Introduction:

A PN junction diode is an electronic component that allows the flow of electric current only in one direction. For which it is used in the rectification process where the AC signals converted into DC signals.  If you do not know much about it, this article for you. Here, we are going to know the details of the PN junction diode. In this article, you will know

1. What is the PN junction diode?
2. What is Semiconductor?
3. What is an N-type semiconductor?
4. What is a P-type semiconductor?
5. What is the PN junction?
6. What is Depletion region of PN junction diode
7. Reverse and forward biasing of PN junction diode
8. Diode VI characteristics curve
9. Applications of PN junction diode

What is the PN junction diode?

A PN junction diode is a two-terminal electronic component that allows the flow of electric current only in one direction. It allows the flow of electric current only in forward biased condition, in the reverse-biased it blocks the flow of electric current.

PN junction diodes are made with semiconductors such as Germanium(Ge) and Silicon(Si). When a p-type semiconductor is joined with an n-type semiconductor then a PN junction semiconductor is formed.

What is Semiconductor?

The semiconductor is a type of material in which the electrical conductivity of it falling between the conductor( such as copper, gold) and insulator( such as wood, glass). These types of materials are found in Group 14 of the periodic table. In the electronics, the materials such as  Germanium(Ge), Silicon(Si) Gallium arsenide(GaAs) are used as semiconductors.

There are four valence electrons in these types of material's atom and they are capable of bonding with the other four electrons of other material's atom. The energy gap between the conduction band and the valence band is less than the energy gap of the insulator. For this reason, very low energy can pick up the valence electron from the valence bond to the conduction band.

What is an n-type semiconductor?

An n-type semiconductor is a type of extrinsic material that is created by introducing impurity elements that have five valence electrons( Such as Arsenic, antimony, and phosphorus) to a Si or Ge material by the process of doping. Every four electrons of the impurity atoms create four covalent bound with the pure Si atoms.

There is, however, an additional fifth electron due to the impurity atom, which is unassociated with any particular covalent bond. These remaining electron, loosely bound to its parent atom, is relatively free to move within the newly formed n-type material. For this reason, the majority carriers in this type of semiconductor, are electrons and the minority carriers are holes.

What is a p-type semiconductor?

A p-type semiconductor is a type of extrinsic material that is created by introducing impurity elements that have three valence electrons( Such as Boron, Gallium, and Indium) to a Si or Ge material by the process of doping. Every three electrons of the impurity atoms create three covalent bound with the pure Si atoms.

There is now an insufficient number of electrons to complete the covalent bonds of the newly formed lattice. The resulting vacancy is called holes that is the absence of the negatively charged electron. And it will accept a free electron. For this reason, the majority carriers in this type of semiconductor, are holes and the minority carriers are electrons.

What is the PN junction?

The PN junction is formed when both the n-type and p-type materials are simply joined together.

What is the depletion region of a PN junction diode?

When both the n-type and p-type materials are simply joined together the PN junction is formed. Then the density of free electrons in the n-region far exceeds that in the p-region. On the other hand, the density of holes in p-region far exceeds that in n-region. Because of these differences in carrier concentrations, as soon as a PN junction is formed electron from n side diffuse into p-side and holes from p-side diffuse into n side.

The electrons leave behind positively charged immobile donor impurity ion on n side and holes leave behind negatively charged immobile acceptor impurity ion on the p-side. After crossing the junctions from n side the free-electron recombines with holes in p-side.

Similarly, the holes that diffuse from p-side to n-side recombine with free electrons in n-side. As a result of these diffusions and recombinations, unneutralized ions appear in the neighborhood of the junction. The region near the junction which is depleted of mobile charges and contains only immobile ionic charges is called the depletion region.

When the depletion region formed, an internal electrostatic potential barrier is developed across the junction which prevents further electron and holes diffusion. In the equilibrium condition, the internal potential barrier becomes high enough to restrain the process of diffusion.

However, it is to be noted that in equilibrium net current through the junction is zero but the electrons and holes flows do not stop entirely. For example, there are always some minority holes in n-side. If these holes, because of their random motion, move close to the junction they can immediately slide down the potential barrier (i.e., they are swept by the electric field existing within the depletion region) to the p side.

This movement of minority holes constitutes a very small (drift) current. Because of the statistical distribution of velocities, there are always a few majority holes that have enough energy to overcome the potential barrier and move from p-side to n-side.

The magnitude of (this diffusion ) current due to this flow of majority holes is governed by the height of the barrier. In equilibrium, the potential barrier assumes such a height that net current due to the movement of holes becomes zero. Similarly, for electrons, there will be two types of flow: minority electrons moving from p-side to n-side and a few of the majority electrons moving from n-side to p-side.

Again these two flows counterbalance each other and yield zero current due to electrons in an unbiased PN junction. Thus in an open-circuited PN junction, a condition of dynamic equilibrium is set in.

The expression for the width of the depletion region is

$\small&space;W=\left&space;[&space;\frac{2\epsilon&space;kT}{e^{2}}\left&space;(&space;\frac{1}{N_{a}}&space;-\frac{1}{N_{d}}\right&space;)&space;\ln&space;\frac{N_{a}N_{d}}{n_{i}^{2}}\right&space;]^{\frac{1}{2}}$

And t
he expression for the height of the potential barrier across the junction is

$\small&space;V_{b}=\frac{kT}{e}\ln&space;\frac{N_{a}N_{d}}{n_{i}^{2}}$

Where 𝜖 = The absolute permittivity of the medium
e = Charge of the electron
k = Boltzmann constant
T = Temperature
Na = Concentration of acceptor ion in p side
Nd = Concentration of doner ion in n side
The value of the potential barrier for Ge and Si often in the range of 0.5V to 0.7V.

Biasing of PN junction diode

Forward biasing of a PN junction diode

In an unbiased PN junction there exist an intrinsic potential barrier(Vb) which tends to oppose the movement of majority holes from p-side to n-side and that of majority electrons from n-side to p-side. Now if an external voltage (V) is applied in such a way that p-side becomes positive and n-side becomes negative then the intrinsic potential barrier (Vb) is reduced to Vb-V.

The junction is then said to be forward-biased. Now a good number of holes from n-side will be able to overcome this reduced potential barrier and diffuse into n-side. Similarly, a good number of electrons from n-side will be able to cross the reduced barrier and diffuse into the p region.  These two flows constitute a large current in the same direction, called forward current which increases exponentially with the applied voltage.

Reverse biasing of a PN junction diode:

Now if the polarity of the voltage (V) is such that p-side becomes negative and n-side becomes positive, the height of the intrinsic potential barrier is increased to Vb+V. The junction is said to be reversed biased. The increased barrier height makes the current due to the flow of majority carriers negligible.

Minority carriers (electron in p-side and holes in n-side) move down the potential barrier and hence their motion is not affected by the barrier height. They constitute a small current in the reverse direction, called reverse saturation current.

Applications of PN junction diode

1. Rectification: A PN junction diode is allowed easy to flow of charge in one direction but restrains the flow in the opposite direction. It indicates that a PN junction diode can act as a rectifying element for the conversion of AC to DC. So it is used as a rectifier in the electronic circuit.

2. The PN junction diodes are used in the clamping circuit for DC restoration.

3. The PN junction diodes are used in the clipping circuit for waveshaping.

4. It is used in voltage multiplier and voltage regulator.

5. And also it is used in demodulation circuits.

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