Basics of Semiconductor—Definition

This chapter will discuss semiconductors along with their workings and principles. We will study two types of semiconductors namely intrinsic and extrinsic semiconductors. Semiconductors are a type of crystalline solid with conductivity between conductors and insulators, helping electricity to flow through them. This crystalline solid is conductive due to impurities present in it, or it might become conductive due to a change in the temperature. This phenomenon of adding impurities is known as doping. Extrinsic semiconductors are especially crucial to electronics. 

Basics of semiconductors

As mentioned earlier, semiconductors are intermediate materials with conductivity between conductors (metals) and insulators. They are mostly used in devices as they are 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. This in turn makes the resistance of semiconductors lower than that of insulators. 

Types of semiconductors

There are mainly two types of semiconductors:

1. Intrinsic semiconductors
2. Extrinsic semiconductors

Intrinsic semiconductors

Intrinsic semiconductors are conductive due to their surrounding temperature. At room temperature, intrinsic semiconductors have low conductivity. However, due to the aforementioned properties, intrinsic semiconductors are unstable in electronic devices.

In intrinsic semiconductors, there are no other intentionally doped materials (i.e. impurities such as Silicon (Si) or Germanium (Ge)). 

Atomic number and electronic configuration of Si and Ge are as follows:

The above table indicates that Si and Ge have four remaining electrons in their outermost orbitals, i.e. their valence shells. These electrons are called valence electrons and are responsible for the conductive properties of these semiconductors.

What are intrinsic semiconductors?

Intrinsic connotes to inherent or natural. Intrinsic semiconductors display features of semiconductor substances themselves, not impurities or dopants. As Si and Ge are elemental semiconductors,  they are most widely used as intrinsic semiconductors. They were among the first semiconductors to be widely researched and utilised. The distinctive features of semiconductors are possible due to their electrical structure, which distinguishes them as a peculiar kind of material.

Intrinsic semiconductors—also referred to as pure or undoped semiconductors—are flawless semiconductor crystals that are free from the flaws and impurities of other elements. All semiconductor materials, including those doped with other components, have intrinsic features as the doping components introduce the desired qualities.

What is fermi energy?

Fermi energy is in the band hole (Ec + Ev)/2 in addition to a little remedy that relies directly upon the temperature. The remedy term is little at room temperature since Eg ~ 1 eV while kBT ~ 0.025 eV.

Extrinsic Semiconductors

These are semiconductors that are prepared by adding sufficient amounts of impurities. Even adding them as a few parts per million (ppm) can exponentially increase the conductivity of extrinsic semiconductors.

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

Dopants used in Extrinsic Semiconductors

The dopants used in the crystals of Si and Ge can be classified as,

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

Majority carriers 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 an ‘electron’ and the minority charge carrier is a ‘hole’, the material as a whole is still electrically neutral as the electrons are produced by adding neutral pentavalent impurity atoms. Therefore, there is no addition of either a positive or negative charge. 

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

The most commonly used semiconductors are

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

• Silicon can bear a much higher temperature than that of 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 as follows: 

• 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 for electrical circuits

Conclusion

Semiconductors are one of the most crucial topics of electronics. Semiconductors can be classified into two types, extrinsic and intrinsic semiconductors. The doping in the semiconductors increases their conductivity. Extrinsic semiconductors are further classified into two types, i.e. N-Type and P-Type semiconductors. The extrinsic semiconductors become intrinsic at very high temperatures as the band to band transition dominants over impurity ionisation.