Semiconductor doping basics: effect of adding a trivalent impurity When a trivalent (group III) impurity such as boron, aluminium, or gallium is introduced into a tetravalent semiconductor (for example silicon or germanium), the resulting semiconductor becomes:

Introduction / Context:
Doping is the intentional introduction of impurity atoms to control carrier concentrations in semiconductors. Understanding how donor and acceptor dopants change conduction is foundational for diodes, transistors, and integrated circuits.

Given Data / Assumptions:

• Host crystal is tetravalent (e.g., Si or Ge).
• Impurity is trivalent (group III), such as B, Al, or Ga.
• Thermal equilibrium and moderate doping (nondegenerate) are assumed.

Concept / Approach:
A trivalent atom has three valence electrons. In a tetrahedrally coordinated lattice requiring four covalent bonds, one bond around a trivalent atom lacks an electron, creating an electron vacancy called a hole. Such dopants are called acceptors because they can accept an electron from the valence band, leaving behind mobile holes that carry positive charge.

Step-by-Step Solution:
A trivalent dopant contributes one fewer valence electron than the host requires for bonding.This deficit forms an acceptor level slightly above the valence band edge.At ordinary temperatures, electrons thermally excite from the valence band into the acceptor level.This process leaves mobile holes in the valence band, increasing hole concentration p.Since p ≫ n after such doping, holes are majority carriers and the material is p-type.

Verification / Alternative check:
Mass-action law states n * p = n_i^2. If acceptors raise p well above n_i, then n = n_i^2 / p becomes small, confirming holes as majority carriers and electrons as minority carriers.

Why Other Options Are Wrong:
n type: Requires donors (group V), not acceptors. Intrinsic: Undoped, with n ≈ p. Neutral: Doping purposefully shifts carrier balance. Degenerate semiconductor: Implies very heavy doping that moves the Fermi level into a band, not assumed here.

Common Pitfalls:
Confusing valence count (3 vs 5) and mixing up donors with acceptors; assuming doping always increases electrons rather than considering the dopant’s valence relative to the host.

Final Answer:
p type