Semiconductors all have a reasonably sized
band gap which enables them to
conduct or not conduct current depending on
applied voltages. An n-type semiconductor (as opposed to a
p-type semiconductor) is one in which
doping or the natural flaws in the crystalline structure promote
electron generation in the conduction band.
Because there are a lot (relatively) of electrons in the conduction band, they are the major contributers to current. This becomes important when creating advanced devices such as p-n junctions, where one p-type semiconductor is joined with an n-type semiconductor.
The electrons are generated because some flaw or dopant creates a single allowed energy state in the normally forbidden band gap close to the energy level of the conduction band (a perfect semiconductor lattice would be neither n nor p type).
Energy
^
|
| Conduction Band
| O
| -----------------------
| - O - - O - O - O -
| \
| Band Gap energy state
| caused by dopant
| (full of electrons)
| -----------------------
| O O O O O O O O O O O
| | Valence Band \
| \ \
| hole electron
|
+------------------> x direction
This graph shows holes as blank spaces and electrons as an 'O'. Note how the electrons in the energy state caused by the dopant can travel up into the conduction band where they contribute to current.
I'm not sure if my graph is too confusing. /msg me and let me know. Also if there is any additional information you would like to see, I am more than happy to add it.