Conductors, semiconductors, insulators

Materials can be catagorised into conductors, semiconductors or insulators by their ability to conduct electricity. It is a popular belief that insulators do not conduct electricity because their valence electrons are not free to wander throughout the material. In fact they are free to move around, however, in an insulator there are as many electrons as there are energy levels for them to occupy. If an electron swaps place with another electron no change is made since electrons are indistinguishable. There are higher energy levels, but to promote the electrons to these energy levels requires more energy than is usually practical.

Metals conduct electricity easily because the energy levels between the conduction and valence band are closely spaced or there are more energy levels available than there are electrons to fill them so very little energy is required to find new energies for electrons to occupy. The resistivity of a material is a measure of how difficult it is for a current to flow. Semiconductors have a resistivity between 10^-4 < r < 10⁸ Ohms m although these are rough limits. The band theory of materials explains qualitatively the difference between these types of materials. Electrons occupy energy levels from the lowest energies upwards. However, some energy levels are forbidden because of the wave like properties of atoms in the material. The allowed energy levels tend to form bands. The highest filled level at T=0 K is known as the valence band . Electrons in the valence band do not participate in the conduction process. The first unfilled level above the valence band is known as the conduction band . In metals, there is no forbidden gap; the conduction band and the valence band overlap, allowing free electrons to participate in the conduction process. Insulators have an energy gap that is far greater than the thermal energy of the electron, while semiconductor materials the energy gap is typically around 1eV. The diagram shows the differences in metals, semiconductors and insulators in terms of the how the energy bands are separated.

Conductors

Most of the conductors used in electronics are metals like copper, aluminium and steel. Conductors are materials that obey Ohm's law and have very low resistance. They can 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. They help ensure that most of the signal's power reaches it's destination instead of warming up the wires in between!

In fact, although it sounds odd, modern resistors are also made of conductor materials. However, they use very thin pieces of conductor which don't pass current too easily.

Insulators

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 electric wires to prevent them from 'shorting together' or giving you a shock. Silk and cotton are also good insulators (when they're dry!!) and some of the mains wiring in very old houses once used them - but by modern standards this was pretty dangerous since you could get a shock when wet or a spark would set them alight when dry!

Modern insulators like PVC (PolyVinylChloride) are much better and safer.

Insulators are also very useful to fill the 'gap' in between the metal plates of a capacitor. Very old capacitors (and some modern radio ones) use air gaps since air is a good insulator.

Semiconductors

All the transistors, diodes, integrated circuits, etc. used in modern electronics are built using a range of semiconductors. The basic property of a semiconductor 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.

Some materials are intrinsic semiconductors. The semiconducting properties occur in these materials naturally. However, most of the semiconducting materials used in electronics are extrinsic. This means that left to themselves they are excellent insulators. 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!