Semiconductor doping — effect of pentavalent (donor) impurities When a pentavalent impurity (for example, phosphorus, arsenic, or antimony) is added to a pure semiconductor crystal, the resulting semiconductor becomes:

Introduction / Context:Doping controls carrier type and concentration in semiconductors. Pentavalent dopants in group-IV semiconductors provide extra electrons near the conduction band and are central to forming n-type regions in diodes and transistors.

Given Data / Assumptions:

• Host crystal: group-IV (e.g., Si, Ge).
• Dopant valence: 5 (donor), such as P, As, Sb.
• Low to moderate doping so the material remains nondegenerate.

Concept / Approach:Pentavalent atoms substitute for a host atom and form four covalent bonds, leaving one loosely bound electron. This donor electron is thermally ionized at room temperature, contributing a free electron to the conduction band and creating a positively charged donor ion. Thus electrons become majority carriers and the material is n-type.

Step-by-Step Solution:Identify dopant valence: 5 → donor.Ionization releases an electron to the conduction band.Carrier balance shifts: n ≫ p → n-type behavior.

Verification / Alternative check:Energy band diagrams show donor levels slightly below the conduction band; at room temperature most donors are ionized, raising electron concentration.

Why Other Options Are Wrong:p-type requires acceptors (trivalent B, Al, Ga). “Intrinsic” ignores the dopant. “Neutral with no change” is false; carrier concentration changes markedly. “Degenerate p-type” is unrelated to donor doping.

Common Pitfalls:

• Confusing donor and acceptor terminology.
• Assuming all dopants create the same carrier type.

Final Answer:n-type