Why does n-type semiconductors have so many electrons than holes?
The elements of group V are pentavalent such as arsenic (As), phosphorus (P), bismuth (Bi), and antimony (Sb). These have five electrons in their valence band. When one of these materials is added to a semiconductor like Si, four of its valence electrons form four covalent bonds with neighboring Si atoms and the fifth electron becomes free.
The number of free electrons is equal to the number of impurity atoms added. So plenty of free charge carriers (electrons) are available for conduction. As the electrons have a negative charge, this material is called n-type material.
In n-type material, electrons are the majority carriers. In this way, an n-type semiconductor has so many electrons than holes.