Insulators

In an insulator the valence and conduction bands are separated by a very large forbidden gap, approximately 6 eV. Because this gap is so wide, no realistic external electric field or increase in temperature can supply enough energy to lift an electron from the valence band into the conduction band, so the number of free electrons is essentially nil and electrical conduction is not possible. Consistent with this, an insulator's resistivity is enormous, in the range $10^{11}$ to $10^{19}\ \Omega\text{m}$. Common examples are glass, rubber and mica, where …