Page 957 - Basic Electrical Engineering

stops any further diffusion of charge carriers. This initial diffusion of charge
carriers at the junction, and the development resultant barrier voltage take
place when a p–n junction is formed during the manufacturing process. The

barrier voltage depends upon the amount of doping, charge carriers, and the
junction temperature. For germanium the barrier voltage is 0.3 V and for

silicon the barrier voltage is 0.7 V at room temperature (25°C). The shaded
portion on both sides of the p–n junction is having only immobile ions of

opposite polarities which creates a potential difference, i.e., barrier voltage.
This portion is devoid of any electron or hole, i.e., any charge carriers. This

region is depleted of any charge carrier, and hence is called the depletion
region. It may be noted that the thickness of the depletion region in Fig. 14.5
(b) has been shown expanded. In fact, this layer is very thin, of the order of

micrometer.
The p-side of the depletion region acceptor impurity atoms that have lost

their holes associated with them by accepting electrons have become
negatively charged ions. Similarly the n-side of the depletion region consists

of donor atoms (pentavalent atoms) that have lost their associated free
electrons and have become positive ions. As shown in Fig. 14.5 (b), the

depletion layer is equally divided on both sides of the p–n junction. This is
because both the p-type and the n-type materials have been equally doped.
Equal percentage of doping materials have been added on both sides. If the

doping is different, the width of the depletion region on the two sides will be
different. Application of some voltage across the p–n junction is called

biasing. Depending on the polarity of biasing the width of the depletion layer
will change.

14.4.4 Biasing of p–n Junction

Biasing of a p–n junction means application of some external voltage across
the two sides of the p–n junction. When the p-side is connected to the

positive terminal of a battery and the n-side is connected to the negative
terminal, the p–n junction is said to be a forward-biased junction. If the
positive terminal of the battery is connected to the n-side and the negative