Band gap facts for kids
A band gap is like an energy "no-go zone" inside certain materials. Imagine electrons as tiny cars driving around. In some materials, there are certain speeds (energy levels) these cars just can't be at. The band gap is the range of these forbidden speeds.
This idea is super important in solid-state physics and chemistry. You find band gaps in materials called insulators and semiconductors.
Think of it this way:
- The valence band is where electrons usually hang out, close to their atoms. They're like cars parked in a garage.
- The conduction band is where electrons can move freely and help carry electricity. These are like cars driving on the open road.
The band gap is the energy difference between the top of the valence band and the bottom of the conduction band. It's the "jump" an electron needs to make to go from being stuck to an atom to being able to move freely and conduct electricity.
Materials with a large band gap are usually insulators. This means their electrons need a lot of energy to jump into the conduction band, so they don't conduct electricity well. Materials with smaller band gaps are semiconductors. Their electrons need less energy to jump, so they can conduct electricity under certain conditions. Conductors (like metals) either have very tiny band gaps or no band gap at all, meaning their electrons can move freely very easily.
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How Band Gaps Affect Electricity
Scientists use the band gap to figure out if a material will conduct electricity. Most electrons, called valence electrons, are held tightly by the nucleus of just one atom. But if an electron gets enough energy, it can break free from its atom. Then, it can join the flow of electric current through the material. These free electrons are part of the conduction band.
In semiconductors and insulators, electrons can only exist at certain energy levels, like steps on a ladder. They can't be in between these steps. The band gap is the empty space between the "steps" of the valence band and the conduction band.
Electrons can jump from the valence band to the conduction band. But they need a specific amount of energy to make this jump. The amount of energy needed is different for different materials. Electrons can get this energy by absorbing heat (called a phonon) or light (called a photon).
A semiconductor is a material with a small band gap. At very cold temperatures (like absolute zero), it acts like an insulator. But if you add a little heat, some electrons get enough energy to jump into the conduction band. This makes the semiconductor able to conduct electricity. Materials with a large band gap are insulators and don't conduct electricity easily. In conductors, the valence and conduction bands might even overlap, so there's no band gap at all, making it easy for electrons to move.
The ability of a semiconductor to conduct electricity depends a lot on its band gap. The only electrons that can conduct are those that have enough energy to cross this gap.
Scientists can even "engineer" band gaps! This means they can control or change the band gap of a material. They do this by mixing different semiconductor materials together. This is used to design things like laser diodes and solar cells.
It's sometimes hard to tell the difference between a semiconductor and an insulator. You can think of semiconductors as insulators with a very narrow band gap. Insulators usually have a band gap bigger than 3 eV.
The band gap energy of semiconductors usually gets smaller when the temperature goes up. This is because atoms vibrate more when it's hotter, which changes how electrons interact.
In a normal semiconductor crystal, the band gap is fixed. But in tiny crystals called quantum dots, the band gap can change depending on their size! This is called the quantum confinement effect. Band gaps also depend on pressure.
Solar Cells and Band Gaps
Electrons can get excited by light, not just heat. The band gap of a material decides which colors (or parts) of sunlight a solar cell can absorb. If a material's band gap is too big, it can't absorb much of the sunlight. If it's too small, it absorbs light but doesn't use all the energy efficiently. Solar cells work best when their band gap is just right for absorbing sunlight and turning it into electricity.
Common Band Gaps
Here's a list of band gaps for some materials:
| Material | Symbol | Band gap (eV) @ 302K |
|---|---|---|
| Silicon | Si | 1.11 |
| Selenium | Se | 1.74 |
| Germanium | Ge | 0.67 |
| Silicon carbide | SiC | 2.86 |
| Aluminium phosphide | AlP | 2.45 |
| Aluminium arsenide | AlAs | 2.16 |
| Aluminium antimonide | AlSb | 1.6 |
| Aluminium nitride | AlN | 6.3 |
| Diamond | C | 5.5 |
| Gallium(III) phosphide | GaP | 2.26 |
| Gallium(III) arsenide | GaAs | 1.43 |
| Gallium(III) nitride | GaN | 3.4 |
| Gallium(II) sulfide | GaS | 2.5 |
| Gallium antimonide | GaSb | 0.7 |
| Indium antimonide | InSb | 0.17 |
| Indium(III) nitride | InN | 0.7 |
| Indium(III) phosphide | InP | 1.35 |
| Indium(III) arsenide | InAs | 0.36 |
| Iron disilicide | β-FeSi2 | 0.87 |
| Zinc oxide | ZnO | 3.37 |
| Zinc sulfide | ZnS | 3.6 |
| Zinc selenide | ZnSe | 2.7 |
| Zinc telluride | ZnTe | 2.25 |
| Cadmium sulfide | CdS | 2.42 |
| Cadmium selenide | CdSe | 1.73 |
| Cadmium telluride | CdTe | 1.49 |
| Lead(II) sulfide | PbS | 0.37 |
| Lead(II) selenide | PbSe | 0.27 |
| Lead(II) telluride | PbTe | 0.29 |
| Copper(II) oxide | CuO | 1.2 |
| Copper(I) oxide | Cu2O | 2.1 |
Other Types of Gaps
The idea of "gaps" isn't just for electrons!
- In photonics, there are "photonic band gaps" where certain light frequencies (photons) can't pass through a material. These materials are called "photonic crystals".
- Similar ideas apply to phonons (vibrations or heat energy) in a "phononic crystal".
See also
In Spanish: Banda prohibida para niños
