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GaP Biased Detector Chapter 4: Operation
Rev A, December 27, 2017 Page 5
Photoconductive
In photoconductive mode, an external reverse bias is applied, which is the basis
for our DET series detectors. The current measured through the circuit indicates
illumination of the device; the measured output current is linearly proportional to
the input optical power. Applying a reverse bias increases the width of the
depletion junction producing an increased responsivity with a decrease in
junction capacitance and produces a very linear response. Operating under
these conditions does tend to produce a larger dark current but this can be
limited based upon the photodiode material.
Note
The DET detectors are reverse biased and cannot be operated under
forward bias conditions.
Photovoltaic
In photovoltaic mode the photodiode is zero biased. The flow of current out of the
device is restricted and a voltage builds up. This mode of operation exploits the
photovoltaic effect which is the basis for solar cells.
4.4. Dark Current
Dark current is leakage current which flows when a bias voltage is applied to a
photodiode. Photoconductive mode tends to generate a dark current that varies
directly with temperature. It can be inferred that dark current can approximately
double for every 10 °C increase in temperature, and shunt resistance can
decrease to approximately half for every 6 °C rise. Of course, applying a higher
bias will decrease the junction capacitance but will increase the amount of dark
current present.
The dark current present is also affected by the photodiode material and the size
of the active area. Silicon devices generally produce low dark current compared
to germanium devices which have high dark currents. The table below lists
several photodiode materials and their relative dark currents, speeds, sensitivity,
and costs. Please note that sensitivity values in the table are typical values;
Thorlabs offers photodetectors with sensitivity ranges that vary from those shown
below.