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Conductors: All conductors share the property that they're able to carry an electric current (a flow of electronic charge) in response to an applied difference in end-to-end voltage. Inside the material it is the electric field (the voltage difference/distance over which it is applied) that forces any free charges to move. In most cases the vast majority of the electrons inside the material are fixed in place — tightly held by the atom they're orbiting. However, in all conductors some of the electrons find a way to move. In some materials the band-gap energy is quite small compared with the average thermal energy available per particle. This means that random thermal motions cause the atoms & electrons to jiggle around, providing some electrons with enough energy to jump the gap. This produces a thermalised population of free electrons in the conduction band and an equal number of freely movable holes in the valance band. These free charge carriers (-ve electrons and +ve holes) can then move in response to an applied field and the material conducts. Some materials have a very small band gap, or even a non-existent gap. Then the conduction and valance bands touch or overlap. This means an electron only requires a tiny amount of extra energy to reach a free energy level.
Insulators: The valence band of an insulator is completely full. The electrons in this band are a bit like guests at a very overcrowded party. They cannot move, no matter how hot it gets, because they are trapped by the other guests squeezed up against them. You probably know how this feels: you want to cross the room to get a drink or meet someone, but you can't move because of all the people! In an insulating material the band gap energy is very large compared with the typical thermal energy available per particle. As a result, none of the electrons in the valence band can get enough energy to reach the conduction band.
The conduction band is completely empty. So even if an electric field is applied there are no electrons to move and so no conduction can take place.
The valence electrons cannot move because the valance band is packed tight, so the insulator refuses to let any electrons move when we apply an electric field. The result - no current.
Semiconductors: Doping is the process by which engineers change an insulating material into a semiconductor. The basic process inserts a small 'population ' of a foreign element into the crystal lattice of the insulator. For example, we might insert some boron atoms into a lump of - otherwise very pure - silicon. It is conventional to call the main material the bulk and the small number of foreign atoms the dopant.
The energy levels available for electrons orbiting in the dopant atom are different to those in the bulk material. The dopant also has a different number of electrons per atom since it is a different element. By choosing a suitable dopant we can achieve one or other of the two situations described below...
1. Here we have chosen a dopant which has an energy level just below the bulk material's conduction band. Left to itself the dopant has an electron sitting in this level. Since this dopant level is very close to the bulk conduction band we only require a small amount of energy to free the dopant's electrons and they can move freely in the conduction band. The effect of doping is to provide the bulk material with a population of free electrons which have been 'borrowed' from - or donated by - the dopant. Semiconductors manufactured in this way are called n-type because the free charge carriers we have created are negative (electrons) and there are no corresponding holes in the valence band. The dopant atoms are called Donors because they donate their electrons for conduction. Each of the donor atoms end up with an overall positive charge because it has lost an electron. This positive charge is effectively unable to move since the atom is stuck between its neighbours.
2. The alternative way to create a semiconductor is to add dopant which has a, normally empty, energy level just above the valence band of the bulk material. Electrons in the valence band find it relatively easy to hop into these new levels, opening up a population of holes in the valence band. For obvious reasons, dopant atoms which act like this are called Acceptors. They pick up and hold a fixed negative charge, creating a free population of holes to conduct through the valence band. Semiconductors made this way are called p-type because conduction can now take place when the positive hole moves in response to an applied field.
By choosing the right doping atoms and the number of them that we inject per cubic centimetre, we can alter or engineer the electronic properties of the semiconductor. The ability to manufacture 'designer' materials is one of the reasons that semiconductor engineering is such a very useful technique
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Glasgow Physics 1. Semiconductorsby glesgaphysics39,015 views
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