Semiconductors have an electrical resistivity that is in between those of good conductors and those of good insulators. Both silicon and germanium, which are the two basic semiconductors, have four electrons in the outermost electron subshell. In formation of the lattice structure of the silicon or germanium, all the valence electrons are involved in the bonding, so the material should be an insulator. However, an unusually small amount of energy is needed to break one of the bonds and set an electron free to roam around the lattice. This energy is approximately 1ev. This energy corresponds to the energy gap between the valence and conduction bands. In an insulator, this energy gap is very high to approximately 5ev. No electron can naturally attain 5ev. At room temperature, a substantial number of electrons are dislocated from their parent atom in a semiconductor. This number increases with increasing temperature so we can say that semiconductors have higher conductivity at higher temperatures.
When an electron is removed from a covalent bond, it leaves a hole and this hole can travel through the lattice and serve as an additional current carrier. The current mostly comes from the electrons that are out of the lattice structure. A hole behaves like a positively charged particle. In a pure semiconductor, holes and electrons are always present in equal numbers.
Devices such as transistors and diodes are fabricated using impurity semiconductors prepared by adding small quantities of foreign atoms, such as arsenic or gallium, to an intrinsic semiconductor. The added foreign atom only accounts for a few part per million. The process of this is known as doping. This process produces two distinct kinds of systems. When silicon is doped with a five valence electron atom as arsenic, the fifth electron is not locked in place so it does not fit and can move around freely within the crystal. These electrons stay in an energy level just below the conduction band, into which can easily be made to jump. Because these charge carriers are negative, the system is referred to as an n-type semiconductor. When the silicon is doped with a three valence electron atom like gallium, there will be a missing electron in the lattice structure. In effect, there would be a hole in the negative distribution of electrons. An outer electron from a nearby silicon atom can drop out of its cloud and drop into the hole, but this created another new hole in the place where the electron originally was. Because the carriers are positive, this system is referred to as a p-type semiconductor. When the n-type semiconductor is formed, there is a new level formed in between the valence band and the conduction band called the donor level. In a p-type semiconductor, this level is called the acceptor level.