DAY 4 OF ELECTRONICS

SEMICONDUCTOR DIODES

P-N diode, from the name itself we can understand that it contains a P-type and an N-type semiconductor.

In last session we discussed about the N-type and P-type semiconductor, by using those semiconductor we can construct P-N diode.

But it should not formed by joining the pieces of P-type and N-type because forming like this will lead to discontinuity in the structure.

So it should be done within a single crystal by adding P-type impurities in one side and N-type impurities in other side.

The boundary between P-type and N-type material is called P-N junction.

In a P-N diode, majority of holes & minority of electrons exist in P-side. Similarly majority of electrons & minority of holes exist in N-side.

Let us see the working of P-N diode under biasing and non biasing conditions........

  

UNBIASED P-N JUNCTION:

The state without any external potential is known as unbiased state (ordinary P-N diode at the initial stage).

In this state holes in the P-region cross the junction and enter N-region where there is scarcity of holes.

Similarly electrons in the N-region cross the junction and enter P-region where there is scarcity of electrons.

When a hole from P-region crosses the junction it recombines with free electrons (donor ion) in N-region then free electron vanishes.

In this manner, a large number of uncovered positive charges produced near the junction in N-region. The donor ion is called an uncovered positive charge.

In the same way, when a electron from N-region crosses the junction and recombines with holes (acceptor ion) in P-region.

Likewise, a large number of uncovered negative charges produced near the junction in P-region. The acceptor ion is called an uncovered negative charge.

This process continues, after some time the transportation of charges stops due to negative electric field in the P-region and positive electric field in the N-region.

The region near the junction is depleted hence it is called as depletion region or depletion layer.

The depletion region contains immobile oppositely charged ions called as barrier potential.

This is the functionality under unbiased condition.

BIASED P-N JUNCTION:

Biasing of diode involves two kinds,

            Forward Bias                                                       

            Reverse Bias

FORWARD BIASED DIODE:

If the positive terminal of the voltage source is connected to the P-side and the negative terminal of the source is connected to the N-side of the diode, then the diode is said to be forward biased.

As the holes are positively charged, they are repelled by the positive terminal of the battery. Likewise the electrons are negatively charged they are also repelled by the negative terminal of the battery.

Therefore, under forward bias condition majority carriers in both the sides of P-N junction are forced to move towards the junction.

Under influence of external potential, these majority carriers acquire higher energy and enter into the depletion region.

This reduces the effect of barrier potential because carriers acquire higher than this.

MECHANISM OF CURRENT FLOW:

When external voltage is connected, majority carriers in both the side cross the junction and recombination of electron-hole pairs takes place.

Two different operations take place in P-side and N-side. Let us see that one by one...

In N-region, for every recombination one electron vanishes and one electron enters in to the same region. This electron moves towards the junction due to negative potential.

In P-region, for every recombination one hole vanishes at the same time one covalent bond breaks it produces an electron and a hole. So the produced hole move towards the junction and the electron moves towards the positive terminal of the battery.

When an electron reaches positive terminal, a new electron produces in the negative terminal.

So under forward bias there will be a continuous flow of electron from negative terminal to positive terminal through the diode, because of this there will be large current flow.

This is the working of diode under forward bias condition.

REVERSE BIASED DIODE:

If the negative terminal of the voltage source is connected to the P-side and the positive terminal to N-side of the diode, then the diode is said to be reverse biased.

It’s working also little bit similar to that of forward bias....

The holes in the P-region get attracted by negative terminal also the electrons in the N-region attracted by positive terminal.

Therefore under the reverse bias condition the width of the depletion region increases as a result of this number of immobile charges in the depletion region increases, so that the barrier potential decreases.

MECHANISM OF CURRENT FLOW:

As the barrier potential increases there will be no current flow across the junction due to majority carriers.

But in practical case there will be a little current flow due to minority carriers in it.

This is the working of diode under reverse bias condition.