1 electron = 1 photon?

Started Apr 12, 2013 | Discussions thread
Eric Fossum
Contributing MemberPosts: 699
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Re: 1 electron = 1 photon?
In reply to Jack Hogan, Apr 14, 2013

As many persons have answered, there are several physical processes involved.  I tend to summarize things this way.

1.  First a photon has to make it into the silicon and not be reflected or absorbed by layers above the silicon.

2.  Second, the photon needs to break a silicon-silicon covalent bond.  The higher the photon energy, the higher the probability, and there is a minimum energy required as well - about 1.1 eV - also called the bandgap energy.  Thus, the lower the energy (redder), the lower the probability of breaking a bond and the average absorption depth is deeper.  Red photons could still be absorbed right at the surface though.  It is just a matter of probabilities.  There are some other mechanisms for carrier generation but they are much more rare and can be ignored.  For Backside Illumination (BSI) the total thickness of the silicon typically corresponds to a red photon's absorption length.  In BSI, there could be a frontside reflector (metal wiring) that gives the photon a 2nd chance to be absorbed.

3.  Third, the photocarrier (usually an electron, but possibly a hole) needs to be collected and stored by the photosite.  In frontside illuninated devices, the deeper parts of the silicon are doped such that carriers generated there never get collected for one reason or the other.  Carrier collection efficiency is usually an important ingredient of the "quantum efficiency".  Sometimes carriers are collected to an adjacent photosite giving rise to color crosstalk. (Red photoelectron gets counted as a green photoelectron, for example).

4.  The QE is typically the inverse ratio of photons striking the surface of the sensor over the full area of a pixel to the number of photoelectrons successfully collected and is wavelength dependent.

5.  Amps per Watt is an ancient metric useful perhaps for solar cells or optical communication devices, not so much for sensors.  But as was said, multiply by the number of electrons collected per second by the electronic charge, and multiply the # photons incident per second by their energy and you get to Amps/Watt.

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