Properties of pn Junction
At the instant of pn-junction formation, the free electrons near the junctions near the
junction in the n region begin to diffuse across the junction into the p region where they
combine with holes near the junction.
The result is that n region loses free electrons as
they diffuse into the junction. This creates a layer of positive charges (pentavalent ions)
near the junction. As the electrons move across the junction, the p region loses holes as
the electrons and holes combine.
The result is that there is a layer of negative charges
(trivalent ions) near the junction. These two layers of positive and negative charges form
the depletion region (or depletion layer).
The term depletion is due to the fact that near
the junction, the region is deplete (i.e. empty) of charge carries (free electrons and
holes) due to diffusion across the junction. It may be note that depletion layer is form
very quickly and is very thin compare to the n region and the p region. For clarity, the
width of the depletion layer is shown exaggerate.
Once pn junction is formed and depletion layer created, the diffusion of free
electrons stops. In other words, the depletion region acts as a barrier to the further
movement of free electrons across the junction. The positive and negative charges set
up an electric field. This is shown by a black arrow in Fig. 1. The electric field is a
barrier to the free electrons in the n-region. There exists a potential difference across
the depletion layer and is called barrier potential (V o ). The barrier potential of a pn
junction depends upon several factors including the type of semiconductor material, the
amount of doping and temperature. The typical barrier potential is approximately:
For silicon, V o = 0.7; For germanium, V o = 0.3 V
Fig. 1 shows the potential (V o ) distribution curve.
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