Forbidden Zone
Definition and meaning of Forbidden Zone in chemistry.
Forbidden zone (also called the band gap or forbidden band) is the range of electron energies between a solid's valence band and conduction band in which no electron states exist. Electrons cannot occupy energies within this zone; they must acquire enough energy to jump from the top of the valence band to the bottom of the conduction band.
In more detail
In band theory, overlapping atomic orbitals in a crystal lattice split into continuous, closely spaced energy bands separated by gaps where no wavefunction solutions satisfy the periodic potential of the lattice. The width of the forbidden zone determines electrical behavior: metals have no gap (bands overlap or are partially filled), semiconductors have a narrow gap that thermal energy or absorbed light can bridge, and insulators have a wide gap that effectively blocks conduction at ordinary temperatures. Thermal energy or photon absorption can supply the energy needed to promote electrons directly across this gap, while doping instead introduces additional donor or acceptor energy levels within the forbidden zone (close to the conduction or valence band edges) that lower the effective energy barrier for conduction without eliminating the intrinsic gap itself.
Key facts
| Field | Physical Chemistry |
|---|---|
| Also known as | Band gap; forbidden band |
| Silicon gap (300 K) | ≈1.1 eV |
| Diamond gap (insulator) | ≈5.5 eV |
Silicon has a forbidden zone of about 1.1 electronvolts at room temperature, so electrons need at least this much energy (from heat or absorbed light) to jump from the valence band into the conduction band, which is why silicon behaves as a semiconductor rather than a conductor or insulator.
Frequently asked questions
Is the forbidden zone the same thing as the band gap?
Yes, "forbidden zone" and "forbidden band" are alternative names for the band gap in solid-state band theory.
Why can't electrons have energies inside the forbidden zone?
The periodic potential of the crystal lattice permits only certain electron wavefunctions; energies between the valence and conduction bands correspond to no allowed standing-wave solutions, so no stable electron states exist there.