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For information on how semiconductors are used as electronic devices, see semiconductor device.
In the parlance of solid-state physics, semiconductors (and insulators) are defined as solids in which at 0 K (and without excitations) the uppermost band of occupied electron energy states is completely full. It is well-known from solid-state physics that electrical conduction in solids occurs only via electrons in partially-filled bands, so conduction in pure semiconductors occurs only when electrons have been excited--thermally, optically, etc.--into higher unfilled bands.
At room temperature, a proportion (generally very small, but not negligible) of electrons in a semiconductor have been thermally excited from the "valence band," the band filled at 0 K, to the "conduction band," the next higher band. The ease with which electrons can be excited from the valence band to the conduction band depends on the energy gap between the bands, and it is the size of this energy bandgap that serves as an arbitrary dividing line between semiconductors and insulators. Semiconductors generally have bandgaps of approximately 1 electron-volt, while insulators have bandgaps several times greater.
When electrons are excited from the valence band to the conduction band in a semiconductor, both bands contribute to conduction, because electrical conduction can occur in any partially-filled energy band. The current-carrying electrons in the conduction band are known as "free electrons," though often they are simply called "electrons" if context allows this usage to be clear. The free energy-states in the valence band are known as "holes." It can be shown that holes behave very much like positively-charged counterparts of electrons, and they are usually treated as if they are real charged particles.
One of the main reasons that semiconductors are useful in electronics is that their electronic properties can be greatly altered in a controllable way by adding small amounts of impurities. These impurities, called dopants, add extra electrons or holes. A semiconductor with extra electrons is called an n-type semiconductor, while a semiconductor with extra holes is called a p-type semiconductor.
The most common n-type dopants for silicon are phosphorusPhosphorus is the chemical element in the periodic table that has the symbol P and atomic number 15. A multivalent, nonmetal of the nitrogen group, phosphorus is commonly found in inorganic phosphate rocks and in all living cells but is never naturally fo and arsenicArsenic is a chemical element in the periodic table that has the symbol As and atomic number 33. This is a notorious poisonous metalloid that has three allotropic forms; yellow, black and grey. Arsenic and its compounds are used as pesticides, herbicides. Notice that the latter two elementGenerally, an element is a basic part that is the foundation of something. For a long time, elements classical element were believed (by the Pythagoreans and alchemists for example) to be the building blocks of all matter in the universe. Similarly, Chines are in Group V of the periodic tableThe periodic table of the chemical elements is a tabular display of the known chemical elements. The elements are arranged by electron structure so that many chemical properties vary regularly across the table. Each element is listed by its atomic number, and silicon is in Group IV. When silicon is doped with arsenic or phosphorus atoms, these dopant atoms replace silicon atoms in the semiconductor crystal, but since they have one more outer-shell electron than silicon they tend to contribute this electron to the conduction band. By far the most common p-type dopants for silicon is the Group III element boronBoron is the chemical element in the periodic table that has the symbol B and atomic number 5. A trivalent metalloid element, boron occurs abundantly in the ore borax. There are two allotropes of boron; amorphous boron is a brown powder, but metallic boro, which lacks an outer-shell electron compared with silicon and thus tends to contribute a hole to the valence band.
Heavily doping a semiconductor can increase its conductivity by a factor greater than a billion. In modern integrated circuits, for instance, heavily-doped polycrystalline silicon is often used as a replacement for metals.