indirectbandgap

An indirect bandgap is a property of some semiconductors in which the conduction band minimum and the valence band maximum occur at different points of crystal momentum (k) in the Brillouin zone. In such materials, optical transitions near the band edge cannot conserve momentum with a photon alone and must involve a phonon (a quantum of lattice vibration) to balance momentum.

Because momentum conservation requires a phonon, indirect-gap transitions are phonon-assisted and occur less readily than direct-gap transitions. This makes absorption near the band edge weaker and radiative recombination slower, which in turn reduces light-emitting efficiency.

Common examples and implications: Silicon and germanium are classic indirect-bandgap materials. Direct-bandgap materials, which enable efficient light emission, include gallium arsenide, gallium nitride, and indium phosphide. Some materials or nanostructures can exhibit both indirect and direct characteristics depending on conditions such as strain, temperature, or quantum confinement, and engineering can modify the gap type.

Applications and relevance: Indirect-bandgap semiconductors dominate electronic devices and many photovoltaic applications due to mature fabrication, strong absorption over a range of wavelengths, and favorable electronic properties. However, their poor light emission makes them less suitable for LEDs and laser diodes. Ongoing research explores strategies to enhance light emission in indirect-gap materials, including strain engineering, nanostructuring, and phonon management, while leveraging their excellent electronic performance for integrated devices.