Photodetectors Physics
There are at least 10 types of optical detectors or in other words, optical sensors. They are built using two main different technologies (gas tubes and solid state devices) and all of them have different sensitivities and different peak detection wavelengths depending on their main use. Optical detectors can be used to detect infrared, visible light up to X or gamma rays.
Between the optical sensors that use gas tubes there are the photomultipliers and gaseous ionization detectors. In this category could enter also bolometers and pyroelectric detectors used to measure temperature by comparing the color of the heated object with a standard color whose temperature is known.
1) Photomultipliers use a light sensitive cathode that emits electrons when illuminated. Each photon emits one electron, but this electron is being consecutively accelerated between electrodes and creates other secondary electrons (it multiplies) by ionization of the gas inside the tube. These devices combine the photoelectric effect with the secondary multiplication of electrons effect. Thus the sensitivity of the photomultipliers is very good, of the order of 1-100 mA/W and their peak wavelength is somewhere in the visible spectrum (from red to blue depending on the material on which the photocathode is made). Between the solid state devices that are used as optical detectors the two most important types are the photodiodes and the photoresitors.
2) The photodiodes are made from a PN junction that is reversely polarized (when they are not polarized these devices are called solar cells). When a photon is captured in the PN junction region the reverse resistance of the device drastically decreases due to the fact that the photon generates an electron-hole pair. This pair is separated by the internal electric field of the junction and thus separately the electron and hole take place in the active electrical conduction. The sensitivity of photodiodes is usually lower than that of photomultipliers but there are devices with comparable sensitivity (about 100 mA/W). The peak wavelength depends on the type of semiconductors used to make the PN junction but is usually somewhere in infrared (about 1000 nm).
3) The photoresitors are the less sensitive devices from all. They are built from a bulk semiconductor on to which two metallic contacts are deposited. When illuminated the surface resistance of the semiconductor decreases because each photon (having enough energy) generates a pair of electron-hole. If the recombination rate is lower than the generation rate, some of these electro-hole pair will be separated by the external electric field and will lower the overall resistance of the device. The power of photoresistors is usually small (100-250 mW) and therefore the external applied voltage cannot be too big. This fact and the presence also of the recombination give the lowest sensitivity of all photodetectors. Under illumination the variation or resistance is between 10-50% but the sensitivity depends on the maximum possible applied voltage (a few volts) and current (a few mA). Because of this they are used only together with amplifying transistors.
