Valence electrons of a trivalent impurity (semiconductor doping) In semiconductor terminology, a “trivalent impurity” has how many valence electrons?

Introduction / Context:
Doping silicon or germanium with substitutional impurities changes carrier type. Trivalent dopants create acceptor states and promote p-type behavior. Recognizing the valence of dopants is essential for device physics.

Given Data / Assumptions:

• Host crystal: group-IV semiconductor (Si/Ge) with four valence electrons.
• “Trivalent impurity” means group-III dopant such as B, Al, Ga, or In.
• Substitutional incorporation on a lattice site.

Concept / Approach:
A group-III atom has three valence electrons. When it replaces a group-IV atom, one covalent bond remains short of an electron, creating an acceptor level. Thermal energy can promote an electron from the valence band to fill this bond, leaving behind a mobile hole (the majority carrier in p-type material).

Step-by-Step Solution:
Identify trivalent dopants: 3 valence electrons.Substitute into group-IV lattice → one bond is deficient by one electron.This produces an acceptor state and generates a hole upon ionization.

Verification / Alternative check:
Common p-type dopants: boron (3e−), aluminum (3e−), gallium (3e−). Their chemistry and device behavior confirm the 3-valence electron characterization.

Why Other Options Are Wrong:
5 and 4 correspond to pentavalent/group-V and group-IV atoms; 1 or 2 are not relevant for standard substitutional dopants in Si/Ge.

Common Pitfalls:

• Confusing “trivalent” with “triply ionized”; here it refers to valence count.

Final Answer:
3