Three types of materials
Conductors like copper or aluminum have a ‘cloud’ of free electrons at all temperatures, since the valence electrons are very loosely bond to their nuclei. Or, in other words, they have sufficient energy to escape from their nucleus. If an electric field is applied across this material, then these electrons can move freely – like an electric current – from lower to higher potential. In isolators, like many ‘plastics’, these valence electrons are very strongly bonded to their nuclei. They have not sufficient energy to escape from their nuclei, so that they are not capable of moving like free electrons in an electric field. The electric current is zero or negligibly small. At low temperatures, semiconductors behave like isolators: Currents are negligibly small. But when temperature is increased, there is more energy in the system. And this energy can be used by the electrons to escape from their nuclei. These free electrons are capable of moving in an electric field: the semiconductor then behaves like a conductor.
The most popular material for semiconductors is Silicon (Si), but also Germanium (Ge) is frequently used, or the more exotic Gallium Arsenide (GaAs). The figure above shows the crystal structure of Silicon in two dimensions at low temperatures. Silicon has four valence electrons, but the outer electron shell can contain up to eight electrons. Moreover, the atom is the most stable when the outer shell is ‘full’. In pure Silicon, every atom shares four valence electrons with four neighboring atoms. As a consequence, every atom has eight, though shared, electrons in its outer shell. This type of bond is called a covalent bond, and results in a very stable structure.
At low temperatures, the covalent bond is too strong for electrons to be able to conduct. At higher temperatures however, the crystal structure will be broken at some places due to thermal vibrations. This enables some electrons to leave a covalent bond. In that covalent bond, a ‘hole’ remains, ready to accept another electron. At each ‘fracture’, an electron-hole pair emerges (see the figure below). The higher the temperature, the more electron-hole pairs, the lower the electric resistance of the material.
These ‘holes’ also behave like electric charge carriers, and move from high to low potential, in the opposite direction of free electrons. Note that a ‘moving hole’ is nothing more than an electron that fills the hole, and thereby leaves another hole. When a hole moves over several atoms, it is each time another electron that fills the hole. This is illustrated in the slide show below. Because it is always the same hole that is moving, one speaks of hole conduction. Compare it to the bubbles in beer or soda pop. These move from the bottom to the top, but actually these are the molecules of the fluid moving in the opposite direction.
So, temperature is one way of turning a semiconductor into a conductor. Obviously, it would not be practical to use temperature as a way to change the conductivity in practical circuits. To increase the conductivity of a material, impurities are added to the pure material. The process is called doping. These impurities are atoms that can fit in the crystal structure, but that have less or more valence electrons. As an example, take Phosphorus (P) in Silicon. Phosphorus has five valence electrons, and four of them will join in a covalent bond with neighboring Silicon atoms. The fifth electron can not join in a bond, and will behave like a free electron. These donor atoms donate extra free electrons to the material, which is now called an n-type semiconductor. The ‘n’ stands for ‘negative’, since the charge of an electron is by definition negative.
Another possibility is adding acceptor atoms, e.g. like Boron (B) in Silicon. These atoms have less valence electrons. In the case of Boron, there are three, so there are no four covalent bonds, and a ‘hole’ is generated. This ‘hole’ can also participate in an electric current, as described above. This type of semiconductors is of the p-type. The ‘p’ stands for ‘positive’, since a hole corresponds to the lack of a negative electron. Note that the effect of doping is rather drastic. A concentration of one ‘impure’ atom on one billion, already results in a very low resistance.