We will discuss now, working principle of diode
An n-type semiconductor block will have a significant number of free electrons, and
very little number of holes. Holes present in n-type semiconductor due to generation of electron hole pairs mainly
caused by thermal excitation
A p-type semiconductor block will have a significant number of holes, and very little number of
free electrons. Free electrons present in p-type semiconductor due to generation of electron hole pairs mainly
caused by thermal excitation. Let us create a simple p-n junction diode
Let us join an n-type, and a p-type material together. The majority carriers in n-type material are free electrons
The majority carriers in p-type material are holes. Let us see, what happens, at the junction of these two materials
Adjacent to this junction edge, free electrons in the n-type material will recombine with
holes in the p-type material. As a result there will be lack of free electrons, and holes across this region
This region is called depletion region. We know that, an n-type material is electrically neutral, because, the number of free electrons
and positive donor ions are same in an isolated n-type material. Similarly, a p-type material is electrically neutral, because the number of holes, and
negative acceptor ions are same in an isolated p-type material. After absorption of free electrons in n-type side of in-depletion region, uncovered positive
donor ions will remain there
After absorption of holes in p-type side of in-depletion region, uncovered negative acceptor
ions will remain there
This region of uncovered positive, and negative ions is called the depletion region due to
the, depletion, of free carriers in the region. Due to the presence of these positive, and negative ions, a static electric field called
as, barrier potential, is created across the junction. It is called as, barrier potential because, it acts as a barrier, and opposes the further
migration of holes, and electrons across the junction. Now, we connect lead to the ends of each material
We made the basic p-n junction diode. Now, we connect positive terminal of the voltage source to the lead connected to p-type side
And, we connect negative terminal of the voltage source to the lead connected to n-type side
In this condition, diode is said to be in forward biased condition
We already told that, there is a barrier potential across the junction
This barrier potential acts in the opposite of the forward applied voltage
So, there is no forward current through the diode unless the magnitude of the applied
voltage becomes more than that, of barrier potential. This voltage we refer as, forward biased voltage
For silicon, forward biased voltage is 0.7 volts. For germanium, forward biased voltage is 0.3 volts
If we apply more forward voltage than forward biased voltage of the diode, the diode behaves
as closed switch. Now it gives rise to an uncontrolled forward current through the diode
There is no more voltage drop across the diode beyond this forward biased voltage
We can only limit the current by connecting external resistances series with the diode
Now, we connect negative terminal of the voltage source to the lead connected to p-type side
And, we connect positive terminal of the voltage source to the lead connected to n-type side
In this condition, diode is said to be in reverse biased condition
Due to attraction of positive potential, the free electrons in n-type region go away from
the junction. Due to attraction of negative potential, the holes in p-type region go away from the junction
This makes the depilation layer of the junction wider with increasing reverse biased voltage
In this situation, the current does not flow through the diode
This is because, free electrons in n-type region, and holes in p-type region cannot
cross this wide depletion region due to electrostatic force of the applied reverse voltage
As we told earlier there are always some thermally generated holes in n-type semiconductor
and free electrons in p-type semiconductor. We call these charge carriers as minority charge carriers
These minority charge carriers also present in depletion region. Due to electrostatic force of applied reverse biased voltage, these minority carriers will
smoothly flow through the diode. Because, the polarity of reverse biased barrier potential does not oppose the flow of holes
from n-type to p-type region, and free electrons from p-type to the n-type region
As the number of minority carriers is very few, a tiny current flows from n-type end
to p-type end in the diode. This current is called reverse saturation current
If we ignore this tiny current, we can say there will be no current flowing through the
diode in reverse biased condition. So, we have seen there are significant current through the diode when it is in forward biased
It blocks current when it is reverse biased. Hope, you got an idea
Thank you