Semiconductors are essential components of modern electronic devices and so are one of the fundamental building blocks of modern technology. To be suitable as a semiconductor material, a substance needs to have electrical conductivity that lies between that of insulators, which conduct very little electricity, and conductors, which allow electricity to flow very easily. Most semiconductor materials are crystalline inorganic solids, though semiconductors made from amorphous solids and liquids also exist. Common semiconductor materials include silicon, gallium arsenide, and gallium nitride, though others also exist. In addition to these primary materials, semiconductors often also contain small amounts of other substances, known as dopants.
A semiconducting material's conductivity can be increased by subjecting it to electrical energy, magnetic fields, or other stimuli that increase the energy levels of the material's electrons, causing some of them to move from the low-energy valence band to the higher-energy, less crowded conduction band. This allows the energized electrons to move through the material more freely while creating positively charged gaps in the valence band called electron holes. This allows electricity to flow through the semiconductor. By manipulating a semiconductor's conductivity, it can be used as a switch. Semiconductors are also used for solar power generation and light-detecting sensors, because they can produce a flow of electrical current when suitably energized by incoming photons of light.
The most commonly used semiconductor material is silicon, the 14th element on the Periodic Table and one of the most common elements in the Earth's crust. Most silicon semiconductors have a regular crystalline structure in which their atoms are arranged, but noncrystalline, or amorphous, silicon can also be used. Amorphous silicon semiconductors have inferior performance compared to crystalline silicon, but amorphous silicon can be deposited in much thinner layers, which can decrease material cost.
The next most common semiconductor material is the compound gallium arsenide (GaAs). Gallium arsenide is superior to silicon in a number of respects, such as faster switching and greater resistance to heat. It is also costlier and more difficult to process, however, and so is usually used only for applications in which silicon is inadequate. It also suffers from higher power consumption. Gallium arsenide is commonly used for purposes such as high-speed electronics and high-efficiency photovoltaic cells.
Another gallium compound used for semiconductors is gallium nitride (GaN), which can function at very high temperatures and voltages and so is often used for applications involving microwaves. Gallium nitride is also used in light-emitting diodes (LEDs) and high-frequency laser diodes, as well as some military radars. It can also be combined with another semiconductor material, indium nitride (InN), to produce a mixture called indium gallium nitride. Indium gallium nitride is commonly used in LEDs and can also be an extremely efficient material for solar cells.
Semiconductors frequently contain small amounts of dopants to alter their conductive properties according to their function. Common dopants in silicon include the elements boron, phosphorus, and arsenic. Gallium arsenide and gallium nitride doped with metals such as manganese have both semiconducting and ferromagnetic properties.