Energy difference between valence and conduction bands.

• A. Band width
• B. Fermi energy
• C. Band gap
• D. Band alignment

Answer: (C)

In a solid, electrons fill energy bands formed from atomic orbitals. The energy difference between the valence band (the highest occupied band) and the conduction band (the lowest unoccupied band) is called the band gap. This gap represents the minimum energy required to promote an electron from a bound state in the valence band to a freely behaving state in the conduction band, which is what enables electrical conduction.

The size of this gap has big consequences. A small or zero gap means electrons can be easily excited and the material behaves like a metal or narrow-gap semiconductor. A moderate gap yields semiconductor behavior, useful for devices like diodes and transistors. A large gap results in insulating behavior. The band gap also determines the optical absorption edge: photons with energy at least equal to the band gap can be absorbed to excite electrons across the gap, which explains why materials appear colored or are active in photovoltaics and LEDs.

For contrast, band width describes how spread out the energy levels are within a single band, not the separation between bands. Fermi energy is the chemical potential at absolute zero, indicating the top of the occupied states at 0 K, and band alignment refers to how band energies line up at interfaces between different materials, affecting charge transfer but not the intrinsic gap itself.