Semiconductor doping – valence electrons in an acceptor impurity In silicon technology, an acceptor impurity creates holes by accepting electrons from the lattice. How many valence electrons does a typical acceptor (e.g., boron, aluminum) contribute?

Introduction:
Doping intrinsic semiconductors tailors their electrical properties by introducing controlled impurities. Acceptors create p-type material by providing energy levels that can accept electrons from the valence band, leaving behind mobile holes. Recognizing the valence electron count clarifies why certain periodic table groups act as acceptors in silicon.

Given Data / Assumptions:

• Host lattice: silicon (group IV) with 4 valence electrons.
• Substitutional doping on Si lattice sites.
• Thermal equilibrium at ordinary temperatures (room temperature).

Concept / Approach:

Group III elements such as boron, aluminum, gallium have 3 valence electrons. When substituting into the Si lattice (requiring four bonds), one bond lacks an electron, creating an acceptor level slightly above the valence band. Thermal energy allows electrons to fill this level, leaving a hole in the valence band. Hence, acceptors are trivalent dopants with 3 valence electrons.

Step-by-Step Solution:

Verification / Alternative check:

Device physics texts consistently show B, Al, Ga as acceptors in Si, each trivalent, confirming the valence count.

Why Other Options Are Wrong:

5 valence electrons are donors (group V, n-type); 4 is the host silicon; 1 or 2 do not represent typical substitutional acceptors in Si.

Common Pitfalls:

Confusing donor and acceptor groups; assuming the same dopant roles in different semiconductors without checking band structure.

Final Answer:

3