Photodetectors are characterized by certain key parameters. Among them are spectral response, photosensitivity, quantum efficiency, dark current, forward-biased
noise, noise equivalent power, terminal capacitance, timing response (rise time and fall time), frequency bandwidth, and cut off frequency.
- Spectral response relates the amount of current produced with wavelength, assuming that all wavelengths are at the same level of light.
- Photosensitivity is the ratio of light energy (in watts) incident on the device to the resulting current (in amperes).
- Quantum efficiency is the number of generated electron-hole pairs (Le., current) divided by the number of photons.
Dark current is the amount of current that flows through the photodiode in the absence of any light (dark), when the diode is reverse-biased. This is a source
of noise when the diode is reverse-biased.
- Forward-biased noise is a (current) source of noise that is related to the shunt resistance of the device. The shunt resistance is defined as the ratio voltage (near oV) to the amount of current generated. This is also called shunt resistance noise.
- Noise equivalent power is defined as the amount of light (of a given wavelength) that is equivalent to the noise level of the device.
- Terminal capacitance is the capacitance from the p-n junction of the diode to the connectors of the device; it limits the response of the photodetector.
Timing response of the photodetector is the time for the output signal to climb from 10% to 90% of its amplitude (rise time) and to drop from 90% to 10%
- Frequency bandwidth is the frequency (or wavelength) range in which the photodetector is sensitive.
- Cutoff frequency is the highest frequency (wavelength) at which the photodetector is sensitive.
Concept Of Photodetector:
Photodetectors are devices, in which light form of photon is used to produce electrons for conduction, by having the absorbed photon liberate a bound electron.
The schematic diagram is shown in figure:
Applications of Photo Detectors:
- Photo detectors are used in various different applications such as radiation detection,smoke detection, flame detection and to switch on relays for street lighting.
- The circuits that use photodiodes use either normally closed or normally open contacts depending on the desired operation.
- In a smoke detector circuit the photo diode is attached to a relay switch, this switch is normally closed and attached to the fire alarm.
- When the photo diode conducts it picks up the relay switch, this causes the normally closed switch to open preventing the alarm from activating.
- When the photo diode fails to conduct, the normally closed contact activates the alarm.
- Photo diodes are also used in modern oil burning furnaces as a safety feature.The photo diode is comprised of lead sulphide and is used to detect the flame from the boiler, in the event that the flame goes out or fails to occur the photo diode opens the circuit, cutting power to the motor and step up transformer.
- Another commonly used application is street lights. The photo diode in the circuit uses switch-on relays to turn on the street lights when the diode fails to conduct and turns the lights off with when sufficient light is present.