Question

In the band structure of n-type semiconductor , the free electrons donated by impurity atoms occupy energy levels in

• A. the conduction band.
• B. the valence band.
• C. the band gap and are close to conduction band.
• D. the band gap and are close to valence band.

Hint:

Donor levels lie in the band gap just below the conduction band.

Answer 1

The Correct Option is C

Step 1: Understanding the Concept:
An n-type semiconductor is made by adding pentavalent impurity atoms (donors) such as phosphorus to silicon. Each donor atom has one extra electron that is not needed for bonding.

Step 2: Energy band picture:
The extra electrons occupy a discrete donor energy level in the forbidden gap, very close to the bottom of the conduction band, only about 0.01 to 0.05 eV below it.

Step 3: Detailed Explanation:
Because this level is so close to the conduction band, a very small amount of thermal energy at room temperature lifts these electrons into the conduction band, where they become free electrons for conduction. The donor levels themselves lie in the band gap close to the conduction band.

Step 4: Why the other options are wrong.
The valence band (B) holds the bonding electrons, not the donor electrons. Option (D) describes acceptor levels of a p-type semiconductor, which lie in the gap close to the valence band.

Final Answer:
Donor levels lie in the band gap close to the conduction band, option (C). \[ \boxed{\text{Option (C)}} \]