4. What type of impurity atoms are used to increase the number of conduction band
electrons in intrinsic silicon?
a. bivalent
b. trivalent
c. tetravalent
d. pentavalent

Pentavalent Impurity Atoms Increase Conduction Band Electrons in Intrinsic Silicon

Pentavalent impurity atoms are used to increase conduction band electrons in intrinsic silicon, making the correct answer d. pentavalent. This process creates n-type semiconductors through doping.​

Option Analysis

Intrinsic silicon has four valence electrons per atom, forming a stable covalent lattice with equal electrons and holes. Adding impurities disrupts this balance.

• a. Bivalent (divalent): These have two valence electrons (e.g., magnesium). They act as double acceptors, creating more holes than electrons, unsuitable for increasing conduction electrons.​

• b. Trivalent: Atoms like boron or aluminum (three valence electrons) create acceptor levels near the valence band, accepting electrons to form holes, resulting in p-type semiconductors with majority holes.​

• c. Tetravalent: Silicon itself is tetravalent (four valence electrons), so these maintain intrinsic properties without net charge carrier increase.​

• d. Pentavalent: Atoms like phosphorus, arsenic, or antimony (five valence electrons) donate an extra electron to the conduction band after forming four bonds, significantly boosting free electrons as majority carriers in n-type silicon.​

Doping Mechanism

In intrinsic silicon, thermal excitation minimally populates the conduction band. Pentavalent doping introduces donor levels ~0.05 eV below the conduction band, easily ionizing at room temperature to supply electrons: Si (group IV) + P (group V) → four covalent bonds + one free electron. Conductivity rises dramatically, with electrons as majority carriers and holes as minorities.​

Pentavalent impurity atoms increase conduction band electrons in intrinsic silicon, transforming it into an n-type semiconductor essential for electronics like diodes and transistors. This doping technique boosts conductivity by adding free electrons from group V elements.​

Why Pentavalent Atoms?

Silicon’s tetrahedral structure needs four electrons for bonding. Pentavalent atoms (P, As, Sb) provide five, leaving one loosely bound electron that jumps to the conduction band, creating majority electron carriers. Concentration is typically 1 dopant per 10^6 silicon atoms for optimal control.​

Comparison of Impurity Types

Trivalent doping suits p-n junctions’ hole side, while pentavalent handles electron flow.​