Avalanche detectors and their structure, Materials, Working principle and Characteristics
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Avalanche photodiode works on internal gain mechanism so that the photoelectric current is amplified within the detector. It is very useful when low levels of light are to be detected. The structure of Avalanche photodiode is shown in the figure below and is known as p+ π p n+ reach-through structure. The π region is the intrinsic region.
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Carrier Multiplication
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Avalanche photodiode
The device is a reverse biased p-n junction. The p+ and n+ are heavily doped semiconductors and have a very low resistance. The π region is very lightly doped and is nearly intrinsic in nature. Under reverse biased condition a depletion region is present between p and n+ region and it covers most of the π region. Under sufficient reverse bias, the junction approaches breakdown condition. A electric field is present in the junction region due to immobile charge carriers.
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A photon that enters through the p+ region is absorbed in the intrinsic region and the resulting electron-hole pair that is generated is separated by the electric field in the π region. The hole drifts towards the p+ region and do not take part in multiplication process. The electrons drift through the π region to the p n+ junction. The electric field present in the p n+ region accelerates the electrons which in turn ionize neutral atoms in its path. The effect is cumulative and builds up into an avalanche. Thus there is carrier multiplication and internal amplification. This amplification process enhances the responsivity of the detector.