Where do Conduction Electrons Reside?

[logearav]

Homework Statement

Electrons present in the outermost orbit are valence electrons. If we supply sufficient energy to these electrons they make their transition from valence band to conduction band and electrons in conduction band are termed as conduction electrons. My question is, take for example Silicon whose atomic no is 14 which has 2, 8, 4 as its electronic configuration and has 4 valence electrons. Now we supply energy to these valence electrons they move to conduction band and they become free or conduction electrons. Does it mean that they move to the next higher energy orbit, in my case, from M Shell to N shell or to any other higher shell. Or they go some where else? Where exactly does this conduction electrons reside?

Homework Equations

The Attempt at a Solution

 

[phyzguy]

When you talk about the shells or energy levels, you are referring to the electron configuration of an isolated Si atom. When you take a large number of Si atoms and join them into a Si crystal, the atoms interact and so the energy levels don't stay the same. Instead of a small number of discrete levels, you have a very large number of closely spaced levels, and this number increases as the number of atoms in the crystal increases. For a macroscopic crystal, with perhaps 10^23 atoms, there are so many levels, and they are so closely spaced, that we can approximate them as a continuous band. It turns out that for crystals with uniform spacing, like Si, the bands have gaps in them ( see, for example - http://en.wikipedia.org/wiki/Kronig–Penney_model, or http://en.wikipedia.org/wiki/Bloch_wave). When you talk about promoting an electron from the valence band to the conduction band, you are talking about promoting it from the lowest energy band, across the energy band gap to the next highest energy band.