2.2. ENERGY BANDS
21
Conduction
Band
Figure 2.11. Energy bands of (a) an insulator, (b) an intrinsic semiconductor, and (c) a
conductor. The cross-hatching indicates the presence of electrons in the bands.
the heat content of the material at room temperature can bring about the thermal
excitation of some electrons from the valence band to the conduction band where
they carry current. The density of electrons reaching the conduction band by this
thermal excitation process is relatively low, but by no means negligible, so the
electrical conductivity is small; hence the term semiconducting. A material of this
type is called an intrinsic semiconductor. A semiconductor can be doped with donor
atoms that give electrons to the conduction band where they can cany current. The
material can also be doped with acceptor atoms that obtain electrons from the
valence band and leave behind positive charges called holes that can also carry
current. The energy levels of these donors and acceptors lie in the energy gap, as
shown in Fig. 2.12. The former produces n-type, that is, negative-charge or electron,
conductivity, and the latter produces p-type, that is, positive-charge or hole,
Figure 2.12. Sketch of the forbidden energy gap showing acceptor levels the typical distance AA
above the top of the valence band, donor levels the typical distance AD below the bottom of the
conduction band, and deep trap levels nearer to the center of the gap. The value of the thermal
energy kBT is indicated on the right.