n-type semiconductor can be produced by doped some pentavalent atoms into a semiconductor.
Pentavalence atoms are atoms that have 5 electrons in the valence shell. Examples of pentavalent atoms include, antimony, and phosphorus.
Figure above shows how the silicon crystal appears after doped with a phosphorous atom, which is pentavalence.
We can see that, the pentavalence atom form 4 covalent bonds with the silicon atoms around. Since a pentavalence atom has 5 electrons, there is an extra electron left over and it is a free electron.
Each pentavalent in the silicon crystal produces one free electron. Therefore, the pentavalence atom is called the donour.
The more pentavalence impurity that is added, the more free electrons in the semiconductor, and hence the greater the conductivity of the semiconductor.
Some holes will also formed in the semiconductor when some electrons are promoted to shell with higher energy level.
The free electrons outnumber the holes, hence they are called the majority carrier and the holes are called the minority carriers.
Since the negative charge carrier (the electrons) outnumber the positive charge carrier (the holes), the semiconductor is called an n-type semiconductor, where the n stands for negative.
