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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