Biomedical Engineering Reference
In-Depth Information
Fig. 10.15
Relative number of electrons in valence and
conduction bands of (a) an insulator and (b) a semiconductor
with
T
>0.
holes contribute to the observed conductivity. The diagram in Fig. 10.16 represents
an intrinsic (pure) semiconductor. Its inherent conductivity at room temperature
is restricted by the small number of electron-hole pairs, which, in turn, is limited
by the size of the gap compared with
kT
.
The conductivity of a semiconductor can be greatly enhanced by doping the crys-
tal with atoms from a neighboring group in the periodic system. As an example, we
consider the addition of a small amount of arsenic to germanium.
1)
When a crystal
is formed from the molten mixture, the arsenic impurity occupies a substitutional
1 The reader is referred to the Periodic Table at
the back of the topic.
