Conductors Insul & Semi

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Conductors A substance or object that allows electricity to flow through it with low resistance is called conductor. In a conductor current can flow freely. They contain free electrons. “Conductor” implies that the outer electrons of the atoms are loosely bound and free to move through the material. EXAMPLES Metals are very good conductors of electricity, such as silver, copper, iron and aluminum. A macro scale example is a Resistivity (ohm m) copper wire while certain kinds of nanotubes are Glass 1012 molecular conductors. In copper, the valence electrons are essentially free and strongly repel 9 x 1013 each other. Any external influence which moves Mica one of them will cause a repulsion of other Quartz (fused) 5 x 1016 electrons which propagates, "domino fashion" through the conductor. Resistivity (ohm m)

The conductivity of the conductors is of the order 107 mho/m.

Copper

1.7 x 10-8

Insulators Most solid materials are classified as insulators because they offer very large resistance to the flow of electric current. In most materials even the outermost electrons are so tightly bound that there is essentially zero electron flow through them with ordinary voltages. Some materials are particularly good insulators and can be characterized by their high resistivities: EXAMPLES

Resistivity (ohm m)

Glass Mica Quartz (fused)

Dry wood, diamond, glass, mica and polythene and most of the non-metals are good insulators. The conductivity of insulators is very low, ranging between 10-10 to 10-20 ohm/m. their electrical resistivity is of the order of 1010 to 1020 ohm/

1012 9 x 1013 5 x 1016

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Semiconductors Semiconductor is a material that has a resistivity value between that of a conductor and an insulator. The conductivity of a semiconductor material can be varied under an external electrical field. Devices made from semiconductor materials are the foundation of modern electronics, including radio, computers, telephones, and many other devices. In a metallic conductor, current is carried by the flow of electrons. In semiconductors, current can be carried either by the flow of electrons or by the flow of positively-charged "holes" in the electron structure of the material. EXAMPLES Germanium and silicon are important semiconductors. Their conductivity lies between insulators and conductors. They have conductivity in the range of 10-4 to 10-6 mho/m .

Energy bands Energy bands consisting of a large number of closely spaced energy levels exist in crystalline materials. The bands can be thought of as the collection of the individual energy levels of electrons surrounding each atom. The energy levels, which were sharp for individual atoms are broadened to become energy bands with forbidden energy bands, known as valence bands, and the higher unoccupied bands are known as conduction bands. Conductors, insulators and semiconductors can now be classified on the basis of these energy bands.

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1-Conductor Energy Bands In terms of the band theory of solids, metals are unique as good conductors of electricity. This can be seen to be a result of their valence electrons being essentially free. In the band theory, this is depicted as an overlap of the valence band and the conduction band so that at least a fraction of the valence electrons can move through the material.

2- Insulator Energy Bands Most solid substances are insulators, and in terms of the band theory of solids this implies that there is a large forbidden gap between the energies of the valence electrons and the energy at which the electrons can move freely through the material (the conduction band).

3-Semiconductor Energy Bands For intrinsic semiconductors like silicon and germanium, the Fermi level is essentially halfway between the valence and conduction bands. Although no conduction occurs at 0 K, at higher temperatures a finite number of electrons can reach the conduction band and provide some current. In doped semiconductors, extra energy levels are added. The increase in conductivity with temperature can be modeled in terms of the Femi functions, which allows one to calculate the population of the conduction band .

The End

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