Biomedical Engineering Reference
In-Depth Information
While the study of crystals is a complex subject, some insight into their electrical
properties can be gained by pursuing these simple physical concepts. With isolated
bands present, the upper hatched area on the far right in Fig. 2.7 is called the
con-
duction band
, and the lower hatched area, the
valence band
. Electrically, solids can
be classified as
insulators, semiconductors
,or
conductors
. The distinction is manifest
in the mobility of electrons in response to an electric field applied to the solid. This
response, in turn, is intimately connected with the particular atomic/molecular
crystal structure and resultant band gap. The three classifications are represented
in Fig. 10.13. Like most ionic solids (Section 2.9), NaCl, for example, is an insu-
lator. The pairing of the univalent sodium and chlorine atoms completely fills the
valence band, leaving no vacancies into which electrons are free to move. In prin-
ciple, thermal agitation is possible and could provide sufficient energy to promote
some valence electrons into the unoccupied conduction band, where they would be
mobile. However, the probability for this random occurrence is exceedingly small,
Fig. 10.13
Band structure of insulators, semiconductors, and
conductors. Starting from zero at the bottom of the valence
band, the vertical scale shows schematically the energies
spanned by the valence and conduction bands and the
forbidden gap. The energy at the top of the valence band is
denoted by
E
0
. The energy at the bottom of the conduction
band is
E
C
=
E
0
+
E
G
,where
E
G
is the size of the band gap.
