What is a Semiconductor ?
In the previous section (Band Theory) an explanation of conductors and insulators was given based upon an understanding of the Energy Bands within the structure of the material.
There is, however, a third class of materials called Semiconductors. Semiconductors are materials that act as an insulator when pure, but can act as a conductor when impurities are added to them.
These materials have properties of both conductors and insulators, and can also be explained using Band Theory.
In a conductor there are no band gaps between the Valence and Conduction Bands. In some metals the Conduction and Valence bands partially overlap. This means that Electrons can move freely between the Valence Band and the Conduction Band.
The Conduction Band is only partially filled. This means there are spaces for Electrons to move into. When Electrons for the Valence Band move into the Conduction Band they are free to move. This allows conduction.
An insulator has a large gap between the Valence band and the Conduction band.
The Valence Band is full as no Electrons can move up to the Conduction Band. As a result, the Conduction Band is empty.
Only the Electrons in a Conduction Band can move easily, so because there aren't any Electrons in an insulator's Conduction Band, the material can't conduct.
In a Semiconductor, the gap between the Valence Band and Conduction Band is smaller. At room temperature there is sufficient Energy available to move some Electrons from the Valence Band into the Conduction Band. This allows some conduction to take place.
As a higher Temperature would give more Energy, there will be more Electrons in the Conduction Band, and as such as Temperature increases, the Resistance of a Semiconductor decreases.
Semiconductor Structure and Doping
As mentioned previously, Semiconductors in their pure form are insulators, due to their crystal structure. The diagram below shows the crystal structure of a pure form of the most common Semiconductor, Silicon:-
Silicon is a group 4 element on the Periodic table, which mean that each Silicon atom has 4 Electrons in its outer shell. In order to create a stable structure, each Silicon atom will bond Covalently with 4 other Silicon atoms to "fill" its outer shell (as can be seen in the above diagram).
In this pure form, all Electrons are tightly bound in full shells, so there are no free Electrons to carry Current, making this material an insulator.
If the Semiconductor is not in a pure form then the electrical properties of the material change. By Doping the material with a group 5 element such as Arsenic, the following structure is formed:-
As can be seen in the diagram above, the Silicon atoms again bond covalently with 4 surrounding atoms. By bonding in this way with the Arsenic atom, however, when the outer shell is full, there is an extra Electron. This Electron is free to move throughout the crystal and therefore can carry Current.
As this free charge carrier is a negatively-charged Electron, this is an N-Type Semiconductor.
Note - Even though there are free Electrons which are free to move through the crystal structure, as there is the same number of Protons and Electrons, the Semiconductor has a neutral charge.
If the Semiconductor is not in a pure form then the electrical properties of the material change. By Doping the material with a group 3 element such as Indium, the following structure is formed:-
As can be seen in the diagram above, the Silicon atoms again bond Covalently with 4 surrounding atoms. By bonding in this way with the Indium atom, however, there is a missing Electron. This gap can be thought of as a positively charged "Hole" within the crystal structure. Electrons from neighbouring atoms can move across and occupy this gap, making the Hole "move" throughout the crystal and therefore can carry Current.
As this free charge carrier is a positively-charged Hole, this is an P-Type Semiconductor.
Note - Even though there are apparently "missing" Electrons, as there is the same number of Protons and Electrons within the material, the Semiconductor has a neutral charge.
If P-Type and N-Type Semiconductors are brought into contact, a P-N Junction will form spontaneously at the point of contact.
The diagram below shows a P-N Junction, giving a view of "Free Electron" or "Free Hole" positions:-
When the P-Type and N-Type Semiconductors join, Electrons from the N-Type Semiconductor migrate across to "fill" Holes in the P-Type Semiconductor, causing Holes to migrate across into the N-Type Semiconductor. This produces a region of almost no charge-carriers, called the Depletion Layer.
As there are now excess Electrons in the P-Type and excess Holes in the N-Type Semiconductor, a Potential difference (~ 0.7 V) forms across the Depletion Layer. As there are no charge-carriers within the Depletion Layer, nothing can move to cancel this effect out and the Potential difference remains in place.
The video below shows another example use of Semiconductors, a Transistor:-
Note - The use of semiconductors as Transistors is non-examinable at Higher level.