Band Theory of Conduction in Solids

Band Theory of Conduction in Solids
In this theory, all materials have two bands, the valency band and the conduction band separated by a gap called the forbidden gap. The conduction band is a high energy band while the valency band is a low energy band where electrons are easily found. As the name suggests, the forbidden gap cannot have electrons- electrons can only be found in the valency or conduction band. 

The conductivity of a material depends on the availability of electrons in the conduction band. This means that a material only conducts if there are electrons in the conduction band. However, it should be understood that electron would always occupy the low energy valency band-naturally.

Therefore, materials can be classified as conductors, insulators and semiconductors depending on the distance separating the bands (the forbidden gap) and the possibility of getting free electron in the conduction band.

In insulators the electrons in the valence band are separated by a large gap from the conduction band, in conductors like metals the valence band overlaps the conduction band, and in semiconductors there is a small enough gap between the valence and conduction bands that thermal or other excitations can bridge the gap. With such a small gap, the presence of a small percentage of a doping material can increase conductivity dramatically.

Band theory in Conductors 

In conductors, the forbidden gap is so small that the gaps overlap.  Since the gaps overlap, the electron in the valency band can easily move into the conduction band and are free and mobile to conduct electric current. No energy is required to overcome the forbidden gap. Hence, in terms of the band theory of solids, metals are unique as good conductors of electricity.

One unique property of conductors is that their conductivity decreases with increase in temperature. When conductors are heated, the present free electrons gain more kinetic energy increasing their collision with each other. These collisions instead increase the materials resistivity to electric current flow which requires that the available free electron flow in one specific direction.

Band theory Insulators

In insulators, there is a large forbidden gap between the valency band and the conduction band. All electrons are in the valenvy band and a large energy would be required to overcome the wide forbidden gap.  This way, insulators cannot conduct electric current.

The only way to help push electrons into the conduction band would be increasing temperature for the electrons to overcome the forbidden gap, but however, insulators will break (burn out) before the this happens- no condition will move electrons in valency band into the conduction band hence remains insulators at any given temperature.

Band theory Semiconductor 

 In semiconductors, the forbidden gap is bigger than that in conductors. When the bands are separated, electrons-under normal conditions- are found in the valency band. With absence of electrons in the conduction and at low temperatures, semiconductors are insulators; their resistance is very high.

When the temperature is increased, the electrons in the valency band gains kinetic energy. If the gained energy is large enough to overcome the forbidden gap, the electrons move into the conduction band increasing its electrical conductivity.(See how semiconductors conduct by doping)

CONDUCTORS, INSULATORS AND SEMICONDUCTORS

Electrical conduction is the movement of electric charge through a transmission medium. The movement can form an electric current in response to an electric field. This charge movement depends on the material.

Conduction, under normal conditions, depends on the availability of free and mobile electrons. This explains why metals which have free electrons are very good conductors while such materials as plastic with no free electrons are poor conductors.

For conduction to be achieved, a material should have these free electrons, and if not, they must be made available through increasing temperature or even introducing them through such processes as doping.

Conduction in metals and resistors is well described by Ohm's Law, which states that the current is proportional to the applied electric field.

Conductors, Insulators and Semi-conductors

Materials can be classified as conductors, insulators or semi-conductors depending on their electrical conductivity.

Conductors

Conductors are materials that allow electric current of flow through them. They obey Ohm's law and have very low resistance. They therefore carry electric currents from place to place without dissipating a lot of power. As a result, metals are useful as connecting wires to carry electrical signals from place to place.

One unique property of conductors is that their conductivity decreases with increase in temperature. When conductors are heated, the present free electrons gain more kinetic energy increasing their collision with each other. These collisions instead increases the materials resistivity to electric current flow which requires that the available free electron flow in one specific direction.

Insulators

Insulators are materials that do not allow electric current to flow through them. They do not have free and mobile electrons essential for charge flow, and are therefore highly resistive to current flow. Glass, most polymers (plastics), rubber and wood are all examples of insulators. These are materials which will refuse to carry an electric current.

 They are useful for jobs like coating (‘insulating’) electric wires to prevent them from 'shorting together' or giving you a shock.

Semiconductors

Semiconductors are materials whose conductivity properties lie between those of conductors and insulators. Semiconductors are generally insulators at low temperatures and conductors as temperatures increases.

We can also look at a semiconductor as is given away by its name - it 'conducts a little bit'. A semiconductor will carry electric current, but not as easily as a normal conductor.

Semiconductors do not have   free electrons naturally and we therefore do not expect them to conduct current. Therefore, they are made to conduct by enhancing the availability of free electrons or any other particles and conditions necessary for conduction. Heating a semiconductor for example, breaks the internal bonds hence freeing some electrons which therefore move freely to conduct current. Other than heating, some impurities can be added to a semiconductor with an aim of introducing free electrons, or holes in some cases, to enable them conduct electric current.

These two methods of making a semiconductor conduct current create two types of semiconductors, the intrinsic semiconductors and the extrinsic semiconductors.

Intrinsic Semiconductors: These are pure (group four elements) semiconductors whose semiconducting properties occur naturally. They can be made to conduct by increasing their temperature. (Read more on Band theory of conduction inn solids)

Extrinsic Semiconductors: These materials are turned into semiconductors by doping them with small amounts of foreign atoms. The number of doping atoms you need to add is very small. If you left all the doping atoms inside the package of a transistor and removed the bulk of the material you'd be left with a vacuum better than exists between the planets of the solar system. (Read more on Doping)