Question

Ge is doped with As. Due to doping,

• A. the structure of Ge lattice is distorted.
• B. the number of conduction electrons increases.
• C. the number of holes increases.
• D. the number of conduction electrons decreases.

Hint:

Doping a semiconductor with a pentavalent impurity (such as As, P, or Sb) produces an n-type semiconductor where electrons become the majority charge carriers.

Answer 1

The Correct Option is B

Step 1: Understanding semiconductor doping.
Doping is the process of adding a small amount of impurity atoms to a pure semiconductor in order to increase its electrical conductivity. Germanium (Ge) is a group 14 element and has four valence electrons. When impurities are added, the electrical properties of the semiconductor change significantly.

Step 2: Nature of arsenic impurity.
Arsenic (As) belongs to group 15 of the periodic table and has five valence electrons. When arsenic atoms are introduced into the germanium crystal lattice, four of its electrons form covalent bonds with neighboring Ge atoms, while the fifth electron remains loosely bound.

Step 3: Formation of n-type semiconductor.
The extra fifth electron from arsenic becomes a free conduction electron. This increases the number of free electrons available for conduction. Therefore, the doped semiconductor becomes an n-type semiconductor, where electrons are the majority charge carriers.

Step 4: Evaluation of options.

• (A) Incorrect. The lattice structure remains nearly the same; only a small impurity atom replaces a Ge atom.
• (B) Correct. Doping with arsenic increases the number of conduction electrons.
• (C) Incorrect. Holes are majority carriers in p-type semiconductors, not in n-type.
• (D) Incorrect. The number of conduction electrons actually increases, not decreases.

Step 5: Conclusion.
Thus, when germanium is doped with arsenic, additional free electrons are produced, increasing the number of conduction electrons.
Final Answer: the number of conduction electrons increases.