In this screencast we will explore the nature
of electron energy bands in metals, insulators
and semiconductors.
Energy bands are the combination of energy
states that electrons cannot keep high in
a bulk material.
They are determined by the electron orbitals
of the atoms and the inner atomic spacing
of the atoms in the solid.
The first band structure we'll discuss is
that found in metals such as copper.
At zero Kelvin the energy occupied by the
electrons is shown here in purple.
These are known as filled states.
There are states that the electrons can occupy,
directly adjacent in terms of energy, shown
here in green.
Electrons can easily move into these states
and thus are available for electrical conduction
in the material.
There's also an energy band at higher energy
in these materials but that band is unused
since there are empty states directly adjacent
to the filled states.
That empty band is shown here in black.
The next type of material I will discuss is
a different type of materials.
Unlike copper which has only one electron
in its 4s orbital and is capable of accommodating
2 electrons in that orbital, thus the empty
states in that electron band, metals such
as Magnesium have a filled 3s orbital.
However, there is an adjacent orbital, the
3p orbital, that can accommodate electrons.
Though this orbital overlaps it does take
a little bit of energy to get electrons into
it and thus Magnesium is a slightly less good
conductor of electricity that copper is.
Again, just to reiterate, unlike copper, magnesium
doesn't have an unfilled state.
It has an adjacent empty band where the band
in terms of energy is adjacent to the filled
states.
Thus it's very easy for the electrons to enter
that band and be available for conduction.
The empty band is shown here in black.
The next type of material I will discuss is
electrical insulators like silicon dioxide.
These materials, often ceramics, have filled
energy states just like the other two.
However, their empty band is significantly
farther away in terms of energy.
This energy differential is called the band
gap and for insulators this band gap has an
energy that's usually much greater than 2
eV.
The final type of material is semi-conductors.
These materials, such as silicon, have a similar
band structure to insulators where again,
they have filled states and an unfilled energy
band that is slightly different in energy.
However, unlike the insulating materials,
the band gap here is very small in terms of
energy where it has about 2 eV or less difference
between the top of the filled states and the
empty band.
