P N Junction - Meaning, Formation, Applications, FAQs

What is PN junction diode?

Junction meaning: A pn junction is defined as a boundary between the two type of semiconductor materials which are p-type and n-type in a single crystal of semiconductor. P-type semiconductors are those semiconductors which are formed by the doping of trivalent element and has holes as the majority charge carriers. N-type semiconductors are those semiconductors which are formed by the doping of pentavalent elements and has electrons as the majority charge carriers.

Formation of PN junction

When n-type and p-type semiconductors are joined to form a pn diode junction, there are several processes which occur in it.

We know that the concentration of electrons is high in n-type semiconductors as compared to p-type semiconductors and the concentration of holes is high in p-type semiconductors as compared to n-type semiconductors.

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When an electron diffuses from n to p, it leaves behind an ionized immobile positive ion which get accumulated at the n-side of the junction. Similarly when a hole diffuses from p ? n ,it leaves behind an ionized immobile negative ion which get accumulated at the p-side of the junction. This charge region on both sides of the junction is called depletion region.

Due to this positive charge region on n-side and negative charge region on p-side of the junction, an electric field directed from positive charge to negative charge develops. Due to this electric field electrons on the p-side moves to the n-side and hole on the n-side moves to p-side. This motion of charge carriers due to the electric field is known as drift current.

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What is PN junction diode working?

A pn junction diode is nothing but a pn junction provided with metallic contacts at both the ends which are used to supply external voltage to the junction. It is a two terminal device. he pn junction diode and its symbolic representation is shown below.

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Biasing in PN junction diode

Biasing is a process of connecting the pn junction diode to the battery in order to provide the external input voltage. There are two different types of biasing methods depending upon which of the two terminals of the p-n junction are connected to which of the two sides of the battery.

1. Forward Bias: When an external voltage V is applied across pn junction diode such that p-side of the pn junction is connected to positive terminal of the battery, n-side of pn junction is connected to the negative side of the battery, then the pn junction is forward biased.

2. Reverse bias pn junction: When an external voltage V is applied across a pn junction diode such that the p-side of the pn junction is connected to the negative terminal of the battery and n-side of pn junction is connected to the positive side of the battery, then the pn junction is reverse bias pn junction.

Vi characteristics of diode of pn junction

The Vi characteristics of diode of pn junction is shown below. It represents the change in current with respect to change in voltage or vice-versa. The x-axis represents voltage and the y-axis represents current. The VI characteristics of pn junction can be explained according to three

Picture source :- Class 12th physics NCERT (Part-2)

Condition :

Zero bias ? In this condition there is no external voltage is applied, hence the potential barrier is high and the circuit current is 0.

Forward bias ?In the forward bias condition, the external voltage is applied which results in reduced potential barrier and hence the current flows in the circuit.

Reverse bias pn junction ? In the reverse bias pn junction condition, the external voltage is applied but this time potential barrier increases. A small amount of reverse saturation current flows but on increasing the applied voltage, the diode will break.

NCERT Physics Notes:

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Applications of PN junction diode

There are some of the common and important application of pn junction diode given below:

• It has the potential to be used as a solar cell.

• When the pn junction diode is reverse-biased, it can be used as a photodiode because it is sensitive to light.

• In forward-biased condition, the pn junction diode can also be used in LED lighting applications.

• It is used in many electric circuits as a rectifier because it allows the current to flow only when the circuit is forward biased.

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