= Photodiode
{wiki}
A <photodiode> is a <semiconductor> <diode> producing <photocurrent> from <photon absorption>. Exciting an <electron> across the <band gap> leaves an <electron hole>. The <depletion region>'s <electric field> separates the <charge carriers>; others can arrive by <diffusion> before <electron-hole recombination>. Contacts complete the <electrical circuit>.<Si photodiodes by Hamamatsu Photonics K.K.>[Si photodiodes, p. 2, §1-1 and Figs. 1-1–1-2: generation, separation and collection]{ref}
The useful bridge is from <light> to a measurable <electric current>: <spectral responsivity> converts incident <radiant flux> into <photocurrent>, while <quantum efficiency> counts collected signal <charge carriers> per incident <photon>. These answer different questions: how much <electric current> one watt produces, and how many incident <photons> contribute to the signal. The worked calculation below makes their relationship explicit.
\Image[/photodiode-absorption-and-charge-collection.png]
{title=<Photon absorption>, junction separation and an external <electric current>}
{description=Original qualitative <p-n junction> schematic with an external <resistor>. Collecting one <electron>–<electron hole> pair transfers one <elementary charge> through the external <electrical circuit>, rather than twice that amount. Dimensions and paths are schematic.}
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