SOLUTION

The addition of pentavalent impurity to semiconductors creates many free Electrons.

• The n-type semiconductor is formed by doping a pure silicon or germanium crystal with a material having five valence electrons.
• Antimony, arsenic, and phosphorous are examples of pentavalent materials.
• If arsenic in very small quantity is added to a silicon crystal, four out of five valence electrons will form covalent bonds with silicon atoms with one electron left free.
• Thus, for each arsenic atom, there will be one free electron. Although the percentage of arsenic added is very small, the number of atoms being very large, a huge amount of free electrons will be available in the n-type semiconductor.
• These electrons being free (not taking part in any covalent bonding) are loosely bound to their parent atom and are free to conduct electricity.