[mjc (Mark)]

Context: CCDs

The energy gap of Silicon is about 1.1eV (at 300 Kelvin). A photon striking silicon with a wavelength shorter than about 1000nm has enough energy to liberate an electron from the valence band to the conductance band - where an outermost electron can escape and can be conducted away.

As I see it - we're left with a positively charged Si atom - and as I see it - we have both an ion (its charged) and a hole (a pseudonym for a single positive charge) so we have an electron - hole (e-h) pair.

Now we have another metric - the energy of ionization - stated to be about 3.7eV for Si at 300 Kelvin - also producing an e-h pair.

Can anyone give me a clue as to why there are two energies that appear to be thresholds of ionization?

It maybe that the former is special and is strictly quantum photo-electric stuff and the latter kinetic (though at quantum scales) - e.g. a stray electron striking a Si atom (I'm clutching at straws).

Its the difference between the two that I am seeking.

Mark C.