A<unk>  is a solid material that has electrical conductivity in between that of a conductor and that of an insulator; it can vary over that wide range either permanently or dynamically.<unk> s are tremendously important in technology.<unk>  devices, electronic components made of<unk>  materials, are essential in modern electrical devices. Examples range from computers to cellular phones to digital audio players.  Silicon is used to create most<unk> s commercially, but dozens of other materials are used as well.<unk> s 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<unk> s at room temperature, just as in insulators, very few electrons gain enough thermal energy to leap the band gap from the valence band to the conduction band, which is necessary for electrons to be available for electric current conduction. For this reason, pure<unk> s and insulators in the absence of applied electric fields, have roughly similar resistance. The smaller bandgaps of<unk> s, however, allow for other means besides temperature to control their electrical properties.<unk> s' 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,<unk> s will conduct electricity nearly as well as metals. Depending on the kind of impurity, a doped region of<unk>  can have more electrons or holes, and is named N-type or P-type<unk>  material, respectively. Junctions between regions of N- and P-type<unk> s create electric fields, which cause electrons and holes to be available to move away from them, and this effect is critical to<unk>  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<unk> s is normally modified dynamically by applying electric fields. The ability to control resistance/conductivity in regions of<unk>  material dynamically through the application of electric fields is the feature that makes<unk> s useful. It has led to the development of a broad range of<unk>  devices, like transistors and diodes.<unk>  devices that have dynamically controllable conductivity, such as transistors, are the building blocks of integrated circuits devices like the microprocessor. These "active"<unk>  devices (transistors) are combined with passive components implemented from<unk>  material such as capacitors and resistors, to produce complete electronic circuits.
In most<unk> s, 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<unk>  laser, both of which are very important commercially. Conversely,<unk>  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 vary with its intensity. This is useful for fiber optic communications, and providing the basis for energy from solar cells.<unk> s may be elemental materials such as silicon and germanium, or compound<unk> s such as gallium arsenide and indium phosphide, or alloys such as silicon germanium or aluminium gallium arsenide.
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