Page 17 - 12-phy-17 PHYSICS OF SOLIDS

17. Physics of Solids eLearn.Punjab

Now an electron is attracted from C to B and a hole is created at C (Fig. 17.12 b,c) and positive
charge appears at C. This process is repeated between the atoms C and D with the result that the

electron moves from D to C and the hole (+ve charge) appears at D (Fig. 17.12 c.d). Thus we notice
that if a hole is present in any valence shell, it cannot stay there but it moves from one atom to

other with the electron moving in opposite direction. Secondly we notice that the appearance of
hole is accompanied by a positive charge. Thus a moving hole is equivalent to a moving positive

charge.
In this example we have considered a special case in which the electron and the hole are moving

in a straight line. Actually their motion is random because positively charged core of the atom can
attract an electron from any of its neighbouring atoms.

Thus, in semi-conductors there are two kinds of charge carriers; a free electron (- e) and a hole (+ e).
When a battery is connected to a semi-conductor, it establishes an electric field across the

semiconductor due to which an opposite flow of
electrons and holes takes place. The electrons drift

towards the positive end, whereas the holes drift
towards the negative end of the semi-conductor (Fig.

17.13) The current / flowing through the semi-conductor
is earned by both electrons and holes. It may be noted

that the electronic current and the hole current add up
together to give the current /. Fig. 17.13

17.4 SUPERCONDUCTORS

There are some materials whose resistivity becomes zero

below a certain temperature T called critical temperature
c
as shown in resistivity-temperature graph in Fig. 17.14.

Below this temperature, such materials are called
superconductors.

They offer no resistance to electric current and are,
therefore, perfect conductors. Once the resistance of a

material drops to zero, no
Do You Know?
energy is dissipated and the
Superconductors are alloys
that, at certain temperatures, current, once established, Fig. 17.14
conduct electricity with no continues to exist indefinitely
resistance.
without the source of an emf.
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