DET08CFC(/M)
Chapter 4: Operation
TTN020103-D02 Page
6
4.3.1. Photoconductive
In photoconductive mode, a reverse external 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 and a decrease in junction
capacitance: a linear response. Operating under these conditions tends to
produce a larger dark current, but this can be limited by selecting an appropriate
photodiode material. (
Note: This detector is reverse biased and cannot be
operated under a forward bias.
)
4.3.2. Photovoltaic
In photovoltaic mode, the photodiode is zero biased. The flow of current out of
the device is restricted causing a build up of voltage. This mode of operation
exploits the photovoltaic effect, which is the basis for solar cells. When operating
in photovoltaic mode, the amount of dark current is at a minimum setting.
4.4. Dark
Current
When a bias voltage is applied to a photodiode, a leakage current, called dark
current, is produced. Photoconductive mode tends to generate a higher dark
current that varies directly with temperature. Dark current approximately doubles
every 10 °C increase in temperature, and shunt resistance doubles every 6 °C
rise. Applying a higher bias will decrease the junction capacitance but will also
increase the amount of dark current present.
The photodiode material, and the size of the active area, also affect the amount
of dark current present. Silicon devices generally produce low dark current
compared to germanium devices, which have high dark currents. The table on
the next page lists several photodiode materials and their relative dark currents,
speeds, sensitivities, and costs.
