Materials T7 - Functional Properties

Conductors

Allow for free flow of electrons (i.e. metals)

Semiconductors

Allow for some flow of electrons (e.g. silicon)

Insulators

Allows little/no flow of electrons (e.g. plastics, ceramics)

Electrical Current

Motion of electrically charged particles from externally applied E field

Solid Metal Energy Band Structures (copper)

Metal Energy Band Structures (magnesium)

Overlap of empty + filled band (3s and 3p bands overlap)

Semiconductor Band Structures

One band filled, separated from empty conduction band w/ small gap

Insulator Band Structures

Same as semiconductor, but w/ large gap

Valence Bands

e- band filled with electrons

Conduction Band

Empty e- bands

Conduction in Metals

Excited e- from filled states promoted to adjacent energy states w/ little energy required

Conduction in Semiconductors and Insulators

Energy gap must be overcome to move e- to conduction band

Inpurities

Reduces mobility and conductivity in metals

Lattice Defects (dislocation and grain boundaries)

Reduces mobility and conductivity in metals

Atomic Vibrations at T > 0K

Reduces mobility and conductivity in

Intrinsic Semiconductor

Pure materials (Silicon, Germanium) - for every e- excited, there is missing e- left behind

Opposite Direction

Movement of e- and holes

Extrinsic Semiconductors

Dictated by impurities, impurity atoms added to increase conductivity

n-type Extrinsic Semiconductor

Impurity atom w/ valence of 5 added as a substitutional impurity, creates filled E level just below conduction band

Donor State

An E level just below the conduction band (no holes are created when conducting)

p-type Extrinsic Semiconductor

Impurity atom with valence of 3 is added, creates E level just above valence band

Acceptor State

An E level just above valence band (only holes are move when conducting)

Material at T = 0K (1)

Conduction band empty

Freeze-Out Region (2)

More e- and holes excited from donor/acceptor states to conduction/valence bands

Extrinsic Region (3)

All e- and holes excited to conduction/valence bands (e- concentration plateaus)

Intrinsic Region (4)

Temperature high enough to excite more e- to conduction band and leave holes in valence band

Mobility of e-

Greater than the mobility of holes (as holes are heavier)

p-n Junction

At depletion zone, e- and holes combine and annihalate, E-field generated which creates a voltage barrier across junction

Forward Bias

Reduces p-n junction’s depletion layer, allows high current flow

Reverse Bias

Increases p-n junction’s depletion layer, blocks current flow

Advantages of Semiconductor Devices

Small size, low power consumption, no warm-up time

Photovotaic Solar Cells

When excited by sunlight, e- attracted toward p-type region and holes attracted toward n-type region

Magnetic Properties

Consequence of magnetic moments from e-

Types of Magnetic Moments

e- orbits around nucleus + e- spin on axis

Relative Permeability

Measure of degree to which material can be magnetised

Diamagnetism

Weak form of magnetism that only persists when external field is applied

Diamagnetic Properties

Induced by change in orbital motion, with extremely small magnitude and direction opposite to applied field

Paramagnetism

Each atom possesses a permanent dipole moment by incomplete cancellation of e- spin and/or orbital magnetic moments

Paramagnetism Properties

No net magnetism without external magnetic field. With external field, dipoles free to rotate indiviudally - dipoles align in external field

Ferromagnetism

Permament magnetic moments result from atomic magnetic moments due to: e- spin, orbital magnetic moments, and coupling interactions

Ferromagnetism Properties

Displayed by transition meetals, magnetic susceptibilities reach up to 10^6

Hysteresis loop

Indicates magnetic energy loss per volume (converts to heat, increases temp)

Large Hysteresis Loop

Hard magnet, energy loss larger

Small Hysteresis Loop

Soft magnetic, energy loss smaller

Increasing Temperature of Magnet

Increases thermal vibration of atoms, leads to dipole misalignment (decrease in saturation magnetisation)

Max Saturation of Magnet

0 Kelvin

Curie Temperature

Transition temperature where certain materials lose their permanent magnetic properties

Heat

Transfer of thermal energy due to a difference in temperature

Thermal Conductivity

Heat transferred from high- to low-temp regions

Direction of Flux

Perpendicular to heat flow direction

Heat Flux

Thermal conductivity x termperature gradient

Phonons

Lattice vibrations, through which heat is transported

Means of Heat Transport

Phonons and movement of free electrons

Vibrational Heat Capacity

Consists of elastic waves with range of distributions/frequencies

Effect of Alloying

Decreases conductivity (impurities slow electron motion)

Light

Electromagnetic radiation consisting of waves made of photons

Forms of EM Radiation

Light, heat, radar, laser, microwave, radio wave, infrared, x-ray

400-800nm

Visible light wavelength

White Light

Mixture of all colours (wavelengths)

Light Transmission

Light completely passes through object (transparent object)

Light Absorption

Atoms absorb photons as energy

Light Reflection

Light changes direction when reaching surface

Electronic Polarisation

Caused by E-field component interacting with electron cloud (some energy absorbed)

Emission of Photon

Simulated electron cannot remain excited, decays to ground state

Transparent

Little/no aborption or reflection

Translucent

Light transmitted diffusely, scattered within interior

Opaque

All light absorbed or reflected

Electrically Insulating Materials (Polymers/ceramics)

Can be made transparent or opaque

Can be opaque or transparent

Semiconductors

Intrinsically Transparent

Made transparent/opaque because of interor reflection and refraction

Sources of Internal Scattering