Donor impurity basics: A donor impurity atom in a group-IV semiconductor contributes how many valence electrons?

Introduction / Context:
In silicon or germanium, substitutional doping with group-V elements (P, As, Sb) creates donor states because these atoms bring five valence electrons to a lattice where only four are required for sp^3 bonding. The extra electron is weakly bound and can be thermally ionized to the conduction band, producing n-type conductivity.

Given Data / Assumptions:

• Host: group-IV semiconductor with four covalent bonds per atom.
• Donor: group-V atom substituting a lattice site.
• Room temperature operation where donors are largely ionized.

Concept / Approach:

Valence electron count determines dopant type: five valence electrons make a donor (n-type), three make an acceptor (p-type). The donor’s fifth electron occupies a shallow hydrogenic level and contributes to conduction when ionized.

Step-by-Step Solution:

Verification / Alternative check:

Common donors: P, As, Sb (all five-valent). Common acceptors: B, Al, Ga, In (three-valent).

Why Other Options Are Wrong:

• 4 corresponds to host atoms (Si, Ge), not donors.
• 3 corresponds to acceptors.
• 1 or 2 do not match main-group substitution chemistry here.

Common Pitfalls:

• Confusing donor/acceptor roles across different hosts (e.g., III–V compounds follow different rules).

Final Answer:

5