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P-N Junction Diode

When a P-type semiconductor is suitably joined to an N-type semiconductor, then the resulting arrangement is called P–N junction or P–N junction diode (Fig. 10).
Fig. 10

Depletion region

On account of difference in the concentration of charge carrier in the two sections of P–N junction, the electrons from N-region diffuse through the junction into P-region and the holes from P–region diffuse into N-region.
Due to diffusion, neutrality of both N- and P-type semiconductors is disturbed. A layer of negative ions appears near the junction in the P-crystal and a layer of positive ions appears near the junction in N-crystal. This layer is called depletion layer.
Fig. 11
  • The thickness of depletion layer is 1 micron = 10–6 m.
  • Width of depletion layer 107009.png.
  • Depletion is directly proportional to temperature.
  • The P–N junction diode is equivalent to capacitor in which the depletion layer acts as a dielectric.

Potential barrier

The potential difference created across the P–N junction due to the diffusion of electron and holes is called potential barrier.
For Ge, VB = 0.3 V and for silicon VB = 0.7 V.
On the average the potential barrier in P–N junction is ~ 0.5 V and the width of depletion region ~ 10–6 m.
So the barrier electric field, 107015.png
Some important graphs
Fig. 12

Diffusion and drift current

Because of concentration differences, holes/electrons try to diffuse from their side to the other side. Only those holes/electrons cross the junction, which have high kinetic energy. This diffusion results in an electric current from the P-side to the N-side known as diffusion current (idf).
As electron–hole pairs (because of thermal collisions) are continuously created in the depletion region, there is a regular flow of electrons toward the N-side and of holes towards the P-side. This makes a current the from the N-side to P-side. This current is called the drift current (idr).

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