It’s Professor Dave, let’s learn about conductivity and semiconductors.
Much of our technology relies on substances that can conduct electricity, meaning that
they allow the flow of an electrical current.
But what is it about a material that allows it to do this?
Conversely, if insulators are things that do not conduct electricity, why is that the case?
And finally, what are semiconductors, and where do they fit in?
Let’s learn about all of these terms now.
So as we said, conductors conduct electricity, and insulators do not, while semiconductors
are materials that have a conductivity in between that of a conductor and an insulator.
Because of this intermediate conductivity, semiconductors can be controlled and used
for technological purposes.
The explanation for the conductivity of these substances requires that we take a look at
molecular orbitals in metals and network solids, so if you need to review this concept, check
out my tutorials on molecular orbital theory first.
Otherwise, in order to put things into perspective, we must understand that these materials are
not small molecules comprised of a handful of atoms.
These are huge arrays of a particular atom or repeating formula unit, stretching endlessly
in every direction.
We know that when two atomic orbitals combine, two molecular orbitals will be generated,
one lower-energy bonding orbital, and one higher-energy antibonding orbital.
Thus as more and more covalent bonds form, more molecular orbitals will be generated
to house all of these electrons.
Here we can see the molecular orbitals involved as many atoms come together to make a network
solid that will be a conductor.
As the number of atoms increases, from one, to two, to four, to a number so large that
it can be compared with infinity for practical purposes, the number of orbitals will increase
in the same way, and as the number of orbitals becomes incredibly large, they will begin
to resemble one continuous band.
There are some important things to note about this model, which we call band theory.
First, the atoms are held together by electrons in the lower-energy bonding orbitals, as these
are filled first, while the higher-energy antibonding orbitals remain vacant.
Second, in a conductor, the difference in energy between the highest occupied molecular
orbital and the lowest unoccupied molecular orbital is infinitesimally small, which means
that electrons can move freely between orbitals, and this is the key feature of a conductor
that allows it to conduct electricity.
Comparing substances with differing conductivities, we will notice something called a band gap,
which is a gap in energy between the bonding orbitals, or valence band, and the antibonding
orbitals, or conduction band, named as such because electrons must access these orbitals
in order for the substance to conduct electricity.
Conductors, as we said, have no band gap, or just a tiny one, so electrons can flow easily.
Insulators have large band gaps, which does not allow for a current to form.
Semiconductors, on the other hand, have a small enough band gap that thermal energy
in the system is able to keep some of the bonding orbitals in the valence band empty
and promote some electrons into the antibonding orbitals in the conduction band.
This is why semiconductors conduct electricity better at higher temperatures, because there
is more thermal energy available to promote electrons into the conduction band and thus
a stronger current is able to form.
So what kinds of materials can act as semiconductors?
Here are some common substances, along with their respective band gaps in electron volts,
so we can get a sense of the range that is permissible.
We will notice that some of these substances are elements, like silicon and germanium,
and some are compounds, like lead sulfide.
Once the band gap becomes sufficiently large, a substance will become an insulator, like
diamond, or aluminum nitride.
Apart from temperature, another way to increase the conductivity of a semiconductor is through
a method called doping, which is where impurities are mixed into a substance, and this impurity
can either have more or fewer valence electrons than the substance.
If the impurity, or dopant, has more valence electrons, the conduction band will begin
to fill, and current can flow.
This is called an n-type semiconductor, where n stands for negative.
If the dopant has fewer electrons, the valence band will end up not completely full, and
current will again be able to flow.
This is called a p-type semiconductor, where p stands for positive.
Electronic components like diodes and transistors utilize both p and n type semiconductors,
so these techniques are very important indeed.
We will dive into these kinds of materials in more detail in a future engineering course,
but for now, let’s move forward and learn more chemistry.