Difference between Energy Gap and Energy of Ionization

[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.

[avandonk]

Try here for all the answers

http://en.wikipedia.org/wiki/Electronic_band_structure

A crystal lattice with added 'doping' atoms is not anything like the isolated atoms that make up that lattice.

This explains the difference between energy of ionization and band gap of a crystal lattice made up of these atoms.

The computer you are using right now exists because some smart blokes at Bell labs were looking at the fundamentals of crystal lattice structure of Germanium and Silicon in the forties. The practical result was the transistor.

Just shows how fundamental research leads to totally unexpected results. Wheras goal oriented reseach just gets you to your forseen goal!

Bert