Brillouin zones and energy bands

cavalier3024

In the semi-classical model, i noticed that all the electrons with values of k that are in the same brillouin zone are considered to be at the same energy band, but i cant quite understand why it is so.
i know that in each brillouin zone the number of allowed states (of k) is the same as the number of states at each band (N), but that doesn't really provide a full explanation.

so if you can briefly explain the idea behind this (or refer me to the relevant pages at Ashcroft) i would be thankful.

Niles

There are 2N states in each band, and for a single k-vector, there are infinitely many energy bands (this can e.g. be seen when looking at the energy bands in the reduced zone scheme).

So when one energy band is filled, electrons start filling the next one, and so on. On what page in A&M have you read this?

cavalier3024

you said that 'when one energy band is filled, electrons start filling the next one', but that's not always the case. sometimes bands overlap and one band may start filling before the previous one finished, giving two partially filled bands.
you can see what i mean at page 224 (chapter 12), figure 12.3. they show that 2N electrons fill part of the first brillouin zone and also part of the second BZ. then they say that this produces two partially filled bands. so what i understand is that all occupied states that are in the first brillouin zone are filling one band, and the ones at the second zone fill another band.
what i don't understand is why the brillouin zones determine the bands. why all the electrons that occupy a certain BZ are considered to be at the same band?