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Electrical resistivity is a term used to describe the extent to which a material resists the flow of electric charges through it. Alternatively, electrical conductivity is the ability of a material to conduct electricity and is given as the reciprocal of resistivity.  A material that has very low electrical conductivity is said to be a conducting material, e.g, metals. A material that completely resists the flow of electric current through it, is called an insulator, e.g., plastics. There is another class of materials called as the semiconductors that possess electrical conductivity values in a range intermediate to that of conductors and insulators, e.g., silicon, germanium etc,. the properties of semiconductors pertain to solid state conduction. Semiconductors are an integral part of electronics.

There are numerous devices that can be fabricated using semiconductors. A few examples include transistors, diodes and solar power cells.

Energy band theory:

The behavior of semiconductors is explained using the energy band theory. Electrons are known to possess energies that are by virtue of the energy level in which they exist. At the ground level electrons possess inherent energy to be bound tightly to atoms in the material. It is also known that electrons can be raised to a higher level of energy by way of supplying external energy in terms of heat. Such a process is called excitation and it is possible to excite an electron to a level where it is displaced from its atom and exists as a free electron. The amount of energy required to convert a bound electron into a free electron is called the excitation energy and depends on the type of atom.

Such energy levels exist in the form of energy bands. The ground level band is termed as the valence band and is composed of electrons that fulfill the valency of the atom. There exists a higher energy level above the valence band called the conduction band where electrons can exist as free electrons and are hence capable of movement. Moving charges can be conducted across a medium to produce electric current. The gap or distance between the valence and conduction bands in an atom is called as the energy gap or band gap. The length of this gap varies depending on the type of conducting material. Good conductors are found to have zero band gap, wherein the valence and conduction bands overlap. Insulators have band gaps greater than 7 eV making it impossible to create free electrons in the material. Semiconductors are found to have band gaps in the range of 2eV to 4eV wherein at slightly elevated temperatures it is possible to generate free electrons in the material and convert them into conductors. This temperature varies with the type of semiconducting material. At room temperatures, semiconductors behave like  insulators.

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