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| | Semiconductors |
 | | Conversely, semiconductor absorption of light in photodetectors excites electrons from the valence band to the conduction band, facilitating reception of fiber optic communications, and providing the basis for energy from solar cells. |
 | | Semiconductors may be elemental materials, such as silicon, compound semiconductors such as gallium arsenide, or alloys, such as silicon germanium or aluminium gallium arsenide. |
 | | Materials in which the band extrema are aligned in k, for example gallium arsenide, are called direct bandgap semiconductors. |
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