PN Junction Diode and its Operation | Forward Bias and Reverse Bias Operation | Forward Current
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#crystal diode
#diode working
#diode
#Forward Biasing of PN junction diode
#Reverse Biasing of PN junction diode
#Hole Current
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#Forward Biasing
#Reverse Biasing
#barrier potential
#forward voltage drop
#reverse saturation current
In this video, we will learn about PN junction diode.
A PN junction forms a popular semiconductor device called PN junction diode or crystal diode.
It is a two-terminal device, namely anode and cathode. ‘Die' means two, and it has two electrodes. The word ‘Die’ and, last four alphabet, from electrode gives us the name diode.
The p-region will act as anode and, n-region acts as cathode.
The outstanding property of a PN junction diode is, it will conduct only in one direction. This property helps us to perform rectification operation.
The symbol of the diode is shown in figure. The arrowhead in the symbol indicates the conventional current flow direction, under forward biased condition.
Let us see the operation of PN junction diode.
Applying external DC voltage to any electronic device is called biasing.
As we discussed, there is no current flow in unbiased p-n junction.
Under biasing, it allows current flow only in one direction.
Biasing is classified as forward biasing and reverse biasing.
If the positive terminal of external DC voltage is connected to anode of the diode, and negative terminal is connected to cathode terminal of the device, then this type of biasing is known as forward biasing.
If the positive terminal of external DC voltage is connected to cathode of the diode, and negative terminal is connected to anode terminal of the device, then this type of biasing is known as reverse biasing.
If a sufficient external voltage that is, forward bias voltage is applied to the diode, the free electrons and holes will cross the PN junction. To achieve this, the external voltage should be more than the barrier potential.
Under forward biased condition, the negative terminal of the external voltage, pushes free electrons, against barrier potential from ‘n’ region to ‘p’ region.
Similarly, the positive terminal of the external voltage, pushes holes, against barrier potential from ‘p’ region to ‘n’ region. These holes and electrons, will cross the PN junction against the barrier potential.
The depletion layer around the PN junction, is decreased under the forward biased condition.
If the forward bias voltage is increased, at a particular value, the depletion region becomes very much narrow because of, the raise in majority carriers cross the junction.
These large number of majority carriers, constitute a current called forward current.
Once the conduction electrons, enter the p-region, they become valence electrons. Then they move from hole to hole, towards the positive terminal of the battery.
The movement of electrons is nothing but, movement of holes in opposite direction, to that of electrons in the p region.
So current in the p region is, the movement of holes which are majority carriers. This current is called as hole current.
Similarly, current in the n region, is the movement of free electrons which are majority carriers. This current is called as electron current.
Hence overall forward current is, due to majority charge carriers. These majority charge carriers can then travel around the closed circuit and, a relatively large current will flow.
Theoretically no current should flow in the external circuit. But in practice, a very small current of the order of a few microamperes flows under reverse bias.
Under the reverse bias condition, the thermally generated holes, in the P-region are attracted towards the negative terminal of the applied DC voltage, and the electrons in the N-region, are attracted towards the positive terminal of the applied DC voltage. Consequently, the minority carriers, that is electrons in the P-region and holes in the N-region, roam over the junction and, flow towards their majority carrier side giving rise to a small reverse current. This current is known as reverse saturation current.
The magnitude of reverse saturation current, mainly depends upon junction temperature because, the major source of minority carriers is, thermally broken covalent bonds.
In the next video, we will discuss the reverse breakdown of the diode.
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