Semiconductor

In this blog on Semiconductors, we will discuss, Absolute Zero, Intrinsic semiconductor, N-Type Semiconductor, P-Type Semiconductor, Doping Process, Conductor, and Insulators.

Absolute Zero Temperature

Before learning about the Semiconductors and Energy Bands, you need to learn about Absolute Zero.

What is Absolute Zero?

It is nothing but Zero Kelvin, the Absolute value of the temperature.

One Kelvin means -273 degrees Celsius.

Types of Energy Band

• Valence Band
• Conduction Band

The bands which contain zero Kelvin are called valance electrons. The bands with higher energies are called conduction bands.

Valence band:-The valance band is the band in which electrons are normally present at absolute zero temperature. The electron present in that state is valance electrons.

Conduction band:-The conduction band is the band of the electron orbitals. When the electrons get excited, they jump up toward the conduction band. And the electrons in these orbitals have enough Energy to move freely in the material.

The valance electrons are attached loosely toward the nucleus (at room temperature). Some of these electrons that leave the valance band are called free electrons.

Forbidden gap:- it is the gap between the valance band and conduction band. As the name suggests, it prohibits electron flow. The flow of electrons from the valance band to the conduction band flows through the gap.

If the gap is greater, electrons are closely bound with a nucleus, which causes no flow of free electrons. And if the gap is smaller (no gap), electrons are loosely bound with a nucleus, which causes the flow of free electrons.

According to the energy bands, the material can be classified into three categories

• Conductor
• Insulator
• Semiconductor

Conductor

In some materials, the outer electrons of each atom or molecule are weakly bound to it. These electrons are free to move throughout the conductor are called free electrons (conduction electrons)

When such material is placed in the electric field, free electrons start moving in the opposite direction of the electric field. Such material is called the conductor.

The forbidden gap between the valance band and conduction band for a conductor is very less (almost no gap present). Ultimately this causes fast travel of free electrons by making the material conducting.

Insulators

Opposite of the conductor is the insulator. All the electrons are tightly bound with an atom or molecule. Effectively, there are no free electrons. When such material is placed in the electric field, the electron may shift opposite the field but the can’t leave their parent atom and hence can’t move through long distances. Such materials are said as insulators.

The forbidden gap in insulators is very high (5ev). As there are fewer free electrons, electrons cannot reach the conduction band .which makes it insulation because there is no flow of electrons.

Semiconductors

The semiconductor behaves like an insulator at absolute zero temperature, but at a higher temperature, a small number of electrons can free themselves, and they respond to the applied electric field.

As the number of free electrons is less than the conductor, its behavior lies between conductor and insulator. Hence, its name is semiconductor.

The forbidden gap is 3ev in semiconductors, which you can say is medium-range compared to conductor and insulator. As the free electrons are not present at 0K(0 kelvin), we increase the temperature, which creates free electrons, causing the flow from the valence band toward the conduction band.

Intrinsic semiconductors

An example of semiconductor is silicon and germanium, which has four valance electrons. They create a covalent bond with the neighboring atom, making the outermost orbit filled with 8 electrons means it is completely filled. Due to this conductivity of material becomes zero. (It will act as an insulator)

But as soon as we provide some energy, the covalent bond between the atoms/molecules breaks and generates holes and free electrons. The absence of an electron in the covalent band is referred to as a hole. Because of the generation of holes and electrons, it has low conductivity at room temperature.

The energy required to break the covalent bond is 1.1ev for silicon and 0.72ev for germanium at room temperature. The concentration of holes and electrons is the same at room temperature for silicon and germanium, .which is why it intrinsic (pure) semiconductor which provides low conductivity at room temperature.

What is Doping Process?

It is the process in which the impurity is added to the semiconductor to increase its conductivity. It is the principal process to be done for creating the Extrinsic Semiconductor. Without doping, it is in the purest form, and then it can be regarded as an intrinsic semiconductor. In reality, Extrinsic Semiconductors are used more than Intrinsic Semiconductors.

There are two impurities added to the Extrinsic Semiconductors during the Doping process,

• Pentavalent (With Valancy of 5)
• Trivalent (With Valancy of 3)

Extrinsic Semiconductors

The conductivity of silicon and germanium can be increased by the doping process doping to add trivalent or pentavalent impurities. And the semiconductor which is doped becomes an extrinsic semiconductor.

So after reading the above two paragraphs, you must have come to know that semiconductors can be classified as intrinsic (pure) and extrinsic (with impurities)

According to the form of impurity (trivalent & pentavalent), let’s move towards the extrinsic semiconductor. It is again classified into two types,

• P-type semiconductor.
• N-type semiconductor.

 N-Type semiconductor

In the case of pentavalent doping, “Penta” means five, so this impurity contains five valance electrons. Which creates a covalent bond with the silicon atom.

 But are u reading this carefully? Silicon has only four valance electrons, so the fifth electron will be loosely attached to the nucleus. As soon as we provide the energy, this fifth electron will detach. This increases the concentration of electrons (one electron/impurity atom) and decreases the concentration of holes.

The resulting semiconductor, which has a huge concentration of electrons, is referred to as an n-type semiconductor. Since the impurity atom donates excess electrons, it is referred to as a donor or n-type impurity.

Examples of pentavalent impurity is phosphorous(p), arsenic(As), Antimony(Sb)

P-Type semiconductors

In the second case, the trivalent impurity, three valance electrons form covalent bonds with the silicon atom, causing the vacancy of one hole.

This increases the concentration of holes(one hole/impurity atom) and decreases the concentration of electrons. And the resulting semiconductor has a large concentration of holes which is referred to as a p-type semiconductor. Since the impurity atom accepts electrons, it is referred to as an acceptor or p-type impurity.

Examples of trivalent impuritiy is Boron(B), Gallium(Ga), Indium(In),aluminium(Al).

Conclusion

From the above discussion, we conclude that in n-type semiconductors, the electrons are the majority charge carriers, and holes are the minority carriers. Whereas p-type semiconductor holes are majority charge carriers, and electrons are minority charge carriers.

In our next blog, we will learn about the fabrication of devices using an extrinsic semiconductor.  

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