Question

An n-type semi-conductor is formed when a trace amount of impurity is added to silicon. The number of valence electrons in the impurity atom must be:

• A. $3$
• B. $5$
• C. $1$
• D. $2$

Answer 1

The correct option is A $3$

An n-type semiconductor is due to the presence of a free electron in the semiconductor. Since silicon is an element having $4$ valence electrons, in order to have a free electron, it must be doped with an element having one more valence electron than silicon.

Hence the number of valence electron in the impurity must be $4+1=5.$

Therefore, option B is correct.