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The Working Method And Perception Of Photodiode Detector

Photoconductive detector A photodetector made from the photoconductive effect of a semiconductor material. 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 or near-infrared band, it is mainly used for ray measurement and detection, industrial automatic control, photometric measurement, etc. In the infrared band, it is mainly used for missile guidance, infrared thermal imaging, infrared remote sensing and so on. Another application of the photoconductor is to use it as a camera tube target. In order to avoid the image blur caused by photocarrier diffusion, high-impedance polycrystalline materials such as PbS-PbO, Sb2S3, etc. are used for the continuous film target surface. Other materials can be embedded in the target surface, and the entire target surface consists of about 100,000 individual detectors.

In 1873, W. Smith of the United Kingdom discovered the photoconductive effect of selenium, but this effect has long been in the exploratory research stage and has not been applied in practice. After the Second World War, with the development of semiconductors, various new photoconductive materials continued to emerge. In the visible light range, by the mid-1950s, good performance cadmium sulfide, cadmium selenide photoresistor, and infrared-wave lead sulfide photodetector have been put into use. In the early 1960s, sensitive Ge- and Si-doped photoconductive detectors in the middle- and far- infrared wavelength bands were successfully developed. Typical examples are Ge: Au doped with gold and Ge: Hg operating in the 3 to 5 micron and 8 to 14 micron bands. Photoconductive detectors. After the end of the 1960s, research on the ternary materials with variable bandgap, such as HgCdTe and PbSnTe, has progressed. Working principles and characteristics Photoconductive effect is a kind of internal photoelectric effect. When the irradiated photon energy hv is equal to or greater than the bandgap Eg of the semiconductor, the photon can excite the electrons in the valence band to the conduction band, thereby generating a conductive electron and hole pair. This is the intrinsic photoconductive effect. Where h is Planck's constant, v is the photon frequency, and Eg is the band gap of the material (in volts). Therefore, the response long wavelength limit λc of the intrinsic photoconductor is λc=hc/Eg=1.24/Eg (μm) where c is the speed of light. The long wavelength limit of intrinsic photoconductive materials is limited by the forbidden band width.

Until the early 1960s, a suitable narrow bandgap semiconductor material had not been developed, and therefore people used extrinsic photoconductive effects. Ge, Si, and other materials have various levels of impurity levels in the forbidden band, and as long as the irradiated photon energy is equal to or greater than the ionization energy of the impurity level, photogenerated free electrons or free holes can be generated. The response long wavelength limit λ of the extrinsic photoconductor is given by λc=1.24/Ei where Ei represents the ionization energy of the impurity level. By the late 1960s, the ternary semiconductor materials such as Hg1-xCdxTe, PbxSn1-xTe, and PbxSn1-xSe were successfully developed and entered a practical stage. Their bandgap varies with the value of the component x. For example, the HG0.8Cd0.2Te material with x = 0.2 can be made into an infrared detector that responds to an atmospheric window with a wavelength of 8 to 14 microns. Compared with Ge:Hg detectors operating in the same band, it has the following advantages:

Operating temperature is high (above 77K), easy to use, and Ge:Hg operating temperature is 38K; intrinsic absorption coefficient, small sample size; easy to manufacture multiple components. Tables 1 and 2 respectively list the Eg, Ei, and λc values of some semiconductor materials.

In general, any semiconductor material suitable for bandgap or impurity ionization has a photoelectric effect. However, factors such as performance, process, price, etc. must be considered in the manufacture of practical devices. Commonly used photoconductive detector materials in the ray and visible light bands: CdS, CdSe, CdTe, Si, Ge, etc.; in the near infrared bands: PbS, InGaAs, PbSe, InSb, Hg0.75Cd0.25Te, etc.; at longer than 8 microns Wavebands include: Hg1-xCdxTe, PbxSn1-x, Te, Si-doped, and Ge-doped; CdS, CdSe, PbS, and other materials can be made into photoconductive detectors in the form of polycrystalline thin films. Visible light-wavelength photodetector detectors CdS, CdSe, CdTe response bands are in the visible or near-infrared region, commonly known as photoresistor. They have a wide bandgap (greater than 1 electron volt) and can operate at room temperature. Therefore, the device structure is relatively simple. Generally, a semi-enclosed bakelite shell is used, a light-transmitting window is added in front, and two leads are drawn at the back. As an electrode. The photoconductive detector used in high temperature and high humidity environment can adopt the metal fully sealed structure, and the glass window and the Kovar metal shell can be melt-sealed.

The sensitivity of the device is represented by the amount of photocurrent generated by each lumen irradiation at a given bias voltage. For example, a CdS photoresistor, when the bias is 70 volts, the dark current is 10-6 ~ 10-8 A, light sensitivity is 3 ~ 10A / lumen. CdSe photoresistors generally have higher sensitivity than CdS. Another important parameter of the photoresistor is the time constant τ, which represents the speed of the device response to light. After the sudden removal of the light, the time required for the photocurrent to drop to a maximum of 1/e (approximately 37%) is the time constant τ. The τ is also calculated as the photocurrent drops to a maximum of 10%; the time constants of the various photoresistors vary greatly. The time constant of CdS is relatively large (in the order of milliseconds). Infrared waveband photoconductive detectors PbS, Hg1-xCdxTe commonly used in the response of the wavelength band in the 1 ~ 3 microns, 3 ~ 5 microns, 8 ~ 14 microns three atmospheric window. Due to their narrow bandgap, thermal excitation at room temperature is sufficient to allow a large number of free carriers in the conduction band, which greatly reduces the sensitivity to radiation.

The longer the response wavelength of light, the more obvious the electrical conductor, where the 1 to 3 micron band detector can work at room temperature (a slight decrease in sensitivity). Three to five micrometer band detectors are divided into three situations:

Working at room temperature, but the sensitivity is greatly reduced, the detection is generally only 1 ~ 7 × 108 cm · watt -1 · He; thermoelectric cooling temperature (about -60 °C), detection of about 109 cm · watt -1 ·Helium; working at 77K or lower, detection up to 1010 cm·W -1 ·H or more. The 8- to 14-micron band detectors must operate at low temperatures, so the photoconductor must be kept in a vacuum dewar. The cooling method consists of injecting liquid nitrogen and using a miniature cryostat.

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

In terms of dynamic characteristics (ie, frequency response and time response), photomultiplier tubes and photodiodes (especially PIN tubes and avalanche tubes) are preferred; in terms of photoelectric characteristics (ie, linearity), photomultiplier tubes, photodiodes, and Photocells are the best; in terms of sensitivity, photomultipliers, avalanche photodiodes, photoresistors, and phototransistors are the best. It is worth pointing out that the high sensitivity does not necessarily mean that the output current is large, while the device with large output current has a large area of photocells, photoresistors, avalanche photodiodes, and phototransistors; the lowest applied bias voltage is the photodiode, phototransistor, and the photocell is not External bias is required; in the dark current, the photomultiplier tube and the photodiode are the smallest, and the dark current is not generated when the photocell is not biased, and the dark current is also greater than the photomultiplier tube and the photodiode after the reverse bias is applied; the long-term operation is stable. In terms of sexuality, photodiodes and photocells are the best, followed by photomultipliers and phototransistors; in terms of spectral response, photomultipliers and CdSe photoresistors are the widest, but photomultipliers respond to the direction of ultraviolet, and photoresistors Response to the direction of the infrared.

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