Understanding Donor Impurities in Semiconductors

In semiconductor physics, doping is the process of adding impurities to a pure semiconductor (like Silicon or Germanium) to change its electrical conductivity. These impurities can be classified as either donor impurities or acceptor impurities.

What is a Donor Impurity?

A donor impurity is an atom added to a semiconductor that contributes extra conduction electrons to the semiconductor's crystal lattice. These atoms typically come from Group 15 (V) of the periodic table, such as Phosphorus (P), Arsenic (As), or Antimony (Sb).

Pure semiconductors like Silicon (Si) and Germanium (Ge) are in Group 14 (IV) and have 4 valence electrons, forming four covalent bonds with neighboring atoms. When a Group 15 impurity atom (with 5 valence electrons) replaces a semiconductor atom, four of its valence electrons form covalent bonds, just like the host atom. However, the fifth valence electron is weakly bound and can easily become a free electron available for conduction. This process creates an n-type semiconductor (n for negative charge carriers - electrons).

What is an Acceptor Impurity?

In contrast, an acceptor impurity is an atom added to a semiconductor that accepts an electron from a semiconductor atom, creating a "hole" in the covalent bond. These atoms typically come from Group 13 (III) of the periodic table, such as Boron (B), Aluminum (Al), or Gallium (Ga).

When a Group 13 impurity atom (with 3 valence electrons) replaces a Group 14 semiconductor atom, it can only form three covalent bonds. This leaves one bond incomplete, effectively creating a deficiency of an electron, known as a hole. This hole acts as a positive charge carrier. This process creates a p-type semiconductor (p for positive charge carriers - holes).

Analyzing the Given Options

Let's look at the options provided and their positions in the periodic table:

• Al (Aluminum): Aluminum is in Group 13 of the periodic table. Atoms from Group 13 are acceptor impurities.
• B (Boron): Boron is in Group 13 of the periodic table. Atoms from Group 13 are acceptor impurities.
• Ga (Gallium): Gallium is in Group 13 of the periodic table. Atoms from Group 13 are acceptor impurities.
• P (Phosphorus): Phosphorus is in Group 15 of the periodic table. Atoms from Group 15 are donor impurities.

Based on this analysis, Phosphorus (P) is the atom that acts as a donor impurity when added to a typical semiconductor like Silicon or Germanium.