directgap

A direct band gap occurs when the minimum of the conduction band and the maximum of the valence band occur at the same crystal momentum k. In this situation, optical transitions between the bands can happen without a change in momentum, allowing efficient radiative recombination of electrons and holes and strong light absorption near the band edge.

In indirect band gap materials, the conduction band minimum and valence band maximum occur at different k-values, so a phonon is required to conserve momentum. This makes radiative emission inefficient and is a primary reason why materials like silicon are poor light emitters, despite their widespread use in electronics and photovoltaics.

Direct-gap semiconductors such as gallium arsenide (GaAs), gallium nitride (GaN), indium phosphide (InP), cadmium telluride (CdTe), and zinc oxide (ZnO) support strong optical transitions and are widely used in light-emitting diodes, laser diodes, and some high-efficiency solar cells. The direct nature of the gap enables bright, efficient emission and gain in optical devices.

Device design often exploits direct gaps through quantum wells, wires, and dots to tailor the effective band gap and emission wavelength. The gap energy can be tuned by alloying, strain, or quantum confinement. Characterization methods include photoluminescence and absorption spectroscopy, with analyses like Tauc plots used to extract the optical band gap.

While direct-gap materials enable efficient light generation, cost and material availability influence their use in photovoltaics, where indirect-gap materials like silicon remain common. Direct gaps remain central to optoelectronics and nanoscale photonics.