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Semiconductors - Electronics .. Physics

Semiconductors

Most of the electronic circuits that exist so far have relied primarily on valves in their composition. However, since the fifties, there has been a strong tendency to replace them with the transistor, which is characterized by its small size, light weight, solidity, and lack of need for heating current, and its ability to operate using a small voltage. Transistors are components made of materials called semiconductors.

It is worth noting that most metals are good conductors of electricity because they contain a large number of free electrons, and that most non-metallic materials contain only a small number of conductive electrons because of their tight connection to the nucleus of the atom itself, and for this reason these materials are called insulating bodies. There is a third type of materials in which the energy of the electrons is sufficient to break the ring that connects them and release them from their places - these materials are called semiconductors such as silicon, germanium and selenium.
In addition, the number of free electrons in semiconductors decreases as their temperature decreases, and disappears completely if their temperature reaches absolute zero. This is because the higher the temperature, the more energy the electrons have, and thus the number of them capable of breaking the bond that binds them increases. For this reason, the resistance of semiconductors decreases as their temperature increases, unlike metals, whose resistance increases with increasing temperature. The crystalline semiconductor is characterized by a small conductivity, but this conductivity increases greatly if some impurities are added to the semiconductor.

It is known that the germanium crystal contains in its outer valence layer four electrons, each of which is linked to the neighboring atom by a covalent valence bond. As for the arsenic atom, for example, it contains in its outer layer only five electrons. If an amount of arsenic is added to the germanium material during its crystallization, one spare electron remains for each of the arsenic atoms present in the network. This electron contributes with the rest of the electrons similar to it to transfer the current. This type of electrical transfer is called . - S (negative) transfer. If indium atoms, each of which has three electrons in its outer shell, are added to germanium, a number of holes remain within the group. Since the valence electrons are not free to drift here and there, they fall into the holes adjacent to them. In this case, the holes are considered positive charges, and the resulting transport is called M- (positive) transport. Both electrons and holes are considered charge carriers. Also, within each type of electrical transport, there are a few charge carriers of the opposite type. For this reason, holes in the M class and electrons in the S class are called the majority carriers.

(Above, left) Movement of a "hole" carrying a positive charge in an electric field. (Above, right) Rectification by M-S coalescence.
(Bottom) Germanium network containing arsenic impurities)

If we connect a semiconductor of type M to a semiconductor of type S and apply an electric voltage to the junction in such a way that the first type becomes positive and the second negative, the charge carriers under the influence of the field are forced to move towards the junction where they combine with each other.

This results in the generation of other charge carriers behind the junction and the current flows as long as the process continues. However, in the case of reversing the voltage, the charges move away from each other and only a very small current flows, so the semiconductor junction is considered an electrical rectifier.