state the features of depletion region in a PN junction diode
Answers
Answer:
A depletion region is a natural feature of doping a semiconductor n-type and right next to it p-type. At the p-n junction (the border of the two types), the mobile electrons in the n-type (this is due to the excess electron provided by the n-dopant) diffuse across the junction, into the p-type area. This then leaves the n-type area with a net positive charge, because the electron left.
Holes (the majority charge-carriers in the p-type, created through doping with an element such as Indium (3 valence electrons)) can be said to 'diffuse' toward the n-type. (I think this is because of the electrons from the n-type causing the electrons in the p-type to repel away, and thus travel away from the n-type, filling existing holes and thus creating holes on their way, however this is not important.)
So we have already established that the n-type is left with a positive charge. The electrons which moved to the p-type area from the n-type combined with the holes in the p-type area, and thus leave the p-type with a negative charge.
So, p-type = net negatively charged and
n-type = net positively charged
(The real answer starts here)
When the electron combined with the hole in the p-type, the outer shell of the atom which previously had the hole is now filled, and thus the atom becomes very unreactive, as it has a stable full outer shell. Therefore, when many electrons fill holes, this happens, and so the area becomes very reactive. ALSO, since the p-type is negative, electrons are repelled away.
In the n-type, atoms have always had a full outer shell, but have gotten rid of their free electron. So now, the p-type region and the n-type region both are charged, so repel charge-carriers, and also are unreactive because of their full outer shells. This -freezes- this region, and no mobile charge-carriers exist here. This is called the depletion zone.
NOTE^^^ This only happens towards close to the boundary of the p-n junction, and so the depletion zone is not infinite, it has a defined width/size.
2nd Question (What happens when a bias is applied):
When you connect the p-n junction to an external voltage, effects happen.
But first, we have to confirm: Since the p-type has a negative charge, and the n-type a positive charge, there is ALREADY A VOLTAGE, the natural voltage. A external battery can create another voltage. When the n-type is connected to the NEGATIVE terminal of the battery, and the p-type to the POSITIVE, then the holes in the p-type (being positively charged) move towards the n-type and the electrons in the n-type (being negatively charged) move towards the p-type. Basically, the holes and electrons move towards the p-n junction, which MAKES THE DEPLETION ZONE SMALLER. Another way to think of this is: the negative terminal connected to the n-type repels the free electrons, which as a result move toward the junction. At the p-type, the positive terminal repels the holes, which move towards the junction.
Then, with a large enough voltage, this happens strongly, causing the depletion zone to become small enough to allow charge-carriers to move freely across the junction, allowing current to be conducted!
Reverse Bias:
The above was forward bias. Reverse bias is connecting the battery the other way, so that the n-type is connected to the positive terminal, and the p-type is connected to negative terminal. This strengthens the depletion zone, and increases the resistance (current is more immobilized/can move less). Only with a Huuuuuge voltage, the diode can break down, and then current can be conducted