Photodiode and Photovoltaic Detectors
When a photon strikes a semiconductor, it can promote an electron from the valence band (filled orbitals) to the conduction band (unfilled orbitals) creating an electron(-) - hole(+) pair. The concentration of these electron-hole pairs is dependent on the amount of light striking the semiconductor, making the semiconductor suitable as an optical detector. There are two ways to monitor the concentration of electron-hole pairs. In photodiodes, a voltage bias is present and the concentration of light-induced electron-hole pairs determines the current through semiconductor. Photovoltaic detectors contain a p-n junction, that causes the electron-hole pairs to separate to produce a voltage that can be measured.
Schematic of semiconductor detector
Photodiode detectors are not as sensitive as PMTs but they are small and robust.
Table 1: Wavelength range
|Si||0.2 - 1.1|
|Ge||0.4 - 1.8|
|InAs||1.0 - 3.8|
|InSb||1.0 - 7.0|
|InSb (77K)||1.0 - 5.6|
|HgCdTe (77K)||1.0 -25.0|
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http://www.scimedia.com/chem-ed/optics/detector/pd.htm, updated 9/12/96