- The PN junction is formed of the P type and N type semiconductor material.
- In P type, the holes are the majority charge carriers
- In N type material, the electrons are the majority charge carriers.
- Therefore at the junction there is a tendency for the free electrons to diffuse over to the P-side and holes to the N-side. This process is called diffusion.
- The diffusion of holes and free electrons is due to the difference in concentration of the two regions.
- This difference in concentration creates a concentration gradient across the junction.
- Due to the process of diffusion the negative acceptor ions in the P region and positive donor ions in the N region are left uncovered in the vicinity of the junction as seen in
- The additional holes, trying to diffuse to the N-region, are repelled by the uncovered positive charge of the donor ions.
- Similarly, the electrons trying to diffuse into the P-region are repelled by the uncovered negative charge of the acceptor ions.
- Thus a barrier is set up near the junction, which prevents the further movement of charge carriers. This is called as potential barrier or junction barrier V0.
- As a result, further diffusion of free electrons and holes across the junction is stopped.
- The region containing the uncovered acceptor and donor ions, in the vicinity of the junction is called depletion region.
- Since this region has immobile ions, which are electrically charged, the depletion region is also known as space-charge region.
- The width of the depletion layer depends upon the doping level of the impurity in N-type or P-type semiconductor.
- The higher the doping level, the thinner will b e the depletion layer and vice versa.
- The depletion layer consists of fixed rows of oppositely charged ions on its two sides.
- Because of this charge separation, an electric potential is established across the junction, even when no external voltage being applied.
- This electric potential is called junction or potential barrier.
WORKING AND VI CHARACTERISTICS: UNDER FORWARD BIAS CONDITION:
When positive terminal of the battery is connected to P-type and negative terminal to N-type of the PN junction diode, the bias applied is known as forward bias.
- Under forward bias, the applied positive potential repels the holes in P-type region so that the holes move towards the junction.
- The applied negative potential repels the electrons in the N-type region and the electrons also move towards the junction.
- Eventually, when the potential applied exceeds the internal barrier potential, the depletion region and internal potential barrier disappear.
V/I characteristics under forward bias condition
As forward voltage increases, for VF < V0 (potential barrier), the forward current I is almost zero, because the potential barrier the potential barrier prevents the flow of electrons and holes across the depletion region from N
and P regions respectively.
For VF >V0, the potential barrier is overcome and current increases rapidly.
Cut-in or threshold voltage: Below which the current is very small and at the cut-
in voltage the potential barrier is overcome and the current through the junction starts to increase rapidly
UNDER REVERSE BIAS CONDITION:
When the negative terminal of the battery is connected to the P-type and positive terminal of the battery is connected to N-type of the PN junction, the bias applied is known as reverse bias.
Under reverse bias condition
- Under reverse bias condition, holes from P side move towards the negative terminal of the battery.
- The electrons from N side are attracted towards the positive terminal of the battery.
- Hence the width of the depletion region increases.
- The resultant potential barrier also increases, which prevents the flow of majority charge carriers in both directions.
- Theoretically, no current should flow in the external circuit.
- But in practice, a very small current flows under reverse bias condition.
- Due to the absorption of energy by the electrons cause the breaking of the covalent bonds.
- This results in the generation of electron-hole pairs.
- The thermally generated charge carriers cross the junction and giving rise to what is known as reverse saturation current.
- For large applied reverse bias, avalanche effect takes place, leading to very large reverse current.
- This leads to the breakdown of the junction.
- Breakdown voltage: The reverse voltage at which the junction breakdown occurs is called breakdown voltage.
V/I characteristics under reverse bias condition
BREAKDOWN IN REVERSE BIASED
Though the reverse saturation current is not dependent on the applied reverse voltage, if reverse voltage is increased beyond particular value, large reverse current can flow damaging the diode. This is called reverse breakdown of a diode. Such a reverse breakdown of a diode can take place due to the following two effects,
1. Avalanche effect and
2. Zener effect
BREAKDOWN DUE TO THE AVALANCHE EFFECT
Though reverse current is not dependent on reverse voltage, if reverse voltage is increased, at a particular value, velocity of minority carriers increases. Due
to the kinetic energy associated with the minority carriers, more minority carriers are generated when there is collision of minority carriers with the atoms. The collision makes the electrons to break the covalent bonds. These electrons are available as minority carriers and get accelerated due to high reverse voltage. They again collide
with another atom to generate more minority carriers. This is called caner
multiplication. Finally large number of minority carriers move across the junction, breaking the p-n junction. These large number of minority carriers give rise to a very high reverse current. This effect is called avalanche effect and the mechanism of destroying the junction is called reverse breakdown of a p-n junction. The voltage at which the breakdown of a p-n junction occurs is called reverse breakdown voltage. The series resistance must be used to avoid breakdown condition, limiting the reverse current.
BREAKDOWN DUE TO THE ZENER EFFECT
The breakdown of a p-n junction may occur because of one more effect called zener effect. When a p-n junction is heavily doped the depletion region is very narrow. So under reverse bias conditions, the electric field across the depletion layer
is very intense. Electric field is voltage per distance and due to narrow depletion region and high reverse voltage, it is intense. Such an intense field is enough to pull
the electrons out of the valence bands of the stable atoms. So this is not due to the collision of carriers with atoms. Such a creation of free electrons is called zener effect which is different than the avalanche effect. These minority carriers constitute very large current and mechanism is called zener breakdown.
The breakdown effects are not required to be considered for p-n junction diode. These effects are required to be considered for special diodes such as zener diode as such diodes are always operated in reverse breakdown condition.
