the valence band
The valence band is the highest range of electron energies that is normally full of electrons at low temperature — the top occupied shelf on the energy bookshelf. It is built from the valence electrons, the outermost electrons of each atom that do the bonding. In a semiconductor or insulator at absolute zero, the valence band is completely filled: every allowed seat is taken.
A completely full band, counterintuitively, cannot carry any net current. The reason is a nice picture: for current to flow, electrons must be able to speed up in one direction more than the other, which requires empty states for them to move into. In a full band there are no empty seats, so for every electron drifting one way there is another drifting the opposite way and the two cancel exactly. This is why a filled valence band, on its own, is electrically dead — a perfectly packed parking lot where no car can move.
Two things bring the valence band to life. First, an electron can be lifted out of it across the band gap into the empty conduction band, leaving behind an empty state — a hole — in the valence band. That hole acts like a mobile positive charge and lets the remaining valence electrons shuffle and carry current. Second, in p-type doping we deliberately remove some valence electrons, creating holes on purpose. So the valence band is not just a filled reservoir; it is where holes live, and holes are half of what makes a semiconductor work.
In pure silicon at room temperature, thermal energy kicks a few electrons out of the valence band into the conduction band. Say 10^16 per cubic meter make the jump: that leaves 10^16 holes behind in the valence band. Both the promoted electrons and the holes they left now carry current, which is why even 'pure' silicon conducts a little.
A full valence band carries no current; it comes alive only when holes appear in it.
A hole is not a real particle — it is the absence of an electron in the nearly full valence band. But it moves and carries charge like a genuine positive particle, and it is far simpler to track the one empty seat than the billions of electrons shuffling around it.