How does a Light Dependent Resistor (LDR) vary its resistance with light intensity?
1 Answer
A Light Dependent Resistor (LDR), also known as a photoresistor, is a type of semiconductor device that exhibits a change in resistance with varying light intensity. Its resistance decreases with increasing light intensity and increases as the light intensity decreases.
The underlying principle behind this behavior lies in the semiconductor material used in the LDR. It typically consists of a light-sensitive semiconductor, such as cadmium sulfide (CdS) or lead sulfide (PbS). These materials are known as intrinsic semiconductors, and their electrical conductivity depends on the number of charge carriers (electrons and holes) present in the material.
When light strikes the surface of the LDR, photons of light are absorbed by the semiconductor material, and this absorption generates electron-hole pairs. In the case of cadmium sulfide, for example, the incident light energy excites electrons from the valence band to the conduction band, leaving behind holes in the valence band.
Now, here's how it affects the resistance:
High Light Intensity: When there is a high intensity of incident light, more electron-hole pairs are generated. The increased number of charge carriers results in higher electrical conductivity, and thus, the resistance of the LDR decreases. As a result, the LDR allows more current to pass through it when exposed to high light levels.
Low Light Intensity: Conversely, when the incident light intensity is low or absent, fewer electron-hole pairs are generated. As a result, the number of charge carriers decreases, leading to lower electrical conductivity. This higher resistance restricts the current flow through the LDR.
In summary, the resistance of a Light Dependent Resistor varies inversely with the intensity of incident light. As the light intensity increases, the resistance decreases, and as the light intensity decreases, the resistance increases. This characteristic makes LDRs useful in various applications, such as light sensors, automatic street lights, and exposure control in cameras.
The underlying principle behind this behavior lies in the semiconductor material used in the LDR. It typically consists of a light-sensitive semiconductor, such as cadmium sulfide (CdS) or lead sulfide (PbS). These materials are known as intrinsic semiconductors, and their electrical conductivity depends on the number of charge carriers (electrons and holes) present in the material.
When light strikes the surface of the LDR, photons of light are absorbed by the semiconductor material, and this absorption generates electron-hole pairs. In the case of cadmium sulfide, for example, the incident light energy excites electrons from the valence band to the conduction band, leaving behind holes in the valence band.
Now, here's how it affects the resistance:
High Light Intensity: When there is a high intensity of incident light, more electron-hole pairs are generated. The increased number of charge carriers results in higher electrical conductivity, and thus, the resistance of the LDR decreases. As a result, the LDR allows more current to pass through it when exposed to high light levels.
Low Light Intensity: Conversely, when the incident light intensity is low or absent, fewer electron-hole pairs are generated. As a result, the number of charge carriers decreases, leading to lower electrical conductivity. This higher resistance restricts the current flow through the LDR.
In summary, the resistance of a Light Dependent Resistor varies inversely with the intensity of incident light. As the light intensity increases, the resistance decreases, and as the light intensity decreases, the resistance increases. This characteristic makes LDRs useful in various applications, such as light sensors, automatic street lights, and exposure control in cameras.