I've been attending talks in some condensed matter groups, they always talk about "subbands", but what exactly are they? What's the difference between the "normal" conduction and valence bands? Thank you.
There are typically many bands for electron states in a material, because there are many electrons per atom.
In a semiconductor, all the bands with higher energy than the band gap are the “conduction band,” and all the states with lower energy are the “valence band.” The bands making up the conduction band and valence bands are the subbands.
Usually, the interesting behavior results just from electrons being in the conduction band or holes being in the valence band. But sometimes, the subbands matter. There can be low energy electron transitions between subband states (inter-subband transitions), for example. In these, if the electron was in the “valence band”, it remains in the “valence band” but is now in a different subband.
If you have more information of the context in which you heard the term, I could probably expand this a little further.