Semiconductors usually have four electrons in the outer electron shell. They are tetravalent. These are called valence electrons. The structure that is formed is a tetrahedral structure. This forms a crystalline structure. There are two types of semiconductors. One type is the intrinsic semiconductors and the extrinsic semiconductors. Intrinsic semiconductors are elements and compounds that are semiconductors. Examples include Silicon and Cadmium. When they are in a solid state, the silicon atoms form a covalent bond among themselves to form a lattice.
Since they form a covalent bond, they will not conduct electricity as there are no free electrons that will be used to carry the current. So when the lattice is in solid state at 0 K is an insulator. When the temperature reaches room temperature, the valence electron gains enough thermal energy so that the energy that binds them to the nucleus. The covalent bond breaks and so the electron leaves the atom that it is in. This leaves a hole in the atom and it becomes positively charged.
Once this happens it is capable of attracting an electron from another atom and the electron from this atom.
So now this hole moves from atom to atom and as this happens, the electrons also move from atom to atom, as a result the electrons move along the lattice and thus carry current. So we can deduce that from this that in semiconductors the resistance decreases as the temperature increases. This we already know from this applies to all the semiconductors.
Extrinsic semiconductors are made from intrinsic semiconductors. Adding tiny amounts of another substance makes them extrinsic. The amounts added are very little so that the added atoms are well spaced from each other.
This is so that the structure remains the same even if the property changes. These atoms are called impurity atoms and these are used so that it does not form unreliable substances. Since we have dealt with the conduction of the ohmic and non ohmic conductors, I would like to deal specifically with the energy band theory. This theory is linked to the conduction of all these substances we have looked at. To understand energy bands we need to first see what valence electrons and conduction electrons are. Valence electrons are those electrons that are on the outside shell of the atom.
In silicon there are 4 valence electrons. For Hydrogen there is 1 valence electron. So when the atoms come together as a cluster they form a lattice. In this lattice the valence electrons come together to form what we call a valence band. This band consists of electrons from the outermost shell of the atoms that form the lattice. The atoms all do not form a single band. There are bands for the first shell, second shell, etc. This is also known as the energy ladder. This is formed because the thermal energy is increased and so the electrons move up the shells as they gain energy.
The energy levels spread out into bands. The bands increase in size as the temperature increases. This energy band theory can be used to explain the conductivity of semiconductors and metals. In metals the highest band is only partly filled. This allows the electrons to move up the bands to the unfilled layers. So the electrons have now escaped from the atoms and become free to move through the material. This is called the conduction band. The space between the valence band and the conduction band is very little and thus the metals are able to conduct electricity.
In insulators we can assume that the gap between the valence and the conduction band is more because there is no electron jumping and so no conduction as in the metals. This proves that the outer shell of these materials is full and held firmly to the atom. The conduction band is thus empty. This proves why they cannot conduct electricity. In intrinsic semiconductors the gap between the valence band and the conduction band is not too much. The electrons can jump across is enough heat is applied. This is true after absolute zero so they would conduct even at room temperature.
When the temperature applied is more, more electrons jump across to the conduction band and so the semiconductor can conduct better in increased temperature. The electron jumping across to the conduction band corresponds to the electron breaking away from an individual atom. It will be called a conduction electron. The idea of the energy bands is shown below: Conduction band Small Gap Gap but electron shells are partly filled Large Gap Valence band The gaps between the valence and the electron bands are the energy gaps.
Now that we know of the semiconductors. Insulators and ohmic and non ohmic conductors we will carry out experiments by forming circuits to check these. For this we first need to plan out the experiment and then perform it. This is shown next. Now that we have see theoretically what the semiconductors, conductors and insulators are, we have to plan out the experiment. The experiment has to be carried out under certain conditions. Theses conditions we will see now.