Engineering Chemistry S Datta

498 ENGINEERING CHEMISTRY
(i) Pure or intrinsic semiconductors. In such semiconductors the energy gap between
the valence band and conduction band is very small (Fig. 22.23). The electrons
jump across the gap and hence they behave as insulators at absolute zero. But their
electrical conductivity is raised with increase in temperature to break the covalent
bonds (Fig. 22.23(a)) to get conducting electrons (Fig. 22.23(b)).
Let covalent bond is broken at A with increase in temperature [Fig. 22.23(b)] and the
electron has moved leaving behind a hole at A where an electron jumps from B creating a hole
there, and so on. Thus a negative charge moves from G to A. i.e. a positive charge from A to G
alternately. An intrinsic semiconductor may be defined as one in which the number of conduction
electrons is equal to the number of holes.
(ii) Impure or extrinsic semiconductors. Semiconductor behaviour may also be imposed
in certain substances by the deliberate addition of impurities. Such semiconductors
are called extrinsic semiconductors. Example, n-type and p-type semiconductors
(discussed earlier).
Those intrinsic semiconductors to which some suitable impurity or doping agent or dopant
is added in extremely small amounts (about 1 part in 10 8 ) are called extrinsic or impurity
semiconductors.
Usually doping agents are:
l pentavalent atoms (As, Sb, P) or
l trivalent atoms (Ga, In, P, B).
Pentavalent doping atom is known as donor atom as it donates one electron to the
conduction band of pure Ge. The trivalent atom, on the other hand, is called acceptor atom as
it accepts one electron from Ge atom.
Depending on the type of doping agent, extrinsic semiconductors are divided into two
classes:
(i) N-type semiconductors
(ii) P-type semiconductors.
Ge
Ge
Ge
Sb Ge
Ge
. .
. . . . . .
.
excess electron
Ge B Ge
. .
. .
Hole
Ge
. .
. .
Fig. 22.23 (c) N-type semiconductor.
Fig. 22.23 (d) P-type semiconductor.