Hi there - this is the first in a series of three videos and semiconductors. In this first video,
I'll give a simple explanation of semiconductor band theory. We look at intrinsic semiconductors
then move on to doping in order to form n-type and p-type semiconducting materials.
So, first of all we're going to look at the element silicon although other semiconducting materials are available.
Silicon has the symbol Si and an atomic number of 14. This means that silicon atoms contain 14 electrons:
2 in the first shell, 8 in the next then 4 in the outer shell. We refer to these outer electrons as valence electrons.
These are the electrons which can form chemical bonds
Now we'll simplify our model of the silicon atom to show only the 4 outer valence electrons, which take part in bonding.
In individual atoms, the electrons can only occupy discrete energy levels.
The production of line emission spectra and line absorption spectra can be explained in terms of the electrons moving to different energy levels and
either emitting or absorbing a photon
although that's covered in another lesson
When a large number of atoms come together to form a solid, the energy levels become reorganised into bands as the atoms interact.
Each energy band actually consists of many many closely spaced energy levels.
You can see from the diagram on the left that all four of the silicon atoms outer electrons form covalent bonds with the neighbouring atoms.
Now out of interest the diagram on the left shows what's known as the semiconductor bonding model.
At zero Kelvin all of the outer electrons are bonded. The diagram on the right shows the energy band model.
In order to explain the electrical properties of semiconductors,
we only need to consider two energy bands - the valence band and the conduction band
These are separated by a band gap which we'll discuss later.
What I've not mentioned yet is that this is an undoped semiconductor of pure silicon.
What we call an intrinsic semiconductor.
As I said before at zero Kelvin all the outer electrons are bonded. Let's look at what happens when we add some heat.
So an electron has gained enough energy to break the covalent bond and is now free to take part in conduction
meaning that the resistance of the material is reduced.
The absence of the negatively charged electron in this position leaves behind a net positive charge which we call a hole.
In terms of energy bands the electron which was initially in the valence band jumps to the conduction band
leaving behind a hole in the valence band.
The band gap or energy gap is just the energy required for the electron to break the covalent bond and jump into the conduction band.
As the semiconductor is further heated more covalent bonds are broken
resulting in more electrons jumping to the conduction band with more positive holes created in the valence band.
So as the temperature of a semiconductor increases its resistance decreases.
due to the motion of the electrons in the conduction band and holes in the valence band.
If we look at the diagram on the left we can see that electrons from neighbouring atoms can move
to fill a hole, leaving behind a hole in their previous position.
Although strictly this was caused by the motion of negative electrons from left to right
We can also describe it as the motion of positive holes from right to left.
Next we'll look at energy bands in a little more detail.
As I said before we only need to consider the valence band and the conduction band in order to explain
electrical properties of semiconductors and for that matter conductors and insulators.
Remember then that in order for a material to conduct it must have
electrons which are free to move within its conduction band or holes in its valence band.
In a conductor, conduction can actually be explained by considering only the conduction band.
The conduction band is only partially filled so electrical conduction is permitted.
Unlike a semiconductor though, increasing the temperature of a metal conductor will actually cause its resistance to increase.
This is because the number of electrons within the conduction band increases so much that they have less freedom of movement.
Remember that in a conductor, the highest occupied energy band (the conduction band) is not completely full.
In insulators, there's a very large band gap between the top of the valence band and the bottom of the conduction band.
At 0 Kelvin, the valence band is completely full and the conduction band is completely empty.
At higher temperatures, electrons don't normally gain enough energy to jump from the valence band to the conduction band.
So with no electrons in the conduction band, conduction isn't possible.
For an insulator then, the highest occupied energy band (the valence band) is full.
Finally, semiconductors have a very narrow band gap between the valence band and the conduction band.
Like insulators, at 0 Kelvin the valence band is completely full and the conduction band is completely empty.
But, since the band gap is so small, as temperature rises some electrons can gain enough energy
to jump from the valence band to the conduction band leaving behind holes in the valence band.
As temperature increases then, the resistance of the semiconductor is reduced.
Next on the agenda is doping. Here on the left, we see the bonding model of an intrinsic semiconductor - silicon again.
With the addition of impurity atoms during the manufacture of the semiconductor we can reduce its resistance. This is done in one of two ways:
By doping with a group 5 impurity atom such as arsenic, as shown here, we end up with an extra electron.
The energy level of this fifth electron is within the bandgap just below the conduction band.
The energy levels associated with the extra electrons are known as donor levels.
Even at room temperature, most of these electrons will gain enough energy to jump into the conduction band thus reducing the resistance of the semiconductor.
We call this an n-type semiconductor since the majority of conduction is due to the negative electrons within the conduction band.
Remember that we previously discussed the heating of an intrinsic semiconductor.
For each electron which jumped from the valence band to the conduction band there was one hole left behind in the valence band.
This meant that, in an intrinsic semiconductor, electrons and holes exist in equal number.
When we dope the semiconductor however, this is no longer the case. In an n-type semiconductor the majority charge carriers are electrons
and holes which are still present due to the process described earlier will be very much in the minority.
Despite the majority charge carriers being negative electrons an n-type semiconductor is still electrically neutral since the impurity atom
Arsenic in this case will have an equal number of electrons and protons.
Now let's look at the p-type semiconductors.
This time the impurity atom or dopant is from group 3 so only three outer electrons are available for bonding with the surrounding silicon atoms.
This results in a positive hole as shown.
The impurity atom in the diagram in case you are wondering is indium
The addition of group 3 impurity atoms leads to additional energy levels in the bandgap just above the valence band.
These are known as acceptor levels.
Electrons in the valence band require very little energy to move to the acceptor levels leaving behind holes in the valence band.
This is where p-type semiconductors get their name because the majority charge carriers are positive holes.
Conduction in a p-type semiconductor is due to the motion of these positive holes within the valence band.
Again, p-type semiconductors are electrically neutral since the impurity atoms have an equal number of electrons and protons.
So that's us for now. In the next lesson, I'll be discussing the formation of a p-n junction and how it performs when forward or reverse biased.
The third lesson will cover applications such as the LED and solar cell. Make sure to watch them too.
For more information on upcoming videos, summary sheets and so on visit physics-podcast.co.uk
Thank you for listening!