Photoconductors and photodiodes

In photovoltaic detectors, also called photodiodes, electrons and holes generated by photons are separated by an electric field formed at a potential barrier within the device. This field is formed at a specifically introduced interface in the material, for instance a p-n junction. Photodiodes used in astronomy are almost always made of intrinsic material. When an electron-hole pair is created at the junction by a photon the electron drifts to the -region and the hole to the p-region. The separation of charge causes a voltage to be generated across the detector terminals, which can be sensed directly or, alternatively, a current can be measured when the circuit is completed. 

When the load resistance greatly exceeds the detector resistance and the signal frequency is low, the signal voltage of a PC detector is 

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In choosing photodetectors for optical sensors, a number of factors must be considered. These include sensitivity, detectivity, noise, spectral response, and response time. Photomultipliers and semiconductor quantum photodetectors, such as photoconductors and photodiodes, are all suitable. The choice, however, is somewhat dependent on the wavelength region of interest. Generally, both types give adequate... 

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