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Photodiode Sensor Physics

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Photodiode Sensor Physics

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A photodiode sensor consists of a semiconductor p-n junction like the laser diode and led described in Laser Diode and LED Physics. Light falling on the junction causes the formation of electron-hole pairs. In photovoltaic mode, i.e., no applied bias, the electron-hole pairs migrate to opposite sides of the junction, thus producing a voltage (and a current, if the device is connected in a circuit). However, most photodiodes are operated in the photoconductive mode where a reverse bias is applied across the junction. Operating in this mode offers a few significant advantages. A reverse bias increases the width of the depletion region, which leads to a larger photosensitive area allowing more light collection. Furthermore, the bias produces a strong field in the junction that sweeps the carriers out quickly, making it less likely for recombination to occur. This ensures a large quantum yield or efficient conversion of photons to charge carriers. There are also advantages in terms of response time (see Photoreceiver Physics). In reverse bias photodiodes, the current produced by the bias and charge carriers is proportional to the incident optical intensity over a wide dynamic range.

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