1 electron = 1 photon?

Started Apr 12, 2013 | Discussions thread
Jack Hogan
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Re: Internal vs external QE
In reply to John Siward, Apr 13, 2013

John Siward wrote:

alanr0 wrote:

Jack:

I suspect it will be difficult to get a reliable indication of penetration depth.  In particular, there may be more going on than simple electron-hole creation at wavelength shorter than 400 nm.

To make a start, I would first calculate QE from the responsivity to remove the inverse wavelength dependence of photon energy.  This will have a much 'flatter' shape than the curve you presented.  You also need to account for incident light which is reflected from the surface of the chip or the package window.  Silicon has a rather high refractive index of around 4 , so there is strong Fresnel reflection unless a suitable anti-reflection coating is applied.  Even with an AR coating, the external quantum efficiency can be significantly lower than the internal QE.

HTH

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Alan Robinson

Good point - the actual sensor is quite a complex thing including classical optics effects (micro-lenses, coatings, etc.) so it would be tricky to de-embed the Si response itself...

J.

Interesting distinction between internal and external QE, Alan, especially the fact that the difference is attributable to reflection, which makes Mike's oscillating R/QE curve all the more relevant in today's multiple layered Bayer sensors, which include IR, AA, CFA, microlenses, coatings and who knows what other reflection inducing barriers.

So to summarize, if we measure shot noise from the standard deviation of a small uniform sample of the Raw data (by image pair difference or at a signal level that is clean of other sources of noise) and use it to obtain the number of electrons in the signal, it's not really a simple matter to model back to the number of incoming photons.

Aside from the question of whether integer or floating point math should be used, for a given sensor the number of photons corresponding to that many electrons (1/EQE) is not a first order function related to wavelength (other than perhaps approximately in the 500 to 600 nm range) but it's a complex, non linear function which also would appear to depend, for instance, on the angle of incident light, where physically the sample came from on the sensor, focal length, f/number etc.  Makes one wonder how important these deviations from the simpler model are and whether manufacturers can/do attempt to compensate for some of these effects.

Jack

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