Materials Chem electrical conductivity

highest occupied band

Valence band

lowest empty band

conduction band

location of the fermi band in a metal

located through a band to produce a partially filled band

location of the fermi band in a semimetal

located right between conduction and valence bands with no band gap

location of the fermi band in a semiconductor/insulator

located in the energy band gap between conduction and valence bands

when electrons are promoted from valence band to conduction band

conductivity

can be used to determine the percentage of electrons that can be promoted to conduction band

fermi-dirac function

electrons that can easily be excited from occupied states into empty states

charge carriers

In the absence of external excitations, excitation happens due to…

thermal energy

replacing the lattice atom with an impurity atom that contains one additional valence electron so that electron can be easily donated to the conduction band

n-doping

n-doping essentially works by…

raising the valence band up

replacing a lattice atom with an impurity atom that contains one less valence electron so that this atom can accept an electron from the valence band to create a hole

p-doping

p-doping essentially works by… 

lowering the conduction band down

in metals, relaxation time is inversely proportional to…

temperature (increased vibration and collisions between electrons and atoms)

in metals, ______ changes very slowly with temperature

carrier concentration

in metals, how are conductivity and temperature related

conductivity decreases as temperature increases

in semiconductors, ____ is inversely proportional to temperature (same for metals)

relaxation time

in semiconductors, carrier concentration increase as temperature goes up due to…

excitations across the band gap

in semiconductors, how are conductivity and temperature related

conductivity increases as temperature increases

How is resistance measured experimentally

applying a voltage across a material and measuring the current (or vice versa)

What is ohms law?

resistance=voltage/current

intrinsic property that quantifies how strongly a material opposes the flow of electric current

resistivity

inverse of resistivity

conductivity

materials that have zero resistance to the flow of electrical current

superconductors

What are the three ways superconductivity is lost

increasing temperature above the critical temperature, increasing the strength of an applied magnetic field beyond the critical field, increasing the electrical current passing through the material beyond the critical current density

Superconductors expel a magnetic field because they exhibit…

perfect diamagnetism

materials with infinite resistance to the flow of electric current

dielectric materials

How are superconductors and dielectric materials related?

opposites

application of a potential across a dielectric material leads to a…

polarization of charge (disappears when voltage is removed)

materials ability to resist n electric field

permittivity

essentially, permittivity is a measure of…

how good a material is at not letting electrons through

material’s ability to store electrical charge

capacitance

capacitance depends on…

the strength of the material’s dielectric property (permittivity)

dielectric materials that have extremely large permittivities

ferroelectric materials

How are ferroelectric materials different from regular dielectric materials?

ferroelectric materials retain a large residual polarization of charge after the applied voltage is removed

A necessary condition for a material to exhibit spontaneous polarization and ferroelectricity is that its space group should not include…

an inversion center

When increasing the temperature of a metal, conductivity decreases. The carrier concentration increases and the relaxation time decreases. Which is more significant?

the decrease in relaxation time (no need to jump the band gap in metals so carrier concentration is less significant)

When increasing the temperature of a semiconductor, conductivity increases. The carrier concentration increase and the relaxation time decreases. Which is more significant?

the increase in charge carriers (more electrons to jump the band gap)