Photon-type detector Principle: made of photons using the external

photoelectric effect The photon-type detector made of photons using the external photoelectric effect is a vacuum electronic device, such as the photomultiplier tube, the infrared imaging tube, and the photomultiplier tube. These devices all have a photoelectric cathode that is sensitive to photons. When photons are projected onto the photoelectric cathode, the photons may be absorbed by the electrons in the photoelectric cathode, and the electrons with enough energy can escape the photoelectric cathode to become free photoelectrons. In the photomultiplier tube, the photoelectrons move under the action of the positively charged anode to form a photoelectric current. The photomultiplier tube differs from the photomultiplier tube in that an intermediate electrode with an electric potential that gradually increases and can generate secondary electrons (called the dynode) is set between the photoelectric cathode and the anode in the photomultiplier tube. The photoelectrons emitted from the photoelectric cathode are accelerated by the dynode voltage and collide with the dynode, causing the multiplication effect, and finally forming a larger photoelectric current signal. Therefore, the photomultiplier tube has much higher sensitivity than the photomultiplier tube. The infrared imaging tube is an infrared-visible image converter that consists of a photoelectric cathode, an anode, and a simple electronic optical system. The photoelectrons are accelerated by the anode while being focused by the electronic optical system at the same time. When they collide with the phosphor screen connected to the anode, they emit green light image signals.


Photonic detectors made using the internal photoelectric effect are solid-state electronic devices made of semiconductor materials, mainly including photoconductive detectors and photovoltaic detectors. 

Photovoltaic detectors are usually composed of semiconductor PN structures, which use the built-in electric field of the PN junction to sweep photo generated carriers out of the junction region and form a signal. When the detector is exposed to light (irradiation) and undergoes intrinsic light absorption within the body, two types of photo generated charge carriers (electrons and holes) with opposite charges are generated. These two types of photo generated charge carriers are initially limited to the illuminated region, and then due to the concentration gradient, some of them diffuse to the PN junction region. Under the action of the built-in electric field in the PN junction, they gather at both ends of the junction, forming a voltage signal. If the two ends of the PN junction are connected into a loop, a current signal is formed. 



Photonic detectors are detection devices that selectively respond to wavelengths. The detector only responds when the energy of the incident photon is greater than the electron activation energy E in the photosensitive material. For external photoelectric effect devices, such as phototubes and photomultiplier tubes, E is equal to the work required for an electron to escape from the photocathode, which is generally slightly greater than 1 electron volt. Therefore, such detectors can only be used to detect near-infrared radiation or visible light. For photovoltaic detectors and intrinsic photoconductive detectors, E is equal to the bandgap width of the semiconductor; For non intrinsic photoconductive detectors, E is equal to the ionization energy of impurities. Due to the large selection range of parameters such as bandgap width and impurity ionization energy, the response wavelength of semiconductor photonic detectors can be adjusted over a wide range. For example, a photoconductive detector made of intrinsic germanium is sensitive to near-infrared radiation; The photoconductive detector made of germanium doped with impurities can be sensitive to both mid infrared radiation (such as germanium doped mercury detectors) and far infrared radiation (such as germanium doped gallium detectors). 

The performance of semiconductor photon type detectors largely depends on the semiconductor materials used to prepare the detectors. Intrinsic semiconductor materials are more useful than doped semiconductor materials. Intrinsic semiconductor materials can be used to make both photoconductive and photovoltaic detectors; Doped semiconductors can only be made into photoconductive detectors. Most semiconductor photon type detectors with longer cutoff wavelengths must operate at lower temperatures, such as 77K, 38K, or 4.2K. The detection rate of the same detector at room temperature is significantly lower than that at low temperature. In order to maintain the normal operation of semiconductor photon type detectors, the detector is often placed in a low-temperature container (Dewar bottle) or a miniature cooler is used to reach a lower operating temperature. 

The ternary alloy semiconductor mercury cadmium telluride (Hg1-xCdxTe) is a detector material with important application value, and its bandgap width varies with composition. By adjusting the x value, the peak response wavelength of the detector can be selected at any wavelength between 1 and 30 microns, with the most important being the material with x=0.20 (corresponding peak response wavelength of the detector is between 8 and 13 microns). The tellurium cadmium mercury material also has the advantages of low dielectric constant, low coefficient of thermal expansion, and high electron mobility, making it suitable for producing high-performance, versatile, and novel photonic detectors. 



The sweep product type photon detector is a novel structure photon detector made of tellurium cadmium mercury material as the preferred material. This is a long strip three terminal device that contains signal delay integration function. It is usually used for delay integration of pixel signals in series scanning of thermal imagers. Its function is equivalent to a linear device with a delay circuit. The working principle is shown in the figure. The light received by the device is a light spot that scans from left to right. Apply bias voltage to both ends of the device. When a light spot irradiated at a certain position excites photo generated carriers, these carriers drift towards the signal readout region under the driving force of bias voltage. Choose the scanning speed of the light spot so that it is strictly equal to the migration speed of photo generated carriers. In this way, the photo generated carriers generated when the light spot sweeps from one end of the device to the other can synchronously accumulate together and enter the signal readout region. Similar to photoconductive detectors, it forms an electrical signal. The bias voltage of the sweep product detector should be large enough to prevent carriers from recombining or diffusing during the drift process. The device cannot be too long, usually only equivalent to the length of a linear device containing 10-20 units.