Briefly introduce the application and working principle of photodetectors

A photodetector is a device that can convert light radiation into electrical quantity. It can use this characteristic to perform display and control functions. Photodetectors can replace human eyes, and because of their wide spectral response range, photodetectors are also an extension of human eyes. Photodetectors use the physical characteristics of the conductivity of irradiated materials changing due to radiation. They are widely used and are mainly used in various fields of military and national economy. What are the applications of photodetectors? The application of photodetectors in the infrared band is mainly in infrared thermal imaging, missile manufacturing and infrared remote sensing; the application in visible light or near-infrared band is mainly in industrial automatic control, photometry and ray measurement and detection. So what are the applications of photodetectors? Next, the editor will introduce you to this knowledge in detail, and I believe it will help you better understand the relevant knowledge.

Photodetector, from its literal meaning, I believe everyone can guess that this detector can convert light signals into electrical signals. There are many types of photodetectors. According to the different working principles of the device or the different ways in which the device responds to radiation, photodetectors are generally divided into two categories: one is a thermal detector and the other is a photon detector.

How Photodetectors Work

The working principle of photoelectric detectors is based on the photoelectric effect, while thermal detectors are based on the fact that the temperature of a material rises after it absorbs light radiation energy, thereby changing its electrical properties. The biggest feature that distinguishes it from a photon detector is that it is not selective to the wavelength of light radiation.

Photoconductive device: A photoelectric detector made of semiconductor materials with photoconductive effect is called a photoconductive device, usually called a photoresistor. Photoresistors are applicable in the visible light band and several windows through which the atmosphere passes, namely the near infrared, mid-infrared and far infrared bands. Photoresistors are widely used in photoelectric automatic detection systems, photoelectric tracking systems, missile guidance, infrared spectroscopy systems, etc.

Photoelectron emission device: Phototube and photomultiplier tube are typical photoelectron emission type (external photoelectric effect) detection devices. Its main features are high sensitivity, good stability, fast response speed and low noise. It is a current amplifier device. In particular, the photomultiplier tube has a very high current gain, which is particularly suitable for detecting weak light signals; but it has a complex structure, high operating voltage and large size. Photomultiplier tubes are generally used to measure weak radiation and have high response speed requirements, such as laser rangefinders and optical radars of artificial satellites.

Cadmium sulfide (CdS) and cadmium selenide (CdSe) photoresistors are the two most commonly used photoresistors in the visible light band; lead sulfide (PbS) photoresistors are the main photoresistors working in the first infrared transmission window of the atmosphere. The response wavelength range of PbS photoresistors working at room temperature is 1.0 to 3.5 microns, and the peak response wavelength is about 2.4 microns; indium antimonide (InSb) photoresistors are mainly used to detect the second infrared transmission window of the atmosphere, and their response wavelength is 3 to 5 μm; the spectral response of mercury cadmium telluride devices is 8 to 14 microns, and its peak wavelength is 10.6 microns, which matches the laser wavelength of CO2 lasers and is used to detect the third window of the atmosphere (8 to 14 microns)

Applications of Photodetectors

Photoconductive detector is a light detection device made by using the photoconductive effect of semiconductor materials. The so-called photoconductive effect refers to a physical phenomenon in which the conductivity of the irradiated material changes due to radiation. Photoconductive detectors are widely used in various fields of military and national economy. In the visible light or near-infrared band, it is mainly used for ray measurement and detection, industrial automatic control, photometry, etc.; in the infrared band, it is mainly used for missile guidance, infrared thermal imaging, infrared remote sensing, etc. Another application of photoconductors is to use them as camera tube targets. In order to avoid image blur caused by the diffusion of photogenerated carriers, continuous thin film target surfaces are made of high-resistance polycrystalline materials, such as PbS-PbO, Sb2S3, etc. Other materials can be made by inlaying the target surface, and the entire target surface is composed of about 100,000 individual detectors.

In 1873, British scientist W. Smith discovered the photoconductivity effect of selenium, but this effect remained in the exploratory research stage for a long time and was not put into practical application. After World War II, with the development of semiconductors, various new photoconductive materials continued to emerge. In the visible light band, by the mid-1950s, cadmium sulfide and cadmium selenide photoresistors with good performance and lead sulfide photodetectors in the infrared band had been put into use.

In the early 1960s, Ge and Si doped photoconductive detectors with high sensitivity in the mid- and far-infrared bands were successfully developed. Typical examples are Ge:Au (germanium doped with gold) and Ge:Hg photoconductive detectors working in the 3-5 micron and 8-14 micron bands. Since the late 1960s, research on ternary materials with variable bandgap widths such as HgCdTe and PbSnTe has made progress. Working Principle and Characteristics The photoconductive effect is a type of internal photoelectric effect.

When the irradiated photon energy hv is equal to or greater than the bandgap width Eg of the semiconductor, the photon can excite the electrons in the valence band to the conduction band, thereby generating conductive electron-hole pairs, which is the intrinsic photoconductivity effect. Here h is Planck's constant, v is the photon frequency, and Eg is the bandgap width of the material (in electron volts). Therefore, the long-wave limit λc of the response of the intrinsic photoconductor is λc=hc/Eg=1.24/Eg(μm), where c is the speed of light. The long-wave limit of the intrinsic photoconductive material is limited by the bandgap width.

Before the early 1960s, suitable semiconductor materials with narrow bandgap width had not been developed, so people used the non-intrinsic photoconductivity effect. There are impurity energy levels of various depths in the bandgap of materials such as Ge and Si. As long as the irradiated photon energy is equal to or greater than the ionization energy of the impurity energy level, photogenerated free electrons or free holes can be generated. The long-wave limit λ of the response of the non-intrinsic photoconductor is obtained by the following formula λc=1.24/Ei, where Ei represents the ionization energy of the impurity energy level.

By the mid-to-late 1960s, ternary semiconductor materials such as Hg1-xCdxTe, PbxSn1-xTe, and PbxSn1-xSe were successfully developed and entered the practical stage. Their bandgap width changes with the value of the component x. For example, HG0.8Cd0.2Te material with x=0.2 can be made into an infrared detector with a response wavelength of 8 to 14 microns in the atmospheric window. Compared with Ge:Hg detectors working in the same band, it has the following advantages:

The working temperature is high (above 77K), easy to use, while the working temperature of Ge:Hg is 38K; the intrinsic absorption coefficient is large, the sample size is small; it is easy to manufacture multi-element devices. Table 1 and Table 2 list the Eg, Ei and λc values ​​of some semiconductor materials respectively.

Generally, any semiconductor material with a suitable bandgap width or impurity ionization energy has a photoelectric effect. However, the manufacture of practical devices also needs to consider factors such as performance, process, and price. Commonly used photoconductive detector materials in the X-ray and visible light bands include: CdS, CdSe, CdTe, Si, Ge, etc.; in the near-infrared band, there are: PbS, PbSe, InSb, Hg0.75Cd0.25Te, etc.; in the band longer than 8 microns, there are: Hg1-xCdxTe, PbxSn1-x, Te, Si doping, Ge doping, etc.; CdS, CdSe, PbS and other materials can be made into photoconductive detectors in the form of polycrystalline thin films.

The response bands of CdS, CdSe, and CdTe photoconductive detectors in the visible light band are all in the visible light or near-infrared region, and are usually called photoresistors. They have a wide bandgap (much greater than 1 electron volt) and can work at room temperature, so the device structure is relatively simple, generally using a semi-sealed bakelite shell, with a light-transmitting window in the front and two pins at the back as electrodes. Photoconductive detectors used in high temperature and high humidity environments can use a metal fully sealed structure, with the glass window and the Kovar metal shell sealed.

The device sensitivity is expressed by the size of the photocurrent generated per lumen of irradiation under a certain bias voltage. For example, a CdS photoresistor, when the bias voltage is 70 volts, has a dark current of 10-6 to 10-8 amps and a light sensitivity of 3 to 10 amps per lumen. The sensitivity of CdSe photoresistors is generally higher than that of CdS.

Another important parameter of photoresistors is the time constant τ, which indicates the speed at which the device reacts to light. After the light is suddenly removed, the time required for the photocurrent to drop to 1/e of the maximum value (about 37%) is the time constant τ. There are also those who calculate τ based on the photocurrent dropping to 10% of the maximum value; the time constants of various photoresistors vary greatly. The time constant of CdS is relatively large (milliseconds).

The common response bands of infrared photoconductive detectors PbS and Hg1-xCdxTe are in three atmospheric transmission windows: 1-3 microns, 3-5 microns, and 8-14 microns. Because their bandgap width is very narrow, thermal excitation is sufficient to make a large number of free carriers in the conduction band at room temperature, which greatly reduces the sensitivity to radiation.

The longer the wavelength of the response, the more obvious this situation is for electrical conductors. Detectors in the 1-3 micron band can work at room temperature (with a slight decrease in sensitivity). Detectors in the 3-5 micron band are divided into three situations:

It works at room temperature, but its sensitivity is greatly reduced, and the detection rate is generally only 1 to 7×108 cm·W-1·Hz; it works at thermoelectric cooling temperature (about -60℃), and the detection rate is about 109 cm·W-1·Hz; it works at 77K or lower, and the detection rate can reach more than 1010 cm·W-1·Hz. Detectors in the 8-14 micron band must work at low temperatures, so the photoconductor must be kept in a vacuum dewar flask, and the cooling methods include perfusion of liquid nitrogen and use of a micro refrigerator.

The time constant of infrared detectors is much smaller than that of photoresistors. The time constant of PbS detectors is generally 50 to 500 microseconds, and the time constant of HgCdTe detectors is in the order of 10-6 to 10-8 seconds. Infrared detectors sometimes need to detect very weak radiation signals, such as 10-14 watts; the output electrical signal is also very small, so a special preamplifier is required.

What are the applications of photoelectric detectors? The above is some introduction made by the editor for you in this regard. Through the introduction of the above article, I believe you have a deeper understanding of photoelectric detectors. In addition, there are some requirements for the application selection of photoelectric detectors. In those applications where the requirements are not very strict, generally any detector can be used. However, in some specific cases, it is necessary to carefully select photoelectric detectors.