# Electron

The electron is a particle with a mass of 9.11×10-31kg and a charge of -1.60×10-19 Coulombs. Electrons can be found in an atom, where they orbit the positively charged nucleus.

Electrons orbit the nucleus of an atom at a given radius: r1, r2, etc. Each orbit has a corresponding energy level: E1, E2 etc. This energy level is the amount of energy required to free the electron from the atom. The equations for these values for the nth orbit are:

```  En = m0q4/(8ε02h2n2)
rn = ε0n2h2/(q2πm0)
```

Here ε0 is the absolute permittivity of a vacuum which is 8.85×10-12 Fm-1, q is the charge of an electron given earlier, m0 is the mass of an electron given earlier, and h is Planck's constant which is 6.63×10-34Js.

Where an atom has energy levels, a solid has energy bands. Again, this energy is the amount of energy required to free an electron. Each of these bands are separated by an energy gap which is the amount of energy required to move an electron from the lower to the higher of the two bands. The two outer bands are known as the valence and the conduction bands. Electrons in the conduction band are called mobile charge carriers.

## References

Fischer-Cripps. A.C., The Electronics Companion. Institute of Physics, 2005.
Roulston. David. J., An Introduction to the Physics of Semiconductor Devices. Oxford University Press, 1999.