Semiconductor conduction band and valency bands

A semiconductor is a solid material that has electrical conductivity in between a conductor and an insulator; it can vary over that wide range either permanently or dynamically.^[1]

Semiconductors are important in electronic technology. Semiconductor devices, electronic components made of semiconductor materials, are essential in modern consumer electronics, including computers, mobile phones, and digital audio players. Silicon is used to create most semiconductors commercially, but dozens of other materials are used as well.

Overview

Semiconductors are very similar to insulators. The two categories of solids differ primarily in that insulators have larger band gaps—energies that electrons must acquire to be free to move from atom to atom. In semiconductors at room temperature, just as in insulators, very few electrons gain enough thermal energy to leap the band gap from the valence band to theconduction band, which is necessary for electrons to be available for electric current conduction. For this reason, pure semiconductors and insulators in the absence of applied electric fields, have roughly similar resistance. The smaller bandgaps of semiconductors, however, allow for other means besides temperature to control their electrical properties.

[edit]Doping

Semiconductors' intrinsic electrical properties are often permanently modified by introducing impurities by a process known as doping. Usually, it is sufficient to approximate that each impurity atom adds one electron or one "hole" (a concept to be discussed later) that may flow freely. Upon the addition of a sufficiently large proportion of impurity dopants, semiconductors will conduct electricity nearly as well as metals. Depending on the kind of impurity, a doped region of semiconductor can have more electrons or holes, and is named N-type or P-type semiconductor material, respectively. Junctions between regions of N- and P-type semiconductors create electric fields, which cause electrons and holes to be available to move away from them, and this effect is critical to semiconductor device operation. Also, a density difference in the amount of impurities produces a small electric field in the region which is used to accelerate non-equilibrium electrons or holes.

In addition to permanent modification through doping, the resistance of semiconductors is normally modified dynamically by applying electric fields. The ability to control resistance/conductivity in regions of semiconductor material dynamically through the application of electric fields is the feature that makes semiconductors useful. It has led to the development of a broad range of semiconductor devices, like transistors and diodes. Semiconductor devices that have dynamically controllable conductivity, such as transistors, are the building blocks of integrated circuits devices like the microprocessor. These "active" semiconductor devices (transistors) are combined with passive components implemented from semiconductor material such as capacitors and resistors, to produce complete electronic circuits.

[edit]Semiconductors and Light

In most semiconductors, when electrons lose enough energy to fall from the conduction band to the valence band (the energy levels above and below the band gap), they often emit light. This photoemission process underlies the light-emitting diode (LED) and the semiconductor laser, both of which are very important commercially. Conversely, semiconductor absorption of light in photodetectors excites electrons to move from the valence band to the higher energy conduction band, thus facilitating detection of light and indicating its intensity. This is useful forfiber optic communications, and provides the basis for energy from solar cells.

The semiconductor silicon does not emit light well in such cases because it has an indirect band gap. This means that the conversion between hole-electron pair and light requires the capture or emission of a phonon (a particle of quantized lattice vibration) to maintain conservation of momentum. Because the required phonon has a particular energy and requirement to balance the momentum difference of the indirect gap, the probability of a random thermal noise vibration having the proper energy and momentum combine at a hole-electron recombination location is small and thus silicon is unable to function as a LED or laser semiconductor.

Other photoelectric applications of silicon such as Charge-Coupled Device (CCD) camera, video arrays, phototransistors, and infrared sensors do not rely on light emission. Instead they involve the conversion of a photon to hole-electron pair plus a phonon. This process is not rate-limited because a phonon of particular energy and momentum is emitted rather than consumed. The resulting phonon simply contributes to heating of the semiconductor.

[edit]Types of Semiconductors

Semiconductors may be elemental materials such as silicon and germanium, or compound semiconductors such as gallium arsenide and indium phosphide, or alloys such as silicon germanium and aluminium gallium arsenide.