CBSE Class 12 · 2006 · All India · SemiconductorsLong Answer

A free atom has sharp, discrete energy levels.
When $N$ atoms form a crystal, each level splits into $N$ very closely spaced levels because of the interaction between neighbouring atoms; these closely packed levels constitute an energy band.
The outer levels form the valence band (highest filled) and the conduction band (next, usually empty), separated by the forbidden gap $E_g$.
Conductor: the conduction and valence bands overlap (or the conduction band is partly filled), so conduction is easy.
Insulator: a large gap ($E_g>3\mathrm{eV}$) keeps the conduction band empty, so conduction is negligible.
Semiconductor: a small gap ($E_g\approx 1\mathrm{eV}$) allows a few electrons to be thermally excited into the conduction band.
$\boxed{\text{Bands from level-splitting; gap: overlap }<\sim 1\mathrm{eV}<>3\mathrm{eV}.}$