Direct bandgap
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In semiconductor physics, a direct bandgap means that the conduction band lies directly above the valence band, in momentum space (see E-k plots). A semiconductor with a direct bandgap can be used to emit light. Indirect bandgap semiconductors such as crystalline silicon cannot do so efficiently.
The prime example of a direct bandgap semiconductor is gallium arsenide - a material commonly used in laser diodes.
See indirect bandgap for an explanation of the connection between bandgap offset and light emission.