A semiconductor is a material that is an insulator at very low temperature, but which has a sizable electrical conductivity at room temperature. The distinction between a semiconductor and an insulator is not very well-defined, but roughly, a semiconductor is an insulator with a band gap small enough that its conduction band is appreciably thermally populated at room temperature.
For information on how semiconductors are used as electronic devices, see semiconductor device.
Fundamental semiconductor physics
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.
Doping of semiconductors
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 phosphorus and arsenic. Notice that the latter two elements are in Group V of the periodic table, 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 boron, 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.
Intrinsic and extrinsic semiconductors
An intrinsic semiconductor is one which is pure enough that impurities do not appreciably affect its electrical behavior. In this case, all carriers are created by thermally or optically exciting electrons from the full valence band into the empty conduction band. Thus equal numbers of electrons and holes are present in an intrinsic semiconductor. Electrons and holes flow in opposite directions in an electric field, though they contribute to current in the same direction since they are oppositely charged. Hole current and electron current are not necessarily equal in an intrinsic semiconductor, however, because electrons and holes have different effective masses (crystalline analogues to free inertial masses).
The concentration of carriers is strongly dependent on the temperature. At low temperatures, the valence band is completely full, making the material an insulator (see electrical conduction for more information). Increasing the temperature leads to an increase in the number of carriers and a corresponding increase in conductivity. This principle is used in thermistors. This behavior contrasts sharply with that of most metals, which tend to become less conductive at higher temperatures due to increased phonon scattering.
An extrinsic semiconductor is one that has been doped with impurities to modify the number and type of free charge carriers.
The purpose of n-type doping is to produce an abundance of carrier electrons in the material. To help understand how n-type doping is accomplished, consider the case of silicon (Si). Si atoms have four valence electrons, each of which is covalently bonded with one of four adjacent Si atoms. If an atom with five valence electrons, such as those from group VA of the periodic table (eg. phosphorus (P), arsenic (As), or antimony (Sb)), is incorporated into the crystal lattice in place of a Si atom, then that atom will have four covalent bonds and one unbonded electron. This extra electron is only weakly bound to the atom and can easily be excited into the conduction band. At normal temperatures, virtually all such electrons are excited into the conduction band. Since excitation of these electrons does not result in the formation of a hole, the number of electrons in such a material far exceeds the number of holes. In this case the electrons are the majority carriers and the holes are the minority carriers. Because the five-electron atoms have an extra electron to "donate", they are called donor atoms.
The purpose of p-type doping is to create an abundance of holes. In the case of silicon, a trivalent atom--such as boron--is substituted into the crystal lattice. The result is that an electron is missing from one of the four possible covalent bonds. Thus the atom can accept an electron from the valence band to complete the fourth bond, resulting in the formation of a hole. Such dopants are called acceptors. When a sufficiently large number of acceptors are added, the holes greatly outnumber the excited electrons. Thus, the holes are the majority carriers, while electrons are the minority carriers in p-type materials. Blue diamonds (Type IIb), which contain boron impurities, are an example of a naturally occurring p-type semiconductor.
A p-n junction may be created by doping adjacent regions of a semiconductor with p-type and n-type dopants. If a positive bias voltage is placed on the p-type side, the dominant positive carriers (holes) are pushed toward the junction. At the same time, the dominant negative carriers (electrons) in the n-type material are attracted toward the junction. Since there is an abundance of carriers at the junction, current can flow through the junction from a power supply, such as a battery. However, if the bias is reversed, the holes and electrons are pulled away from the junction, leaving a region of relatively non-conducting silicon which inhibits current flow. The p-n junction is the basis of an electronic device called a diode, which allows electric current to flow in only one direction. Similarly, a third region can be doped n-type or p-type to form a three-terminal device, such as the bipolar junction transistor (which can be either p-n-p or n-p-n).
Purity and perfection of semiconductor materials
Semiconductors with predictable, reliable electronic properties are difficult to mass-produce because of the required chemical purity, and the perfection of the crystal structure, which are needed to make devices. Because the presence of impurities in very small proportions can have such big effects on the properties of the material, the level of chemical purity needed is extremely high. Techniques for achieving such high purity include zone refining, in which part of a solid crystal is melted. Impurities tend to concentrate in the melted region, leaving the solid material more pure. A high degree of crystalline perfection is also required, since faults in crystal structure such as dislocations, twins, and stacking faults, create energy levels in the band gap, interfering with the electronic properties of the material. Faults like these are a major cause of defective devices in production processes. The larger the crystal, the harder it is to achieve the necessary purity and perfection; current mass production processes use six-inch diameter crystals which are grown as cylinders and sliced into wafers.
- Electrical Engineering Training Series (http://www.tpub.com/content/neets/14179/index.htm) A set of articles on Semiconductors and Transistors
- Semiconductor Technology (http://www.semiconductor-technology.com) Information on the Semiconductor Industry.
- NSM-Archive (http://www.ioffe.rssi.ru/SVA/NSM/Semicond/index.html) Physical Properties of Semiconductors (such as Si, GaAs and others), including band structure, mechanical, electrical, thermal and optical properies
- Semiconductor Concepts at Hyperphysics (http://hyperphysics.phy-astr.gsu.edu/hbase/solids/semcn.html), includes intrisic semiconductors, doping, junctures, band theorry, etc.
- Principles of Semiconductor Devices (http://ece-www.colorado.edu/~bart/book/book/)
- Britney Spears Guide to Semiconductor Physics (http://britneyspears.ac/lasers.htm)
- Semiconductor resource launch page (http://www.semi1source.com/)
- Semiconductor glossary (http://www.semiconductorglossary.com/)