1 electron = 1 photon?

Started Apr 12, 2013 | Discussions thread
alanr0
Senior MemberPosts: 1,261
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Re: 1 electron = 1 photon?
In reply to Jack Hogan, Apr 12, 2013

Jack Hogan wrote:

Simplifying, assuming a modern B&W silicon sensor without a CFA or other filtering and 100% fill factor, if we see one electron produced at the output of a photosite, can we assume that it was the result of one photon making it through to silicon - and that the probability of it dislodging an electron is the Charge Collection Efficiency (QE) of the semiconductor at the wavelength of the incoming photon, so that for a given Exposure

Electrons produced = photons(wavelength) hitting silicon * QE(wavelength)?

If the visible spectrum is between 380 and 760 nm, with a near-infrared photon having half the frequency/energy of a near-ultraviolet photon therefore burying deeper into silicon, does the following responsivity curve merely represent QE at various silicon depths? Or could the fact that the responsivity at 760nm is more than 3 times that at 380nm mean that, for instance, sometimes 1 photon produces two electrons?

https://upload.wikimedia.org/wikipedia/commons/thumb/4/41/Response_silicon_photodiode.svg/500px-Response_silicon_photodiode.svg.png

The response is shown in A/W.  For each electron released, the charge is constant, but the photon energy is proportional to the photon frequency, and inversely proportional to the wavelength.

Efficiency in A/W = (electron charge x wavelength)/(planck's const x velocity of light)

At 100% quantum efficiency (1 electron per photon), you get 0.645 A/W at 800 nm, but only 0.323 A/W at 400 nm.  Your graph shows the quantum efficiency falling from around 84% at 600-800 nm, to around 53% at 400 nm.

In other words, less than 100% QE, with peak performance at 600-800 nm.

HTH

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Alan Robinson

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