Semiconductor Materials: Group IV Elements, Carriers & Doping: 17 MCQs with answers

Its conductivity lies between conductors and insulators.

A semiconductor's conductivity sits between that of conductors ($10^4$–$10^7$ S/cm) and insulators ($10^{-12}$–$10^{-18}$ S/cm), typically $10^{-6}$–$10^3$ S/cm.

A negative temperature coefficient of resistivity

Semiconductors have a negative temperature coefficient: resistivity decreases as temperature rises because more electron–hole pairs are generated.

Changing the colour of the packaging

Doping, temperature, and illumination all change carrier concentration. Packaging colour has no effect on conductivity.

Group IV

Group IV elements like Si, Ge, and Sn have four valence electrons and form covalent lattices, making them common elemental semiconductors.

A missing electron in a covalent bond

A hole is a deficiency of an electron in a covalent bond. It is not a real particle and has no actual positive charge, though it behaves as a positive carrier.

True

The band gap of semiconductors typically ranges from 0.1 to 3 eV, which is why they respond to modest thermal or optical energy.

Both free electrons and holes

In an intrinsic semiconductor, both free electrons and holes contribute to conduction, unlike metals where only electrons conduct.

Phosphorus

Phosphorus is pentavalent and acts as a donor, creating n-type material. Boron, indium, and aluminium are trivalent acceptors.

Holes

Trivalent dopants create acceptor levels, making holes the majority carriers in p-type material.

Valence electrons are locked in covalent bonds.

At 0 K, no thermal energy is available to break covalent bonds, so no free carriers exist and the material behaves as an insulator.

It ranges from about 0.1 to 3 eV.

Semiconductor band gaps range from about 0.1 to 3 eV, small enough for thermal or optical energy to lift electrons into the conduction band.

An insulator

A 6 eV band gap (like diamond) is too large for thermal energy at room temperature to free electrons, so the material remains an insulator.

Between the donor level $E_d$ and the conduction band $E_c$

In n-type material, the donor level $E_d$ sits just below the conduction band $E_c$, and the Fermi level lies between them.

Light frees extra charge carriers.

Illumination with suitable wavelength light generates additional electron–hole pairs, increasing the number of charge carriers and thus conductivity.

Its resistance changes with voltage, temperature, and light.

Semiconductors do not obey Ohm's law because their resistance varies with applied voltage, temperature, and illumination, making current non-proportional to voltage.

Arsenic — donor

Arsenic is pentavalent and acts as a donor. Boron and indium are trivalent acceptors, and phosphorus is a pentavalent donor.

1.8 x 10^16 electrons

To find the number of free electrons, first calculate the number of silicon atoms in 1 cm³: density = $5 \times 10^3$ kg/m³ = $5 \times 10^6$ g/m³, thus 1 cm³ contains $5 \times 10^6$ g/m³ / 28 g/mol = $1.79 \times 10^{23}$ atoms. With a doping concentration of $1 \text{ in } 10^7$, the number of phosphorus atoms (dono…