Understanding Doping in Semiconductors

Doping is a process where impurities are intentionally added to intrinsic (pure) semiconductors like silicon (Si) or germanium (Ge) to alter their electrical properties. This creates extrinsic semiconductors, which can be either n-type or p-type, depending on the type of impurity added.

Trivalent Doping Elements vs. Pentavalent Doping Elements

The type of extrinsic semiconductor created depends on the valence electrons of the doping element compared to the semiconductor material (Silicon or Germanium, which are tetravalent, having 4 valence electrons).

• Trivalent Doping Elements: These are elements that have 3 valence electrons. When a trivalent element replaces an atom in the semiconductor lattice, it creates a 'hole' (a deficiency of an electron) because it only forms three covalent bonds with the four surrounding semiconductor atoms. These elements are called acceptor impurities and lead to the creation of p-type semiconductors. Trivalent elements are typically found in Group 13 of the periodic table.
• Pentavalent Doping Elements: These are elements that have 5 valence electrons. When a pentavalent element replaces an atom in the semiconductor lattice, it forms four covalent bonds with the surrounding semiconductor atoms, leaving one extra electron free. These elements are called donor impurities and lead to the creation of n-type semiconductors. Pentavalent elements are typically found in Group 15 of the periodic table.

Analyzing the Given Options

Let's examine each option and determine its valence and group in the periodic table:

• Arsenic (As): Arsenic is in Group 15. Elements in Group 15 have 5 valence electrons. Arsenic is a pentavalent doping element.
• Antimony (Sb): Antimony is also in Group 15. Elements in Group 15 have 5 valence electrons. Antimony is a pentavalent doping element.
• Boron (B): Boron is in Group 13. Elements in Group 13 have 3 valence electrons. Boron is a trivalent doping element.
• Phosphorous (P): Phosphorous is in Group 15. Elements in Group 15 have 5 valence electrons. Phosphorous is a pentavalent doping element.

Based on this analysis, Boron is the element that has 3 valence electrons, making it a trivalent doping element.

Therefore, the trivalent doping element among the given options is Boron.

Revision Table: Semiconductor Doping Elements

Additional Information about Doping

Doping concentration is crucial in determining the conductivity of a semiconductor. Even a small amount of doping, like 1 impurity atom per $10^6$ or $10^7$ semiconductor atoms, can increase conductivity significantly. The Fermi level, which represents the energy state with a 50% probability of being occupied by an electron, shifts towards the conduction band in n-type semiconductors and towards the valence band in p-type semiconductors. This shift is fundamental to the operation of semiconductor devices like diodes and transistors.

Pure semiconductors are called intrinsic semiconductors and have relatively low conductivity. Doped semiconductors are called extrinsic semiconductors and have much higher conductivity. The choice between trivalent and pentavalent doping elements depends entirely on whether an n-type or p-type material is needed for a specific electronic component or circuit design.