The work I am involved in at Essex is the study of   Electrons in Gallium Nitride. GaN is a semiconductor material which is causing great excitement in the photonics industry and currently undergoing extensive worldwide research. It is a large bandgap material which makes it suitable for the development of devices with short wavelength emission and detection capabilities.
    To the uninitiated, a semiconductor is not a part time ticket collector but in fact a material which at times behaves like an electrical conductor and at other times behaves like an electrical insulator. The conductivity (s) of the material at a specific temperature can be given by the equation s=nem where n is the number of free electrons in the material, e is the electron charge and m is the mobility of the material. The electron charge is negative, -1.6*10-19 Coulombs. The number of free electrons in semiconductors or any material is governed by the number of electrons orbiting in the outermost electron shell of the individual atoms comprising the material. Each electron shell for every atom can only contain so many electrons as a consequence of the Pauli exclusion principle. These shells fill up with electrons starting with the innermost, the outer most full electron shell is called the Valence band. The next shell out from the atom wether it is empty or if it contains any electrons is called the Conduction band. Each electron shell has a different energy required to keep the electrons orbiting the atom, the further out the shell the more energy the electrons have. Atoms inthe conduction band are free, meaning they are free to move from atom to atom within the material until they fall back into any atomic valence band. The special properties of semiconductors is that at approximately room temperatures electrons in the valence band can easily be excited into the conduction band by stimulating them with an external energy source. The electrons leave the valence band and leave behind them a hole that in effect acts like a positive electron (positron). When the free electrons encounter a hole under no energy stimulation they fall back into the valence band (recombine) and can emit a photon with roughly the same energy as the bandgap between the conduction and valence bands of the atoms they are orbiting.
   The simplest semiconductor device is that of the p-n Junction. Electrons and holes can be added to a semiconductor material to make it either p-type with an excess number of holes or n-type with an excess number of electrons. By placing some n-type material and some p-type material close together at a junction (surfaces must be matched at the atomic level) a p-n junction is formed. An external potential difference can then be applied across the junction, if the potential is forward bias the junction conducts much more readily