LDR: Why is conductance proportional to log(intensity)?

[Beam me down]

I am doing a experimental report on various solid state devices and have come unstuck on my work with LDRs. I have data relating conductance of a CdS photo-resistor to intensity of (white) light. The problem is that I am completely unsure as to why there is a log relationship, as has been suggested to me. I understand why there is an increase in conductivity (hence decrease in resistance), as there are more electrons in the conduction band.

Any hints you can provide would be greatly appreciated,.

Cheers,
Matthew

 

[BigJDubs]

The log relationship between the conductance of a CdS photo-resistor and intensity of light can be explained by the photoelectric effect. The photoelectric effect occurs when photons are absorbed by an electron in a material, which then causes the electron to become energized and move to a higher energy level, or conduction band. As the intensity of light increases, more photons are absorbed, thus more electrons become energized and move to the conduction band. Since the conduction band has a higher density of electrons, the overall conductivity of the material increases, resulting in the log relationship between conductance and light intensity.

 

[charng]

The relationship between conductance and intensity in LDRs can be explained by the physics of semiconductors. LDRs, or light dependent resistors, are made of a semiconductor material such as cadmium sulfide (CdS). In semiconductors, the number of free charge carriers (electrons or holes) in the conduction band is directly related to the intensity of light that falls on the material.

As the intensity of light increases, more photons are absorbed by the semiconductor material, causing more electrons to be excited from the valence band to the conduction band. This results in an increase in the number of free charge carriers, leading to higher conductance.

The reason for the logarithmic relationship between conductance and intensity is due to the bandgap energy of the semiconductor material. The bandgap is the energy difference between the valence band and the conduction band, and it determines the amount of energy required for an electron to move from the valence band to the conduction band.

In semiconductors like CdS, the bandgap is relatively small. This means that even a small increase in the intensity of light can excite a large number of electrons from the valence band to the conduction band, resulting in a large increase in conductance. However, as the intensity of light continues to increase, the number of electrons in the conduction band reaches a saturation point, where the bandgap energy becomes a limiting factor for further excitation. This leads to a logarithmic relationship between conductance and intensity, as the increase in conductance becomes less significant with each subsequent increase in light intensity.

I hope this explanation helps you understand the relationship between conductance and intensity in LDRs. Please let me know if you have any further questions or need clarification on any of the concepts. Keep up the good work with your experimental report!