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Photoresistors, also commonly referred to as light-dependent resistors (LDRs), are a class of variables resistors which change their resistance depending on how much light is hitting the resistor. These types of resistor are made of materials which exhibit photoconductivity and are often used in light-sensitive detector circuits, light-activated switching circuits, and dark-activated switching circuits. In this article, we look at the main classes of photoresistor, namely, intrinsic and extrinsic photoresistors.
When it comes to photoresistors, there is not a great deal of difference between the different types, and most photoresistors would by using the same principles. The main difference between intrinsic and extrinsic photoresistors (the two main types) is in the types of materials which the photoresistors are made of—which can sometimes affect the mechanism of resistance change by slight amounts.
Generally speaking, photoresistors work when light is shone onto the photoresistor. When the photons of light hit the photoconductive materials in the photoresistor, the valence band electrons from this material breaks free, leading to a large number of free electrons in the material (and an equal number of positively charged holes where the electrons used to be). The number of free electrons is also proportional to the amount of light being shone on the photoresistor, i.e. the greater the light intensity, the greater the number of free electrons and holes. It should be noted that most photoresistors only work with certain wavelengths of light, and outside of these ranges, the resistance of the device does not change at all.
The separation and free movement of the positively charged holes and the negatively charged electrons enables a current to be carried across the resistor. This is because both electrons and hole can carry current, and both are known as ‘charge carriers’. The amount of current through the resistor depends on the number of free carriers, which is in turn dependent upon the intensity of the light. So, in short, the amount of light on the resistor corresponds to how much light is shone on the device—with a higher light intensity corresponding to a low resistivity, and a low light intensity corresponding to high resistance. This means that when there is no light, the resistors will function as highly resistive, but lowly resistive when it is light. These basic working principles are true for both intrinsic and extrinsic photoresistors, with the main difference between the two being the amount of energy needed to release the charge carriers.
In intrinsic photoresistors, the active material is made of pure semiconductor materials, such as silicon or germanium. These materials have no free electrons in their natural state due to the way in which they are bonded at the molecular level. Because the electrons are held in strong covalent bonds, only a small number of electrons are released when light is shone on the photoresistor. This is because it takes a lot of energy to remove the electrons from the valence band in these photoresistors, so only a few electrons have enough energy to break free. Because these types of photoresistors only release a small amount of charge carriers, they are not sensitive enough for most applications and they are only useable within narrow wavelength ranges.
Extrinsic photoresistors are the more common choice and are made from extrinsic-type semiconductor materials, i.e. semiconductor materials which have been doped with other atoms with a higher number of valence electrons—commonly phosphorus. These semiconductor materials are known as n-type semiconductors, and the presence of higher valence atoms in the lattice leads to more free electrons in the lattice. This is because the extra valence electrons can’t physically form bonds in the (commonly) 4-coordinated silicon (or similar) lattice, so these extra electrons become delocalized within the lattice, leading to a greater concentration of free charge carriers in the lattice.
These free electrons in the lattice then collide with the other atoms in the lattice under photoirradiation, meaning that multiple electrons can be released much easier than in intrinsic photoresistors. The presence of the impurities also creates an extra energy band above the valence band, meaning that a lower amount of energy is required to release the electrons from the atoms, so when the free carriers in the lattice collide with the atoms, it creates an avalanche effect where lots of charge carriers are released, which significantly increases the current. This leads to much higher sensitivities than intrinsic photoresistors and they are much more reliable in real-world applications—where extrinsic photoresistors are generally designed for longer wavelengths of light, such as in the infrared (IR) range, but the specific wavelengths are dependent upon the materials and the doping, where each type of device is tuned to be different.
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