Engineering Chemistry S Datta

SOLID STATE CHEMISTRY 497
Semiconductors
A semiconductor is a material whose electrical properties lie in between those of good
conductors and insulators. For example – germanium (Ge) and silicon (Si). Semiconductors
are neither good conductors nor good insulators, but their electrical properties lie in between.
The resistivity of the semiconductors ranges from 10 5 to 10 –3 ohm-cm and decreases with
increase in temperature according to an exponential law. This is the main difference between
semiconductors and good conductors. The semiconductors which conduct even in chemically
pure state are called intrinsic semiconductors.
In terms of energy bands, semiconductors are the materials which have almost an empty
conduction band and almost filled valence band with a very narrow energy gap separating the
two. The energy gap is of the order of 1 eV.
0
E
Conduction
band
Small
energy gap
Valence band
16
Fig. 22.23
At 0 K, there are no electrons in the condition band and the valence band is filled
completely. As temperature increases, the width of the forbidden gap decreases, so that some
of the electrons are liberated into the conduction band. So, the electrical conductivity of
semiconductors increases with rise in temperature and in this regard they differ from the
metals whose electrical conductivity decreases with rise in temperature. Semiconductors are
of two types:
Ge
Ge atom
. .
G
B
A
Ge
. .
Ge
. .
Ge
F
. .
D
C
Ge
Covalent bond
E
Fig. 22.23 (a) Intrinsic Ge.
Fig. 22.23 (b) Hole formation in semiconductors
(conduction of current).