How long does it take for a valence electron being excited to conduction band?

[zhterran]

We know in silicon a valence electron can be excited to conduction band if it absorbs one photon(one photon absorption) or two photon(two photon absorption),my question is that it must need a certain amount of time for the electron to 'travel' from the valence band to the conduction band(I cannot imagin a instantaneously transition with zero time),then how long is that time?

 

[SpectraCat]

We know in silicon a valence electron can be excited to conduction band if it absorbs one photon(one photon absorption) or two photon(two photon absorption),my question is that it must need a certain amount of time for the electron to 'travel' from the valence band to the conduction band(I cannot imagin a instantaneously transition with zero time),then how long is that time?

I am not sure about transitions in semi-conductors, but for transitions between electronic states in molecules, the timescale is typically on the order of 1 fs (1x10^-15 s). Some of the most recent cutting edge experiments in molecular spectroscopy have started looking at the "motion" of the electron density in atoms and molecules after electronic transitions have been excited with an extremely short (~0.1 fs) pulse. ( For example, check out Nature 466, (2010) p. 739.)