Introduction to Semiconductors

As far as we are concerned with electrical/electronics, materials can be classified into three basic types as, (i) conductors, (ii) semi-conductors and (iii) insulators. Materials having low resistivity are called as conductors and those having very high resistivity are called as insulators. In simple words, good conductors of electricity (such as copper) are called as conductors and non-conductors (such as paper) are called as insulators. There are some materials, which are not good conductors of electricity neither good insulators, such materials are called as semiconductors.

Definition of Semiconductor: A semiconductor is a material whose electrical conductivity (and even resistivity) lies between that of conductors and insulators. For example: silicon, Germenium.

Thinking at atomic level, semiconductors are those materials having four electrons in their outermost shell. It needs eight electrons in the outermost shell for any atom to be stable. Here, in semiconductors, four electrons in the outermost shell of each neighboring atom are shared in such a way that, each atom effectively has eight electrons in their outermost shell. The arrangement is as shown in the figure. This sharing forms a co-valence bond and thereby leaving no valence electrons for conduction.

Importance of Semiconductors

The conductivity of a semiconductor increases with increase in the temperature. The conductivity of a semiconductor can be easily modified by controlled addition of impurities or by some other means. The process of addition of impurities is called as doping which is explained further in this article. In this way, a semiconductor actually can be a good conductor or even a good insulator. Also, combining different types of semiconductors together gives a device with special electronic properties. These all properties of semiconductors make them very important. They are used to manufacture various electronic devices like diodes, transistors, integrated circuits etc.

Types of Semiconductors

Semiconductors can be classified in two main types as (i) intrinsic semiconductor and (ii) extrinsic semiconductor.
Covalent bond of intrinsic semiconductorFigure 1: Covalent bond of intrinsic semiconductor

An intrinsic semiconductor is naturally occurring semiconductor. It possesses poor conductivity as there are no free electrons in outermost shell (because of covalent bond as stated above). Hence, an intrinsic semiconductor alone is of no use.

An extrinsic semiconductor is derived by adding impurities in an pure intrinsic semiconductor. The process of addition of impurities is called as ‘doping‘. An extrinsic semiconductor can be further classified on the basis of do
ping as (a) N-type and (b) P-type semiconductor. Addition of group-5 element (having 5 electrons in outermost shell) in a semiconductor gives N-type. For example, addition of arsenic in silicon – there are five electrons in the outermost shell of arsenic, four of which form covalent bond with silicon atoms and the fifth one is left free. Hence, free electrons are majority charge carriers in an N-type semiconductor. On the other hand, addition of group-3 element (having 3 electrons in outermost shell) give a P-type semiconductor. For example, addition of Aluminium in silicon – there are three electrons in the outermost shell of Aluminium, they for covalent bonds with silicon atoms but fourth electron is absent for the formation of fourth covalent bond. Thus a hole is left in covalent bond structure. Hence, free holes are majority charge carriers in a P-type semiconductors.