Materials Science: Bonding and Band Theory

Chemistry helps engineers understand and design materials by controlling their properties such as electrical, thermal, mechanical, magnetic, and optical behavior.

Electrical conductivity, thermal conductivity, mechanical strength, magnetic properties, and optical properties.

Because circuit boards require a combination of conductors, insulators, and semiconductors to function properly.

An LED is a solid-state semiconductor device that converts electrical energy into light energy.

The type of bonding determines electron mobility, which directly affects electrical conductivity and other properties.

Covalent bonding involves sharing electrons between atoms, resulting in localized electrons.

Because electrons are localized in strong bonds and cannot move freely to conduct electricity.

Ionic bonding involves transfer of electrons and electrostatic attraction between positive and negative ions in a lattice.

Because electrons are confined to ions and cannot move freely through the structure.

Metallic bonding involves a lattice of positive ions surrounded by a “sea” of delocalized electrons.

A model where valence electrons move freely throughout the metal lattice, enabling conductivity.

Because their free electrons can move easily through the lattice and carry charge.

Metals are ductile (can be drawn into wires) and malleable (can be shaped into sheets).

Conductors, semiconductors, and insulators.

Metals and graphite are examples of conductors.

Silicon (Si), gallium arsenide (GaAs), and indium phosphide (InP).

Ceramic oxides, polymers, and paper.

Its electronic band structure, specifically the energy gap between valence and conduction bands.

A band is a group of closely spaced energy levels formed from overlapping atomic orbitals.

By combining atomic orbitals of many atoms, producing molecular orbitals that merge into continuous bands.

The valence band contains lower-energy occupied electrons, while the conduction band contains higher-energy empty states.

A material must have partially filled bands or overlapping valence and conduction bands.

Because electrons cannot move to higher energy states due to lack of available empty levels.

Because their valence and conduction bands overlap, allowing free electron movement.

Small band gap → semiconductor; large band gap → insulator; no gap → conductor.

It explains and predicts electrical behavior, enabling design of electronic devices.

A semiconductor is a material with electrical conductivity between that of a conductor and an insulator, controlled by its band gap.

Intrinsic (pure) semiconductors and extrinsic (doped) semiconductors.

A pure semiconductor with no impurities, where conductivity arises from thermally generated charge carriers.

Because no electrons have enough energy to move from the valence band to the conduction band.

Thermal energy excites electrons into the conduction band, enabling conductivity.

A semiconductor whose conductivity is enhanced by adding small amounts of impurities (dopants).

Doping is the intentional addition of impurity atoms to modify electrical conductivity.

A semiconductor formed by doping with group 15 elements, providing extra electrons as charge carriers.

A semiconductor formed by doping with group 13 elements, creating holes as charge carriers.

Electrons are the majority charge carriers.

Holes (electron vacancies) are the majority charge carriers.

It increases conductivity by introducing additional charge carriers (electrons or holes).

Extra electrons occupy energy levels near the conduction band, making it easier for conduction.

Vacancies (holes) are created near the valence band, allowing electron movement.

A boundary between p-type and n-type semiconductors that enables controlled current flow.

Electrons and holes move toward the junction, allowing current to flow.

Charge carriers are pulled away from the junction, preventing current flow.

Because higher temperature generates more charge carriers by exciting electrons into the conduction band.

Because increased atomic vibrations scatter electrons, reducing their mobility.

The emission of light when electrical energy excites electrons in a semiconductor.

The conversion of light energy into electrical energy in devices like solar cells.

Light excites electrons, creating charge flow across a p–n junction to produce current.

They enable controlled conduction, making them essential for diodes, transistors, LEDs, and solar cells.

An LED is a semiconductor device that emits light when electrons recombine with holes, releasing energy as photons.

Electron–hole recombination in a semiconductor releases energy in the form of light.

Electroluminescence is the emission of light due to electrical excitation of electrons in a material.

At the p–n junction of the semiconductor material.

Electrons and holes move toward the junction and recombine, emitting light.

Because charge carriers are pulled away from the junction, preventing recombination.

The band gap energy of the semiconductor material.

The emitted photon energy equals the band gap energy (E = Eg).

Photon energy is inversely proportional to wavelength.

The Planck relation connects energy, frequency, and wavelength of photons (E = hν = hc/λ).

Small band gap (e.g., GaAs).

Large band gap (e.g., GaN).

They enable efficient photon emission through direct electron transitions.

Because it has an indirect band gap, meaning most energy is lost as heat (vibrations) rather than light.

It allows engineers to tune the band gap and thus the color of light.

It creates the necessary concentrations of electrons and holes for recombination.

Band gap energy, recombination efficiency, and device design (light extraction).

High temperature can reduce efficiency by increasing non-radiative recombination (heat production).

Lighting (bulbs, street lights), displays (TVs, phones), and communication (fiber optics).

They convert electrical energy directly into light with minimal heat loss and have very long lifetimes.