Junction or Barrier Voltage

Even though depletion layer is cleared of charge carriers, it has oppositely-charged fixed rows of ions on its two sides. Because of this charge separation, an electric potential difference VB is established across the junction even when the junction is externally isolated. It is known as junction or barrier potential. It stops further flow of carriers across the junction unless supplied by energy from an external source. At room temperature of 300ºK, VB is about 0.3 V for Ge and 0.7 V for Si. Barrier voltage depends on 

1. doping density, 
2. electronic charge and 
3. temperature. 

For a given junction, the first two factors are constant, thus making VB dependent on temperature. With increase in temperature, more minority charge carriers are produced, leading to their increased drift across the junction. As a result, equilibrium occurs at a lower barrier potential.

The strong field set up by VB causes drift of carriers through depletion layer. Under the influence of this field, holes drift from N-to P-region and electrons from Pto -N region. This drift current must be equal and opposite of the diffusion current because under condition of equilibrium and with no external supply, net current through the crystal is zero.

The processes involved in the formation of a P-N junction are:

1. Holes from the P-side diffuse into the N-side where they combine with free electrons.
2. Free electrons from the N-side diffuse into the P-side where they combine with holes.
3. The diffusion current (also known as recombination current) decays exponentially both with time and distance from the junction.
4. Due to the departure of free and mobile carriers from both sides of the junction, a depletion layer (centred around the junction) is formed. This layer contains only immobile or fixed (also called uncovered) ions of opposite polarity.
5. These uncovered but fixed ions set up a potential barrier across the junction.
6. This potential difference opposes the diffusion of free majority charge carriers from one side of the junction to the other till the process is completely stopped. (Incidentally this potential barrier aids in transfer of thermally generated minority charge carriers from one side of the junction to the other)
7. The width of depletion layer depends on the doping level. For heavy doping, depletion layer is physically thin because a diffusing charge carrier (either free electron or hole) has not to travel far across the junction for recombination (short lifetime). Opposite is the case if light doping is used.