n-type semiconductors
If an impurity element with five outer shell electrons, such as arsenic, is added to silicon in small quantities, (approximately one impurity atom to every one million silicon atoms), the impurity atoms will fit into the crystal structure.
The additional outer shell electron will not be bonded into the valence band of the crystal. This doping affects the electrons' ability to move between energy bands. More electrons are available in the conduction band.
This impurity causes the material to conduct and is called n-type silicon. n-type refers to the negative charge of the extra electron.