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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.

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