Introduction

Before starting with the topic, let’s first understand what semiconductors are.  Semiconductors are materials that have their electric conductivity between conductors and insulators. The properties and conductivity of an intrinsic Semiconductor (pure semiconductor) can be altered by adding impurities; this process of adding impurities into a semiconductor is known as doping. Most common semiconductors can be compounds such as arsenide and gallium or pure elements like germanium and silicon.

What are Semiconductors?

As mentioned earlier semiconductors are intermediate materials that have their conductivity between conductors(metals) and insulators. They are mostly used in devices due to their properties of being compact, reliable, power efficient and cost effective. They are used in electric devices, diodes, transistors and integrated circuits. 

Semiconductors have a small and finite energy band gap due to which some electrons at room temperature escape from the valence band with enough energy to enter into the conduction band which in turn makes the resistance of semiconductors lower than that of insulators. 

Intrinsic Semiconductors

Intrinsic semiconductors are also known as ‘pure semiconductors’. These are substances without the presence of any impurity or doping. 

In intrinsic semiconductors the number of excited electrons and holes are always present in equal amounts i.e, n = p. The electrical conductivity in intrinsic semiconductors is due to crystallographic defects or electron excitation. Using  intrinsic semiconductors in common devices can be problematic as they start conducting at temperatures above the room temperature, this led to the need of finding something better which eventually ended when scientists added impurity to intrinsic semiconductors and a new type of semiconductor, extrinsic semiconductor came to existence. 

Elements like germanium and silicon are the best examples of naturally occurring intrinsic semiconductors. 

In silicon and germanium crystalline structure they tend to share one valence electron of its own and take an electron from their neighbouring atoms. And as the temperature around the crystalline structure is increased, some of these shared electrons break free and tend to behave like conduction electrons. This in turn creates a vacancy in the bond of the crystalline structure and in a result a positive charge  is created which is known as ‘hole’. This newly formed hole eventually behaves as a free charge similar to that of an electron and thus contributes to the overall conduction of the structure. 

It is important to note here that the holes in an intrinsic semiconductor do not actually move in a semiconductor, and the movement created by electrons from valence band to conduction band and to neighbouring atoms is wholly viewed as ‘movement of hole’, and mobility of holes in actual is much lesser than that of electrons.

Extrinsic Semiconductors

These are semiconductors that are prepared by adding a sufficient amount of impurity. Even a few parts per million (ppm) impurity can increase the conductivity of extrinsic semiconductors to many times.

It is important to note that while doping the impurities added in extrinsic semiconductors should not change its lattice structure, and this can be achieved by making sure that the dopant and the size of the atom is the same. 

Dopants used in Extrinsic Semiconductors

The dopants used in the Crystals of Silicon and Germanium are of the following two types:

1. Trivalent (valency 3); like Indium (In), Boron (B), Aluminum (Al), etc.
2. Pentavalent (valency 5); like Arsenic (As), Antimony (Sb), Phosphorous (P), etc.

Majority carrier in extrinsic semiconductors

N-type semiconductors are a part of extrinsic semiconductors as they are made by adding impure substances of valence 5 (As) with pure intrinsic semiconductors. 

Although the majority charge carrier in an n-type semiconductor is ‘electron’ and minority charge carrier is ‘hole’, the material as whole is still electrically neutral as the electrons are produced by adding neutral pentavalent impurity atoms. Thus there’s eventually no addition of either a positive charge or a negative charge. 

Unlike the n-type semiconductors, in p-type the doping is done in valence 3(B), and majority charge carriers are ‘holes’, and electrons are present in minority. 

Power semiconductor devices

• Power devices are used as switches or rectifiers in power electronics. These are usually used in commutation mode in either on or off mode. They handle or process large currents and power. 
• Power devices most commonly used in the power MOSFET, power diode, thyristor, and IGBT. Although power devices work on similar principles of their low power counterparts, unlike them power devices can carry a much larger current and can even withstand much larger reverse-bias voltage in the off-state.
• The small signal power devices are made with lateral structure, however, the majority of the non-integrated or discrete devices are normally built in a vertical structure which in turn makes the current rating of the device proportional to its area and thus giving it a voltage blocking capability. These structural changes in the power devices are made to accommodate the high current densities, high power dissipation and high reverse breakdown voltage.

The most commonly used semiconductors

• The most commonly used semiconductors worldwide are Gallium arsenide, germanium, and silicon
• Although germanium and silicon both are very commonly used in semiconductors, but today silicon is a much preferred material than germanium due to following reasons:

• Silicon can bear much  higher temperature than that of a germanium.
• The leakage current in Silicon is much smaller than that of germanium.

Applications

Some of the most common everyday life Applications of Semiconductors are: 

• Semiconductors are used in Temperature sensor devices  
• Semiconductors are used in 3D printing machines
• Semiconductors are used in microchips and self-driving cars
• Semiconductors are used in calculators, solar plates, computers and other electronic devices.
• Semiconductors are used in transistors and MOSFET which are then used in switches in Electrical Circuits.

Conclusion: 

Semiconductors are devices that have conduction in between that of a conductor and insulators and are used in everyday life in many devices like transistors, zener diodes, solar panels, switches, electric circuits, etc. Semiconductors are divided into two types, intrinsic semiconductors, and extrinsic semiconductors. Intrinsic semiconductors are pure semiconductors and these are substances without the presence of any impurity or doping. Extrinsic semiconductors are semiconductors that are prepared by adding a sufficient amount of impurity.