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When light is absorbed by a material such as a semiconductor, the number of free electrons and electron holes increases and raises its electrical conductivity. To cause excitation, the light that strikes the semiconductor must have enough energy to raise electrons across the band gap, or to excite the impurities within the band gap. When a biasvoltage and a load resistor are used in series with the semiconductor, a voltage drop across the load resistors can be measured when the change in electrical conductivity of the material varies the current through the circuit.
Classic examples of photoconductive materials include :
When a photoconductive material is connected as part of a circuit, it functions as a resistor whose resistance depends on the light intensity. In this context, the material is called a photoresistor (also called light-dependent resistor or photoconductor). The most common application of photoresistors is as photodetectors, i.e. devices that measure light intensity. Photoresistors are not the only type of photodetector--other types include charge-coupled devices (CCDs), photodiodes and phototransistors--but they are among the most common. Some photodetector applications in which photoresistors are often used include camera light meters, street lights, clock radios, infrared detectors, nanophotonic systems and low-dimensional photo-sensors devices.
In 2016 it was demonstrated that in some photoconductive material a magnetic order can exist. One prominent example is CH3NH3(Mn:Pb)I3. In this material a light induced magnetization melting was also demonstrated thus could be used in magneto optical devices and data storage.
The characterization technique called photoconductivity spectroscopy (also known as photocurrent spectroscopy) is widely used in studying optoelectronic properties of semiconductors.
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