Introduction to Semiconductor
Semiconductor, a material whose electrical conductivity is intermediate between that of a good conductor (such as copper) and that of an insulator (such as rubber). Most technologically important semiconductors are crystalline solids. They may be elements or compounds, either inorganic or organic Examples are the elements silicon, germanium, tellurium, and selenium; the inorganic compounds lead sulfide, cadmium sulfide, and indium antimonide, and the organic compounds anthracene, naphthacene, and phthalocyanine.
The semiconductor that is most important commercially is silicon, It is used in such solid-state devices as transistors and rectifiers. Miniature solid-state devices formed with silicon are essential components of integrated circuits (complex electronic circuits manufactured as a unit) that are used in a wide variety of electronic equipment. Silicon is also used in solar cells, which produce an electric current when exposed to light. Other important semiconductors are gallium arsenide, used in LED's (light-emitting diodes), and selenium sulfide and cadmium sulfide, which are used in photographic exposure meters.
Electrical Behavior
The usefulness of semiconductors lies chiefly in the nature of their electrical conductivity; to varying degrees, the conductivity increases with (1) an increase in temperature; (2) exposure to electromagnetic radiation; and (3) the addition of small amounts of certain impurities called dopants. The sensitivity of semiconductors to temperature, radiation, and impurities is a consequence of the semiconductors' atomic structure.
In this discussion silicon and germanium, being typical semiconductors, are used as examples. Atoms of both silicon and germanium have four valence electrons; each atom therefore requires four additional electrons to complete its outermost electron shell, (An atom with a filled outer shell is particularly stable, and atoms tend to combine in such a way as to achieve complete outer shells.)
A model of a small portion of a silicon or germanium crystal is shown in the illustration. Each atom is at the center of a regular tetrahedron of four other atoms, which are called its nearest neighbors. This structure permits each atom to share one of its valence electrons with each of its nearest neighbors, and vice versa. Thus each atom has a complete outer shell of electrons, even though they are all shared, and each is bound to each of its nearest neighbors by a two-electron bond (the shared pair of electrons).
Sensitivity to TemperatureAt all temperatures above absolute zero there is some heat energy, which causes the atoms of a silicon or germanium crystal to vibrate about their average positions and also causes some of the valence electrons (which are only weakly bound to the atoms) to escape. The higher the temperature, the more frequently such escapes will occur.
When an electron escapes, two charge carriers are formed—the electron itself (called a free electron) and a positively charged vacancy (called a hole) left in one of the two-electron bonds. Since the hole is positive, it can attract electrons (which are negatively charged) from a neighboring two-electron bond. If an electron in a neighboring two-electron bond has enough thermal energy, it can shift from its original bond to the hole. This action creates a new hole in the neighboring boring bond. This process can be repeated any number of times. Thus a hole can migrate through the crystal.
A hole and a free electron can recombine or unite whenever they meet to form a normal two-electron bond. In pure silicon or germanium this is the only mechanism by which a free electron or a hole can disappear.
The holes and the free electrons are the current carriers in silicon and germanium. Because the number of holes and free electrons increases with an increase in temperature, the electrical conductivity is greater at high temperatures than at low temperatures. At very low temperatures silicon and germanium behave like insulators, and at very high temperatures, like conductors.
Sensitivity to RadiationElectromagnetic radiation can also cause an electron to be ejected from a two-electron bond. If a photon of electromagnetic radiation has sufficient energy and passes in the vicinity of a two-electron bond in a crystal of silicon or germanium, it may be absorbed and an electron simultaneously ejected from the bond. The result is the creation of a free electron and a hole. Electromagnetic radiation thus increases the number of free electrons and holes (and therefore the electrical conductivity) in a crystal of silicon or germanium.
Sensitivity to ImpuritiesTwo types of impurity atoms that have a profound effect upon the conductivity of silicon and germanium are those having either five valence electrons or three valence electrons.
Arsenic and phosphorus are examples of impurity atoms with five valence electrons. In a crystal of silicon containing a small amount of arsenic, the arsenic atoms randomly replace silicon atoms. Because the concentration of the arsenic is very small, the four nearest neighbors of any arsenic atom are silicon atoms. The arsenic atom forms four two-electron bonds with its four nearest neighbors exactly as a silicon atom would. Its fifth electron, however, cannot be part of a two-electron bond and the arsenic atom has a complete outer shell without it. Consequently this electron is much more easily removed from the atom than are any of the electrons in the two-electron bonds between arsenic and silicon atoms or between two silicon atoms. Thus, a silicon crystal containing arsenic contains many more free electrons at a given temperature than a crystal of pure silicon does.
Impurity atoms such as arsenic that provide free electrons in a semiconductor are called donor atoms. They cause the semiconductor to contain an excess of free electrons over holes. Such semiconductors are called n-type semiconductors. In such a semiconductor the greatest part of an electric current is carried by the free electrons, which in this case are called majority carriers. The holes are called minority carriers.
Boron and aluminum are examples of elements whose atoms have three valence electrons. When introduced as impurities they, too, greatly increase the electrical conductivity of silicon or germanium, but the mechanism of conduction is very different from that in the case just discussed. In a crystal of silicon containing a small amount of boron, the boron atoms substitute randomly for silicon atoms. Because a boron atom has only three valence electrons, it cannot form four two-electron bonds with its four nearest neighbors (which are silicon atoms because of the very low concentration of boron). Thus, one of the four bonds lacks a second electron. Although this structure is electrically neutral, there is a pronounced tendency to form four complete two-electron bonds. Consequently, the neutral boron atom tends to acquire an electron—either a free electron or an electron from a neighboring bond whose thermal energy is sufficient to allow it to jump. In either case the final result is a hole, which, as described before, is free to migrate through the crystal. Hence once again the presence of an impurity greatly increases the likelihood of the formation of charge carriers—in this case, predominantly holes.
Impurity atoms in semiconductors behave like boron in silicon are called acceptor atoms. They cause the semiconductor to contain an excess of holes over free electrons. Such semiconductors are called p-type semiconductors. In such semiconductors the greatest part of an electric current is carried by the positively charged holes. In this instance, the holes are called majority carriers and the free electrons, minority carriers.
If acceptor and donor impurity atoms are simultaneously present in a semiconductor such as silicon or germanium, then whichever one is present in the greatest concentration will determine whether the impure semiconductor is n-type or n-type semiconductor.