When a p-type semiconductor is suitably joined with n-type semiconductor then the surface is called the pn junction.

Formation of PN junction

PN junction is fabricated by special technique known as alloying. In this method, a small block of indium (trivalent impurity) is placed on a n-type germanium slab. The system is then heated to a temperature about 5000 C. The indium and some of the germanium melt to form a small paddle of molten germanium-indium mixture. Then the temperature is lowered. Under proper conditions, the atom of indium impurity will be suitably adjusted in the germanium slab to form a single crystal. Thus a pn junction is formed.

Properties of PN Junction

When the PN junction forms, the free electron near the junction began to diffuse the holes near the junction. So a depletion layer is formed. The term depletion is due to the fact that near the junction, the region is depleted (or emptied) of charge carriers due to the diffusion across the junction.

The depletion layer is formed quickly and very thin compared to n-region and p-region. Once pn junction is formed and depletion layer created the diffusion of free electron stops. There exists a potential difference across the depletion layer and known as barrier potential.

Introduction to Biasing
In relation to a pn junction, there are following two bias condition
1. Forward Bias
2. Reversed Bias

Forward Biasing
When external d.c. voltage applied to the junction is in such a direction that it cancels the potential barrier, thus permitting current flow, is called forward biasing.

To apply forward bias, we have to connect positive terminal of the battery to p-type and negative terminal to n-type. The applied forward potential establishes an electric field which acts against the field due to potential barrier. Therefore the resultant field is weekend and barrier height is reduced at the junction. As potential barrier voltage is very small therefore a small forward voltage is sufficient to completely eliminate the barrier. Once the potential barrier is eliminated, junction resistance becomes almost zero and a low resistance path is established for the entire circuit. This is called forward current.

Reverse Biasing
When the external d.c. voltage applied to the junction is in such a direction that potential barrier is increased, it is called reverse biasing.

To apply reverse bias, we have to connect negative terminal of the battery to p-type and positive terminal to n-type. The applied reverse potential establishes an electric field which acts same direction as the field due to potential barrier. Therefore the resultant field is strengthened and barrier height is increased at the junction. As potential barrier voltage is much therefore prevents the flow of charge carriers across the junction. Thus, a high resistance path is established for the entire circuit and hence current does not flow.

Volt-Ampere (V-I) Characteristics of pn Junction
Volt-ampere or V-I characteristics of a pn junction is the curve between voltage across the junction and the circuit current. The characteristic is given below

Some Important Terms of pn Junction

Breakdown Voltage
It is the minimum reserve voltage at which pn junction breaks down with sudden rise in the reverse current.

Knee Voltage
It is the forward voltage at which the current through the junction starts to increase rapidly.

Maximum Forward Current
It is the highest instantaneous forward current tat a pn junction can conduct without damage to the junction.

Peak Inverse Voltage (PIV)
It is the maximum reverse voltage that can be applied to the pn junction without damage to the junction.