P-N Junction—Working

A P-N junction is an interface or border between the two types of semiconductor material (p-type and n-type). On the positive side of the semiconductor (p side), there are more holes than electrons, while on the negative side (the n side), there are more electrons.

Formation of p-n junction

In a semiconductor, the P-N junction is created by doping (adding impurities). The doping method is used because if the semiconductor is formed by combining different semiconductor materials, it will include a grain boundary that will scatter holes and electrons and stop the electron movement from one side to the other.

The doping process can be described as follows:

Consider the case of a thin p-type silicon semiconductor. With a small amount of pentavalent impurity to the semiconductor wafer, the region now becomes n-type silicon. The wafer now has both p- and n-type regions separated by an interface or junction. Now, diffusion and drift processes take place.

The electrons from the n side diffuse to the p side due to a difference in electron concentration on one side of the junction and a concentration of holes on the other. The perforations on the p side diffuse to the n side in the same way. As a result, there is a diffusion current across the connection.

On the p side, an ionised acceptor is left behind as the holes from the p side diffuse to the n side and a negative charge layer forms on the p side of the connection. In the same way, as electrons from the n side diffuse to the p side, an immobile ionised donor is left on the n side.

On the n side of the junction, a positive charge layer develops as the process continues. The depletion region is the area on both sides of the intersection that is negative and positive. This causes the production of an electric field that travels from positive to negative charge, causing an electron on the p side of the junction to move to the n side.

P- N Junction Working

When the voltage is increased, electrons move from the n side to the p side of the junction. The movement of holes from the p side to the n side of the meeting occurs simultaneously to the rise in the voltage. As a result, a concentration gradient exists between the terminals on both sides. Further, due to the formation of the concentration gradient, charge carriers will shift from higher concentration regions to lower concentration parts. The current flow in the circuit is caused by the movement of charge carriers inside the p-n junction.

Biassing conditions for the p-n junction diode

In a p-n junction diode, there are two active regions namely the p-type and n-type.

The voltage applied, determines one of the three biassing conditions for p-n junction diodes:

• Forward bias: The p-n junction is said to be forward-biassed when the p-type is connected to the battery’s positive terminal and the n-type to the negative terminal.

• Reverse bias: The p-n junction is reverse-biassed when the p-type is linked to the battery’s negative terminal and the n-type is attached to the positive side.
• Zero bias: The p-n junction diode is not exposed to any external voltage.

V-I characteristics of p-n junction diode

A curve between the voltage and current through the circuit defines the V-I properties of p-n junction diodes. The x-axis represents voltage, while the y-axis represents current. Suppose the V-I characteristics of the p-n junction diode are plotted on a graph; we can see that the diode works in three different zones, which are:

1. Zero bias: No external voltage is delivered to the p-n junction in a zero bias condition, as a result, the potential barrier at the junction prevents current flow. Therefore, when V = 0, the circuit current is zero.
2. Forward Bias: The p-type of the p-n junction forward bias is connected to the positive terminal of the external voltage, while the n-type is connected to the negative terminal.

As a result, the potential barrier is minimised.

The forward V-I characteristic of the p-n junction diode shows that the current grows exceptionally slowly at the beginning. The curve is nonlinear because the external voltage delivered to the p-n junction is used to overcome the potential barrier in this region.

The potential barrier is removed once the external voltage surpasses the possible barrier voltage and the p-n junction functions like an ordinary conductor. As a result, the curve AB climbs significantly as the external voltage rises, which is nearly linear.

3. Reverse bias: The p-type of the p-n junction is connected to the negative terminal of the external voltage, while the n-type is connected to the positive terminal.

As a result, the potential barrier at the intersection is enhanced.

The junction resistance rises to an extremely high level and virtually no current flows through the circuit.

In practice, however, a very modest current of the order of micro-amperes travels across the circuit. Because of the minority carriers in the junction, this is known as reverse saturation current.

Conclusion

A p-n junction is the fundamental component of many semiconductor devices such as diodes and transistors. Understanding the development and operation of a p-n Junction is critical to understanding how semiconductor devices work.