Semiconductors are materials that have a conductivity between a conductor (metals) and a non-conductor or insulator. 

Semiconductors can be pure elements, which include silicon or germanium, or maybe they are some other compounds, including alloys such as gallium arsenide or cadmium selenide.

Semiconductors that have many negative charge carriers than positive charge carriers comprise N-type semiconductors. Semiconductors that have a large number of charge carriers as holes are P-type semiconductors.

There are two bands—conduction band and valence band. The conduction bands have electrons in the innermost shell, which may take part in the process of conduction. In the valence band, the electrons are present in the outermost shell, or the valence electrons are present.

In intrinsic and semiconductors (pure silicon), a small fraction of valence electrons is able to reach the conduction band. On reaching the conduction band, these small amounts of valence electrons can drastically change the conduction properties of the semiconductor.

PN junction meaning

PN junction is the boundary between two types of semiconductor material ( one is n-type, and another is p-type). The P side represents the positive side and has an excess of holes, whereas the n side represents the negative side which has an excess of electrons.

Formation of PN junction

A P-N junction is a basic semiconductor device. When the P-type side is placed in contact with the N-type side to form a single piece, the assembly obtained is called a junction diode.

The potential difference developed across the junction due to the migration of majority charge carriers (positive or negative) is called potential barrier.

Forward Biased PN junction

When an external DC source is connected to the junction diode with the P section connected to the positive pole and the N section connected to the negative pole, the junction diode is said to be forward-biased.

Reverse Biased PN junction

The PN junction is said to be reverse-biased when the positive terminal of the external battery in the circuit is connected to the N section, and the negative terminal is connected to the P section.

The flow of the minority carriers across the junction from both the sections of the junction diode is responsible for reverse current.

Characteristics of PN junction diode in forward bias

• When the positive terminal or end of the external battery is connected to the positive pole or the positive side, and the negative terminal is connected to the negative pole or negative side, then the potential difference is in the opposite direction to the barrier potential. 
• When the PN junction is forward biased, the applied positive potential repels the holes in the P region or positive region, and the applied negative potential repels the electron in the negative region, so the charge moves towards the junction.
• As soon as the forward bias becomes greater than the potential barrier of the junction, the forward current increases linearly.
• In forward bias, when the current increases linearly and rapidly over the knee voltage, there is a variation in current, with the increase of forward bias being extremely slow below the knee voltage.
• In forward bias in PN junction, the depletion layer becomes thin because the polarity of the external DC source opposes the fictitious battery that developed the junction.
• It results in the potential drop across the junction and makes the depletion layer thin, which, at a loss, leads to the low resistance of the junction diode.

The action of the P-N junction 

A p-n junction diode is forward biased when the positive side and the negative side are connected to positive and negative terminals of the battery, respectively.

In forward biased PN junction diodes, the electrons travel from the negative terminal of the battery into the inside of the diode, and it crosses over the P site to combine with the holes. Then electrons are emitted from the P side p positive terminal of the battery. This produces more free holes that can travel over the inside, and then it combines with the electrons. Thus, the depletion layer, along with the barrier potential of the junction, decreases and then decreases the overall resistance of the diode.

Conclusion

In a PN junction, when forward biased is increased even slightly beyond the safe limit, it will get damaged. It is because of the large number of charge carriers in forward current. Although charge carriers acquire very small kinetic energy and can produce a very small amount of heat, the total heat generated by all carriers is good enough to rupture the covalent bond. According to our PN junction notes, when a PN junction is forward biased, the width of the depletion layer decreases; as a result, it offers low resistance during forward bias. 

The flow of majority carriers across the junction from both the sections of the junction diode is responsible for forward current.