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What is the physical explanation to hot pixels? What happens to the sensor when a good pixel become an hot pixel?

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2 Answers 2

One of the main causes is exposure to radiation (e.g. it is more likely to get hot pixels when travelling by airplane vs travelling by ship).

Edit: Okay, this was the description for LCDs (view old edits). For camera sensors this is mainly caused by leakage currents, which are electric charges leaking into sensor wells. The ADC interprets this as a high photon count and will result in a bright pixel (hot pixel). I worked as tech support for a camera manufacturer, and the main reason we were given for this was high altitude background radiation which can hit the sensor and knock out the electronics for single pixels.

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That quote appears to refer to LCD screens rather than image sensors, and doesn't really explain what happens at the transistor level... –  Matt Grum Jun 20 '13 at 13:02

Hot pixels in a sensor boil down to slight differences in the "response" of each photodiode, as resulting from the manufacturing process.

Silicon, the semiconductor used in CMOS manufacture, is a "doped" medium. It is a silicon base that has been mixed with a slight amount of impurities in order to support that "semi" conductivity. The distribution of these impurities is generally even, but not perfectly even. Some pixels will have higher concentrations than the average, and others will have lower concentrations than the average, which leads to different response curves for each pixel.

In addition to uneven doping, other manufacturing defects can affect this as well. The surface of an etched silicon wafer is not perfectly flat...so transistors do not all contain exactly the same amounts of silicon. Differences in volume can affect response as well. Nanoscopic scratches and other defects can also mar the surface of a wafer, which again can have an impact on response.

These different levels of impurities affect how easy it is for an electron to be freed within the photodiode. More defects or improper amounts of doping can lead to some photodiodes becoming "hot"...always containing more charge after an exposure than the norm. Electrons can be freed in a number of ways. The appropriate way in a sensor is when a photon strikes the photodiode surface, however increases in heat can affect how easily electrons are freed. The flow of current itself through the circuit can generate heat, and cause electrons to be freed within each photodiode (this is called dark current), and hot pixels tend to accumulate more free electrons from dark current than normal pixels.

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But what about an individual pixel that is stuck all the way on in every photo, even when the surrounding pixels correctly show the background is near black? Such as the shadows outside the spotlight in a theater. –  Michael Clark Jun 21 '13 at 3:25

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