; this mainly determines the linearity of the
photodiode under zero bias conditions.
4.10. Damage Threshold
Exposure to an intense light source can easily damage a photodiode. One of the
main characteristics of a damaged photodiode is the presence of increased dark
current, along with burn spots on the detector active area. The damage threshold
may vary from photodiode to photodiode, as this is generally dependent on
material. Silicon devices tend to be more durable than InGaAs and can handle
higher energy levels.
The formula below calculates the energy of each pulse, using the average power
and the repetition rate. If the pulse width is given, the peak power can also be
determined.
∗