Theory of PN Junction

Theory of P‒N Junction

• In a p‒n junction, on p‒side there are large number of holes while on n‒side there are large number of electrons. Hence the overall there is nonuniform distribution of charge carriers.

• When such nonuniform distribution exists then the charge carriers moving from high concentration area towards the low concentration area. This is called diffusion.

• In unbiased p‒n junction, the majority holes on p side start diffusing into n side while the majority free electrons on n side start diffusing into p side.

• In n region, the holes difusing from n‒side, recombine with free electrons. Thus due to additional positively charged holes, these atoms on n‒side become positive immobile ions, just near the junction in n region.

• In p region, the free electrons diffusing from n‒side, recombine with the holes of the atoms. Thus due to gain of additional negatively charged free electrons, these atoms become negative immobile ions, just near the junction in p‒region.

• As more holes diffuse on n side, large immobile positive charge accumulates near the junction on n side. This positive charge repels the positively charged holes and the diffusion of holes stops.

• Similarly large negative charge accumulates near the junction on p side. This negative charge repels the negatively charged electrons and the diffusion of electrons stops.

• Thus there exists a wall near the junction with negative immobile charge on p side and positive immobile charge on n side. There are no charge carriers in this region. The region is depleted off the charge carriers hence called depletion region, depletion layer or space charge region. The depletion region is shown in Fig. 2.2.1.

• In equilibrium condition, the depletion region gets widened upto a point where no further electrons or holes can cross the junction. Thus it acts as a barrier.

Barrier Potential

• Due to immobile positive ions on n side and immobile negative ions on p side, there exists an electric field across the junction.

• This creates potential difference across the depletion region which acts as a barrier. This is called barrier potential, junction potential, built‒in potential or cut‒in potential of p‒n junction.

• It is denoted as V_J and is shown in Fig. 2.2.2. It is also called height of the depletion region and also denoted as V_O or Vγ.

• The barrier potential depends on,

1. Type of semiconductor

2. Donor impurity added

3. Acceptor impurity added

4. Temperature

Effect of Temperature on Barrier Potential

The physical distance from one side to other side of the depletion region is called width of the depletion region. Practically width of the depletion region is about 0.5 to 1 micron. (1 micron = 10^‒6 m).

• As temperature increases, the width of the depletion region decreases.

• Thus as temperature increases, the barrier potential decreases.

• The barrier potential decreases by approximately 2.5 mV per degree celcius rise in temperature.

Review Questions

1. Explain the theory of p‒n junction.

2. Explain the formation of depletion region in an unbiased p‒n junction, with the help of neat sketch.

3. Briefly explain about depletion region and barrier voltage of a PN junction.