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: Understand the concept:
In an n-type semiconductor, a pentavalent impurity such as phosphorus is added to silicon. Four of its five valence electrons form bonds. The fifth electron is loosely held.

Step 2: Where the donor level sits:
The energy needed to free this fifth electron is very small, about 0.01 to 0.05 eV. So the donor energy level lies just below the bottom of the conduction band. At room temperature these electrons jump easily into the conduction band.

Step 3: Check option (2) and (3):
A level a little above the valence band, or inside it, is the acceptor level of a p-type semiconductor. So both are wrong here.

Step 4: Check option (4):
A level at the centre of the band gap belongs to an intrinsic semiconductor's Fermi level. It is not a donor level, and electrons there would need a lot of energy to reach the conduction band.

Final Answer:
The donor level lies a little below the conduction band, option (1).