PN Junction Diode Construction Working And Uses

Construction-

On a wafer of n-type silicon(usually silicon + phosphorus), an aluminium film is placed and then they are heated at a high temperature. This results in the diffusion of aluminium into silicon and thus our PN junction diode is ready to be used in various semiconductor devices. It can also be prepared by diffusing phosphorus into a p-type semiconductor.

Working of PN junction diode-

Let’s consider an unbiased diode(diode not connected to any voltage source). On seeing the diagram, we observe that the holes are the majority carriers on the left side(p-type) whereas electrons are the majority carriers on the right side(n-type).

Thus, to distribute the concentration over the diode, holes start diffusing towards the right whereas electrons diffuse towards the left. This generates a current flowing from left to right[p-type to n-type] (as direction of current is always OPPOSITE to the direction of flow of electrons and same as the direction of movement of holes). This current is known as DIFFUSION CURRENT.

Therefore,

Idf=Ie+IhIdf=Ie+Ih

i.e. Diffusion current = current due to flow of electrons + current due to movement of holes.

We know that both p-type and n-type semiconductors are neutral initially. Thus, the diffusion of holes and electrons causes an excess positive charge in the n-region and an excess negative charge in the p-region. Thus the system becomes UNSTABLE.

This double layer of charge creates an electric field(direction n-type to p-type[right to left])[because the direction of field is always away from positive charges(right) and towards negative charges(left)]. This field exerts a force on the electrons and holes, against their diffusion. Thus, a potential difference is created in the junction diode with n-region at a higher potential than the p-region. This potential is known as barrier potential. The junction region is now almost void of charge carriers and thus it is known as depletion layer.

When an electron-hole pair is created in the depletion region, the electron is pushed by the barrier electric field towards the n-side and the hole is pushed towards the p-side and this gives rise to a current from n-side to p-side(right to left). This current is known as DRIFT CURRENT and its direction is opposite to that of diffusion current. Thus for an unbiased diode,

Idf=IdrIdf=Idr, i.e. Inet=0Inet=0

Thus, no current flows through an unbiased diode.

Now let’s discuss the working when the diode is FORWARD BIASED.

The diode is in forward bias mode when the positive and negative terminals of a DC voltage source is connected to p-side and n-side of the diode respectively.

This increases the potential at the p-side and decreases the potential at the n-side of the diode. This potential balances the barrier potential which is generated internally in the diode. Thus, the diffusion of electrons and holes is thereby increased and current flows through the semiconductor. As the barrier potential decreases due to the external potential, the depletion layer also becomes smaller.

Therefore when the diode is forward biased,we can conclude that,

Idf>IdrIdf>Idr

and also that the net current flows from p-side to n-side.

The working of REVERSE BIASED junction diode is exactly opposite of that of forward biased.

The diode is in reverse bias mode when the positive and negative terminals of a DC voltage source is connected to n-side and p-side of the diode respectively. This increases the potential at the n-side and decreases the potential at the p-side of the diode. This potential drop is in the same direction as that of barrier potential drop. Therefore, the diffusion of electrons and holes is almost stopped and thus no current flows through the semiconductor.

As the barrier potential increases due to the external potential, the depletion layer also becomes wider.

Therefore when the diode is reverse biased,we can conclude that the net current flows from n-side to p-side as

Idr>IdfIdr>Idf 