Understanding Pentavalent Impurities in Semiconductors

The question asks us to identify a pentavalent impurity from the given options. In semiconductor physics, impurities are intentionally added to pure semiconductor materials (like Silicon or Germanium) to control their electrical conductivity. These impurities are categorized based on their valency, specifically the number of valence electrons they possess.

Pentavalent impurities are elements that contain five electrons in their outermost shell (valence shell). When such an element is introduced into a semiconductor crystal lattice (e.g., Silicon, which has 4 valence electrons), the impurity atom replaces a host atom. Typically, four of the impurity's valence electrons form strong covalent bonds with the four neighboring semiconductor atoms. However, the fifth valence electron is only weakly bound to the nucleus. This extra electron requires very little energy to break free and become a mobile charge carrier, contributing to electrical conduction. Semiconductors doped with pentavalent impurities become n-type semiconductors, where electrons are the majority charge carriers.

Analyzing Options for Pentavalent Impurity

To determine which option is a pentavalent impurity, we need to check the number of valence electrons for each element:

1. Boron (B)

Boron (atomic number 5) has the electronic configuration $2s^2 2p^1$. It possesses 3 valence electrons. When Boron is used as an impurity in Silicon, it forms three covalent bonds and leaves one bond incomplete, creating a vacancy or 'hole'. This makes Boron a trivalent impurity, leading to the formation of a p-type semiconductor.

2. Arsenic (As)

Arsenic (atomic number 33) has the electronic configuration $[Ar] 3d^{10} 4s^2 4p^3$. It possesses 5 valence electrons. When Arsenic is doped into Silicon, four of its valence electrons participate in forming covalent bonds with neighboring Silicon atoms. The fifth valence electron is loosely bound and easily becomes available as a free electron for conduction. Thus, Arsenic is a pentavalent impurity and results in an n-type semiconductor.

3. Gallium (Ga)

Gallium (atomic number 31) has the electronic configuration $[Ar] 3d^{10} 4s^2 4p^1$. It has 3 valence electrons. Similar to Boron, Gallium acts as a trivalent impurity in semiconductors. It accepts an electron to complete its covalent bonds, creating a hole and thus forming a p-type semiconductor.

4. Indium (In)

Indium (atomic number 49) has the electronic configuration $[Kr] 4d^{10} 5s^2 5p^1$. It also has 3 valence electrons. Indium functions as a trivalent impurity, similar to Boron and Gallium, leading to the creation of p-type semiconductors.

Identifying the Pentavalent Impurity Correctly

Summarizing the valence electron counts:

• Boron: 3 valence electrons (Trivalent)
• Arsenic: 5 valence electrons (Pentavalent)
• Gallium: 3 valence electrons (Trivalent)
• Indium: 3 valence electrons (Trivalent)

Based on this analysis, Arsenic is the element with five valence electrons among the given choices, making it the correct pentavalent impurity.