Chapter 4: Operation
13124-S01
Page 6
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 a forward bias.)
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. The amount of dark current is
kept at a minimum when operating in photovoltaic mode.
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 higher 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 double 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.
