1 electron = 1 photon?

Started Apr 12, 2013 | Discussions thread
John Siward
Regular MemberPosts: 215
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Re: Internal vs external QE
In reply to alanr0, Apr 12, 2013

alanr0 wrote:

Jack Hogan wrote:

John Siward wrote:

Your basic point is right: a red photon is much more likely to penetrate deeper into the Si than a blue photon.  This is the principle of operation of Sigma's "Foveon" sensor which stacks three photodiodes vertically to allow the sensor to measure colour without using a colour filter array.

J.

Thank you for walking me through it, John, quite clear.  So could we rearrange the Responsivity diagram's x-axis to represent logarithmic penetration depth, the curve then representing QE at the relative depth?  If so, I would have expected more discontinuities in it, per this interesting introduction.

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.

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