Engineering Chemistry S Datta

SOLID STATE CHEMISTRY 493
Function of the emitter is to inject majority charge carriers into the base, and a collector
is to collect these carriers through the base.
Amplification by a transistor (current amplification or gain)
• Conduction within a PNP transistor is by hole movement from the emitter to the collector.
In an NPN transistor electron moves.
• The collector current is always less than the emitter current i.e. I c
= I e
– I b
:
• The current amplification or gain (α) of a transistor is,
collector current
α =
emitter current = I Ic ′ (α < 1)
e
for both PNP and NPN transistors.
The alloy junction transistor is shown in the following figure (Fig. 22.17).
N-type
germanium
E
C
Indium
B
Fig. 22.17
The alloy junction transistor consists of two beads of indium metal alloyed on opposite
sides of a thin slice of an N-type Ge. Collector is larger in size than the emitter.
Elements of band theory
Metals in general are characterised by certain distinct features, these are:
• High thermal and electrical conductances, the conductance decreases with increase in
temperature.
• High melting and boiling points in most cases.
• Hardness and, at the same time, ductility and malleability.
• Metallic lustre
• Constancy
• Emission of electrons by some metals under the action of heat and light.
All these above properties of metal cannot be explained either by covalency or by an
ionic model. Separate bonding schemes have been proposed for the metals to give an accurate
explanation of the characteristics of metal. But the bond theory is the most satisfactory among
all the proposed theories.
Band theory
Band theory provides the most satisfactory explanation of the characteristics of the
metallic properties in a very natural manner. The theory is an extention of molecular orbital
(M.O.) treatment for a large number of atoms. For homonuclear diatomic species, the
combination of two atomic orbitals gives rise to two molecular orbitals and these are two new
energy levels for the electrons in the two joint fields of the nuclei. If this idea is extended to an