Types of Semiconductor

In Physics, there are certain substances like metals, seawater, graphite etc. that allow free flow of electric current through them. These substances are known as pure conductors. On the other hand, there are certain materials whose composition does not permit electric current to run through them. These are known as insulators. For example cotton, rubber, plastic etc. Semiconductors fall somewhere in between conductors and insulators. These substances somehow function as conductors but do not have the property of supporting the passage of electricity through them as pure conductors do. 

What is a Semiconductor?

Semiconductors or chips are elements that have the characteristics of both conductors and insulators. That means these materials do conduct electricity but not as seamlessly as a pure conductor like copper or iron. Semiconductors are mostly made of silicon – an element that aids in the flow of electricity within the material. The conductive property of a semiconductor can be altered to fit our usage through a process known as ‘doping’. In this method, impurities in small amounts are mixed with pure semiconductors like germanium to find the right ratio as per the requirements. These are widely used in the electrical and electronic industry as they form a major part of almost all electrical devices around us from television to mobile phones.

Semi-conductors can be broadly classified into two categories – intrinsic semiconductors and extrinsic semiconductors. 

What are Intrinsic Semiconductors

Intrinsic semiconductors are also known as pure semiconductors or i-type semiconductors. Silicon and germanium are some common examples of intrinsic semiconductors. These do not have any form of impurities present in them and are formed of only one material. Generally, in intrinsic semiconductors, the number of excited electrons is equivalent to the number of holes, given by n = p. The charge-carrying capacity of this type of semiconductor is dependent on the source material itself. Intrinsic conductors have comparatively poor conducting capacity. The Fermi level of energy lies in the middle of the conduction and valence band in this undoped semiconductor. 

Working of Intrinsic Semiconductor

In the absence of any free electron, intrinsic semiconductors have a conductivity of zero at normal room temperature. However, as the room temperature rises, some electrons are set free from the arrangement which leaves a vacancy known as a ‘hole’ in the position the electrons previously occupied. When an external source of current is passed through the semiconductor, then both the hole as well as the stranded electron help in the current flow as a result of movement, thus giving semiconductors their property. However, the movement of electrons to the conduction band or the possibility of electrical conductivity is also true at normal temperatures. This is given by the Band Theory of Solids. 

What are Extrinsic Semiconductors

Extrinsic semiconductors are non-pure in nature. These have gone through the process of doping and therefore, have certain impurities in the form of aluminium or boron added to it. This process alters the structural composition of the pure silicon or germanium crystal according to the needs of the manufacturer to suit the exact purpose and carry the characteristics it has been built for. The number of holes is not equivalent to the number of electrons in this type of semiconductor. Here, the Fermi level shifts towards either the conduction or the valence band. There are two types of extrinsic semiconductors according to the type of doping agent used. If the doping agent is an electron acceptor then the semiconductor thus formed is known as the p-type. If the doping agent is an electron donor, then the semiconductor formed is called n-type. 

P-type Semiconductor 

During the process of doping (mixing impurities in a pure semiconductor), when elements like aluminium or boron are added to an intrinsic semiconductor then the product formed as a result is known as a p-type semiconductor. These are doped with electron acceptors and because of this, the majority of the charge is carried by the holes. Here, ‘P’ denotes the positive charge of the holes that are formed as a result of the remaining protons in the atom’s hole. P-type semiconductors have more electron holes because of more acceptors and hence, support greater conductivity.  

N-type Semiconductor

If an intrinsic conductor is doped with an electron donor element then the product created is known as an n-type semiconductor. These generally include elements like arsenic and phosphorus. Here, n-type denotes the negative charge of the electron. This is because of the addition of extra electrons provided by the donor element. Opposite to that of the p-type, where the holes are the minority carrier and most of the electric charge is carried by the electrons.

Conclusion

Intrinsic semiconductors are pure in nature. When intrinsic semiconductors are doped with some impurities that alter their conduction properties, they are known as extrinsic semiconductors. Extrinsic semiconductors can be classified as P-type and N-type. P-type semiconductors are doped with an electron acceptor element and n-type are doped with electron donor elements.