The band structures of insulators and semiconductors resemble each other
qualitatively. Normally there exists in both insulators and semiconductors a
filled energy band (referred to as the valence band) separated from the next
higher band (referred to as the conduction band) by an energy gap. If this gap
is at least several electron volts, the material is an insulator, as shown in Figure
11.6c. It is too difficult for an applied field to overcome that large an energy
gap, and thermal excitations lack the energy to promote sufficient numbers of
electrons to the conduction band. But if the energy gap is smaller—typically on
the order of about 1 electron volt—it is possible for enough electrons to be
excited thermally into the conduction band, so that an applied electric field
can produce a modest current. The result is a semiconductor, with the band
structure illustrated in Figure 11.6d. We shall explain this mechanism more
fully in the next section.ขอบคุนที่รับฟัง