PHOTODETECTORS
· The devices which can be used to detect the presence of photons are called as photodetectors.
· They convert incident light into electrical signals.
· Due to the incident light, excess electron-hole pairs are generated. This increases the conductivity of the semiconductor .
· This is the basis of operation of a photodetector
· They are of 5 different types –
1. Photoconductor (LDR)
2. Photodiode
3. PIN Photo diode
4. Avalanche Photodiode
5. Phototransistor
Basic requirements for a photodetector are small size, very high sensitivity and fast response.
· The devices which can be used to detect the presence of photons are called as photodetectors.
· They convert incident light into electrical signals.
· Due to the incident light, excess electron-hole pairs are generated. This increases the conductivity of the semiconductor .
· This is the basis of operation of a photodetector
· They are of 5 different types –
1. Photoconductor (LDR)
2. Photodiode
3. PIN Photo diode
4. Avalanche Photodiode
5. Phototransistor
Basic requirements for a photodetector are small size, very high sensitivity and fast response.
PHOTOCONDUCTIVE CELL (LDR)
LDR is a semiconductor device in which the resistance depends on the intensity of light.Resistance is inversely proportional to the intensity of incident light.
Construction
· A light sensitive material such as Cadmium Sulphide (CdS) is deposited on a ceramic substrate.
· Both are enclosed in a metallic container.
· Contact terminals are brought out for an external connection.
· Light is incident on light sensitive material through the glass cap which is added to the top of the assembly.
Construction
· A light sensitive material such as Cadmium Sulphide (CdS) is deposited on a ceramic substrate.
· Both are enclosed in a metallic container.
· Contact terminals are brought out for an external connection.
· Light is incident on light sensitive material through the glass cap which is added to the top of the assembly.
Operation
When light is incident on a photosensitive material, the following events take place –
· The incident photons collide with the atoms of the light sensitive material and impart energy to them.
· Due to this energy, the valence electrons will cross the energy band gap and enter the conduction band.
· Thus, the conductivity of the material increases.
· Resistance is inversely proportional to the intensity of light.
· ‘Dark Resistance’ is the resistance offered by the LDR in the absence of light
· The value of resistance depends on the memory effect or the light history effect.
When light is incident on a photosensitive material, the following events take place –
· The incident photons collide with the atoms of the light sensitive material and impart energy to them.
· Due to this energy, the valence electrons will cross the energy band gap and enter the conduction band.
· Thus, the conductivity of the material increases.
· Resistance is inversely proportional to the intensity of light.
· ‘Dark Resistance’ is the resistance offered by the LDR in the absence of light
· The value of resistance depends on the memory effect or the light history effect.
Symbol
Spectral response
Applications of LDR
1. Object Counter
By using a light source and LDR detector, it is possible to count no. of objects passing on a conveyor belt.
2. Street Light On/Off System
LDR can be used in automatic On/Off switch circuit used for street lights. During the day time, the intensity of light falling on LDR is high and therefore low resistance is acquired. Voltage across it is not turn on the transistor. The lamp will remain unenergized and it will continue to remain in OFF state.
In the evening, light intensity reduces. Resistance also increases. Therefore, voltage will be enough to turn on the transistor. The relay will be energized and lamp will be in ON state.
Advantages
· Simple construction
· Easy operation
· Low cost
· Good sensitivity
· Linear response
Disadvantages
· Light history effect (resistance changes slowly)
· Slow response of the device
· Small range
· Resistance changes with temperature
· Poor stability
1. Object Counter
By using a light source and LDR detector, it is possible to count no. of objects passing on a conveyor belt.
2. Street Light On/Off System
LDR can be used in automatic On/Off switch circuit used for street lights. During the day time, the intensity of light falling on LDR is high and therefore low resistance is acquired. Voltage across it is not turn on the transistor. The lamp will remain unenergized and it will continue to remain in OFF state.
In the evening, light intensity reduces. Resistance also increases. Therefore, voltage will be enough to turn on the transistor. The relay will be energized and lamp will be in ON state.
Advantages
· Simple construction
· Easy operation
· Low cost
· Good sensitivity
· Linear response
Disadvantages
· Light history effect (resistance changes slowly)
· Slow response of the device
· Small range
· Resistance changes with temperature
· Poor stability