CBSE Class 12 · 2014 · Delhi · SemiconductorsShort Answer

On the basis of energy bands:
Conductor — the conduction and valence bands overlap (or the conduction band is partly filled), so there is effectively no forbidden gap and many free carriers.
Semiconductor — a small forbidden gap ($E_g\approx 1\mathrm{eV}$) separates a filled valence band from an empty conduction band, so a few carriers are thermally excited.
Insulator — a large forbidden gap ($E_g>3\mathrm{eV}$, e.g. 6 eV for diamond) prevents electrons from reaching the conduction band, so conduction is negligible.
$\boxed{\text{Gap: zero (conductor) }<\sim 1\mathrm{eV}(\text{semiconductor})<>3\mathrm{eV}(\text{insulator}).}$