Energiavahe

Energiavahe (engl. energy gap) is the energy difference between two quantum states of a system. In atoms and molecules this often means the gap between the highest occupied and lowest unoccupied molecular orbitals, or between ground and first excited states. In solid materials, the term most commonly refers to the electronic band gap: the energy difference between the valence-band maximum and the conduction-band minimum.

The size and nature of the energy gap influence a material's electrical conductivity and optical behavior. Materials with a large gap are insulators at room temperature; smaller gaps yield semiconductors, which can conduct under illumination or with doping; metals have effectively zero gap. The gap can be direct or indirect in momentum space, affecting how readily photons are absorbed or emitted.

Band-gap values are usually expressed in electronvolts (eV) and vary with temperature, pressure and composition. Common examples at room temperature include silicon about 1.12 eV, GaAs about 1.43 eV, GaN around 3.4 eV, and diamond about 5.5 eV. All are insulators or wide-gap semiconductors; lower gaps enable near-infrared to visible response, higher gaps transmit in the ultraviolet.

In superconductors the term energy gap also appears as the superconducting gap Δ, the energy required to break a Cooper pair. This gap opens below the critical temperature and decreases with increasing temperature, as described by BCS theory; it is probed by tunneling and spectroscopy techniques.

In chemistry and molecular physics, the energy gap between frontier orbitals (the HOMO–LUMO gap) influences optical absorption, reactivity, and color. Smaller gaps shift absorption to longer wavelengths; larger gaps push absorption into the UV.