Atoms of a group of elements called metals are held together by a form of bonding. The type of metal we are concerned with here have single valence electron. For example, copper atom has 29 electrons in orbit round each nucleus. After the first three shells are filled (2+8+18=28), there is one lone electron in the fourth shell. Since copper atom would have been more stable without this valence electron, only a very weak attraction hold it in orbit round the nucleus (unlike the 28 tightly bound electrons which fill the first three shells). As a result, the valence electron often finds itself midway between the nuclei of any two adjacent copper atoms. In this position of balanced attraction, the valence electron is then free to leave its parent nucleus and to move into the fourth shell of a neighbouring atom. The copper atoms donate their valence electrons to form a vast cloud of free electrons, thus becoming stable positive copper ions with their 28 tightly bound orbital electrons. Since the positive metal ions repel one another, they will take up fixed geometric positions within the electron cloud, thus giving the metal object its shape. The atoms are held together by the attraction between the positive metal ions and the common cloud of free electrons. This is referred to as metallic bonding. Free electrons move through the lattice of positive metal ion in the seemingly solid copper conductor. This flow of electrons in an electric conductor represents an electric current.
Insulators
Insulators are materials that have just the opposite effect on the flow of electrons as that of conductors. They do not let electrons to flow very easily from one atom to another. Insulators are materials whose atoms have tightly bound electrons. These electrons are not free to roam around and be shared by neighbouring atoms. Some common examples of insulators are glass, plastic, rubber, air and wood.
Insulators are used to protect us from dangerous effects of electricity flowing through conductors. The rubbery coating on wires is an insulating material that shields us from the conductor inside.
Semiconductors
Semiconductors are materials that exhibit properties that are between a metal and a non-metal. Their electrical properties are part way between a conductor and an insulator – thus they are called “semi-conductors”. (Actually they are much closer to insulators than conductors).
A major difference between semiconductors and conductors is their electrical behaviour at different temperatures. In general, when you heat up a semiconductor its resistance to the flow of electricity decreases (or its conductance increases). In metals the opposite happens. As the temperature of a metal increases, its resistance also increases (or conductance decreases).
Today silicon is the most commonly used semiconductor although germanium was used almost exclusively in early (1950’s) solid state devices.
