Engineering Chemistry S Datta

SOLID STATE CHEMISTRY 499
Emitter junction
Collector junction
P N P
1 mA 0.99 mA
Emitter
Collector
V e
Hole flow Hole flow
0.01 mA
(a)
V c
Base
(b)
Fig. 22.23 (e) Working of a PNP transistor.
N P N
1 mA 0.99 mA
Emitter
Collector
Electron flow
V e
(a)
Electron flow
0.01 mA
V c
Base
(b)
Fig. 22.23 (f) Working of an NPN transistor.
Highlight. Emitter arrow shows the direction of flow of conventional current.
Obviously, electron flow will be in the opposite direction.
Insulators
Insulators are those materials in which valence electrons are bound very tightly to their
parent atoms and thus require a very large electrical field to remove them from the attraction
of the nuclei.
In terms of energy bands, it means that insulators are those materials which have a full
valence band, have an empty conduction band and have very high energy gap between these
two bands (Fig. 22.24).
0
E
Empty conduction
band
Large energy
or forbidden gap
Valence band
16
Fig. 22.24