Question

For a heavily doped n-type semi-conductor, donor level lies

• A. a little below the conduction band
• B. a little above the valance band
• C. a little inside the valence band
• D. at the center of the band gap

Hint:

The fifth electron of a pentavalent donor is loosely bound, so it needs very little energy to reach the conduction band.

Answer 1

The Correct Option is A

Step 1: Idea of donors:
A donor atom gives away one extra electron. For this to happen easily, the extra electron must sit at an energy only slightly less than the conduction band edge.

Step 2: Compare with p-type:
In p-type material the impurity levels are acceptor levels, which sit just above the valence band. Donor levels are the mirror image, so they sit just under the conduction band.

Step 3: Effect of heavy doping:
Heavy doping adds many donor atoms, and the donor levels form a narrow band. That band still stays close to the conduction band, so the position does not change.

Step 4: Pick the option:
Only the first option puts the donor level near the conduction band from below. The others describe acceptor levels or the mid gap.

Final Answer:
The donor level is just below the conduction band. \[\boxed{\text{a little below the conduction band}}\]