# 1 electron = 1 photon?

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

Relative Number of electrons Generated as a Function of Impinging Photon Wavelength

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?

Jack

Hello again, Jack,

These may help . . .

Here's the actual measured QE of a CMOS sensor at 3 different depths:

Guess the Sensor . . .

Here's a graph of absorption versus depth:

Each curve is for a different wavelength . .

Here's a normalized absortion versus depth graph for silicon:

Looks quite a bit like a RC discharge curve . .

. . because it's an exponential function, I reckon.

So, for a 100 perfect photons in, there will be exactly 63.212 photons absorbed (well, in a world of fractional photons there would be).

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Ted http://kronometric.org
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