N-Type semiconductor
- When a small current of Pentavalent impurity is added to a pure semiconductor it is called as n-type semiconductor.
- Addition of Pentavalent impurity provides a large number of free electrons in a semiconductor crystal.
- Typical example for pentavalent impurities are Arsenic, Antimony and Phosphorus etc. Such impurities which produce n-type semiconductors are known as Donor impurities because they donate or provide free electrons to the semiconductor crystal.
To understand the formation of n-type semiconductor, consider a pure silicon crystal with an impurity say arsenic added to it as shown in figure
- We know that a silicon atom has 4 valence electrons and Arsenic has 5 valence electrons. When Arsenic is added as impurity to silicon, the 4 valence electrons of silicon make co-valent bond with 4 valence electrons of Arsenic.
- The 5th Valence electrons finds no place in the covalent bond thus, it becomes free and travels to the conduction band as shown in figure. Therefore, for each arsenic atom added, one free electron will be available in the silicon crystal. Though each arsenic atom provides one free electrons yet an extremely small amount of arsenic impurity provides enough atoms to supply millions of free electrons.
Due to thermal energy, still hole election pairs are generated but the number of free electrons are very large in number when compared to holes. So in an n-type semiconductor electrons are majority charge carriers and holes are minority charge carriers . Since the current conduction is pre-dominantly by free electrons( -vely charges) it is called as n-type semiconductor( n- means –ve).
