båndgabets

Båndgabets, known in English as the band gap, is the energy difference between the valence-band maximum and the conduction-band minimum in a crystalline solid. The size and nature of the band gap determine whether a material behaves as a conductor, insulator, or semiconductor at practical temperatures. In metals the bands overlap, so a finite band gap does not exist.

Bandgaps can be direct or indirect. A direct band gap allows electrons to transition between the valence and conduction bands without a change in momentum, enabling efficient light emission and making direct-gap materials suitable for LEDs and laser diodes. Indirect band gaps require a phonon to conserve momentum, which reduces radiative efficiency as seen in silicon.

Bandgaps are typically expressed in electronvolts (eV) and depend on temperature, chemical composition, and crystal structure. They can be engineered by alloying (for example, aluminum gallium arsenide), by quantum confinement in nanoscale structures, or by applying mechanical strain to the lattice.

Applications rely on the band gap to control optical absorption and electrical behavior. Solar cells absorb photons with energy above the gap; LEDs and lasers emit photons with energies near the gap; wide-bandgap materials enable high-voltage power electronics and devices that operate at high temperatures.

Representative values include silicon with an indirect gap of about 1.12 eV at room temperature and gallium arsenide with a direct gap of about 1.42 eV. Wide-bandgap materials such as GaN and SiC have gaps in the 3–4 eV range. Real materials may contain defect states in the gap and excitons that influence the optical response.