Energy Bands and Band Gaps
How energy bands describe solids
From single atoms to solids
When many atoms join into a crystal, their energy levels spread into bands. The two most important bands are the valence band, which contains bound electrons, and the conduction band, where electrons can move freely through the material.
The band gap
The band gap is the energy difference between the valence band and the conduction band. In semiconductors, this gap is small enough that electrons can sometimes jump across it with added energy from heat or light.
Why the band gap matters
A small band gap means the material can become conductive under the right conditions, while a large band gap keeps electrons locked in place.
Band picture of materials
| Material type | Band gap | Typical behavior |
|---|---|---|
| Conductor | No effective gap | Electrons move freely |
| Semiconductor | Small gap | Conductivity is controllable |
| Insulator | Large gap | Very poor conduction |
What is the band gap?
The band gap is an energy difference, not a physical distance.
Correct answer: The energy difference between valence and conduction bands
Which band contains electrons that can move freely through a solid?
Electrons in the conduction band are able to contribute to electric current.
Correct answer: conduction band
Why can semiconductors conduct better at higher temperatures?
Extra thermal energy can excite electrons from the valence band into the conduction band.
Correct answer: Heat can give electrons enough energy to cross the band gap