A semiconductor is neither a complete conductor nor a complete insulator. Before we go into the details of these two, we need to understand another term. It is called conductivity. The reason being a semiconductor and insulator are both defined in terms of their conductivity. Hence to proceed further without a proper understanding of conductivity would not be of much help. So, what is conductivity? The degree or extent to which a material allows heat or current to conduct through it is called conductivity.
Based on whether it is electricity or heat that passes through a material it is called electrical conductivity or thermal conductivity respectively. So, it is a measure of the tendency of a material to allow current or heat to flow through it. The resistivity decreases as the conductivity increases. In metals, it is assumed that all the outer electrons are free and available to carry a charge and the only impedance is if they bump into each other.
Now let us get into understanding semiconductors and insulators. A material whose conductivity lies between that of a metal like copper or gold and an insulator like glass is called a semiconductor. The semiconductors’ resistance decreases with increase in temperature. With metals, it is the opposite where temperature rise causes increased resistance. The conducting property of semiconductors can be improved by selectively adding impurities. This is called "doping". By introducing them into the crystal structure of the original material the conductivity can be improved. In the same crystal, when two differently or rather oppositely doped regions exist, there is created a semiconductor junction. In modern electronics, diodes, transistors etc: show different behavior depending on this junction. Some of the semiconductors are silicon, germanium.
The two types of junctions mentioned earlier are the p-type and n-type junctions. The "p" type is the positive side and "n" type is the negative side. The positive side has an excess of holes and the negative side has an excess of electrons. The p-n junction is created by the process of doping. There are different processes, like implantation, diffusion of dopants and by growing a layer of crystal doped with one type of over a layer of crystal that is doped with another type. The last process is called epitaxy. The p-n junctions are considered to be the basic building blocks of semiconductors. These are used in LEDs, transistor circuits, diodes etc: A p-doped semiconductor is a better conductor. Even a n-doped one is a good conductor. The junction between them, however, is comparatively non-conductive and depends on the relative voltages of the two semiconductor regions. When you apply a voltage across a p-n junction it is called a BIAS. When the current flow is easy it is called forward bias and when current flow is less or difficult it is called reverse bias. At the junction region, the free electrons of the n-type are attracted to the positive holes in the p-type. They diffuse, canceling each other out. This results in an uncharged and electrically neutral region.
In a Forward bias, the p-type is connected to the positive terminal and the n-type is connected to the negative terminal. When connected in this way, the positive holes and the negative electrons are pushed towards the junction. The positive charge repels the holes and the negative charge repels the electrons. Although the electrons penetrate only for a very short distance into the p-type material, current flow will continue without interruptions.
Connecting the P-type to the negative terminal and N-type to the positive terminal causes a reverse bias. Very little current will flow and the diode will break down.
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Because the positive holes are pulled towards the negative terminal and electrons pulled towards the positive terminal, very small current alone can flow. This will cause this to behave like an insulator with very little current flowing. As the reverse bias voltage increases, the strength of the depletion zone also will increase.
Insulators are resources that oppose the flow of electric current. The same is true for heat also. Wood is an excellent insulator of heat while metal is a good conductor. Insulators are embedded in household appliances as protection devices. The internal electric charges in an insulator will not flow as freely as in a conductor. The resistivity of insulators is very high and consequently very low conductivity. The atoms of insulating material do not have free-flowing electrons. Their electrons are tightly bound causing them to be bad conductors. It is important to note that there is no perfect insulator. When a sufficiently large voltage is applied across an insulator, even the tightly bound electrons are pulled and they start to conduct. The voltage at which the electrons are pulled away and they get to conduct is called the breakdown voltage of an insulator. Some materials have very high resistivity. Example: Glass, paper are excellent insulators. Polymers and plastics made by way of thermosetting or thermo plasticizing are also good insulators. We see these in the protection on electric wires and in switches of appliances. Insulators are used to support and separate the conductors so that there is no physical harm to the user.
Uses of Insulators:
As mentioned before, a flexible coating of insulating material is applied on electric wires. This is called insulated wire. When conducting wires touch each other, it can create a short circuit and damage to appliance hence insulation of the wires is important. Also, wires that expose voltages higher than 60v can cause shock to human beings and hence should be insulated. In electronic systems, the printed circuit boards are made from epoxy materials which are good insulators.
In high voltage overhead transmission wires, they are bare and not protected with any plastic coating but they are insulated by air. Insulation supports are used while fixing long distance transmission of high tension wires. Insulation materials for such high voltage lines are made of ceramic, glass or porcelain.
Dirt, pollution, salt or water on the insulator can cause a conductive route and this cause leakage currents and flashovers. There are different types of insulators like pin type, post insulator, strain insulator etc: For high voltage transmissions, the suspension insulator strings are used.
The first electrical systems that used insulation were the telegraph lines. It is because when the wires were directly attached to the wooden poles during damp weather it gave bad results in terms of performance. First glass was used as an insulator and then ceramic was introduced in the industry by manufacturers in London.
The most important insulation material that is naturally available is air. A multitude of solid, liquid and gaseous materials are also used. Sometimes polymer varnish films are used in motor coils. Earlier asbestos was also used as an insulator but then given the carcinogenic nature of asbestos it is now banned.
Finally, there are two classes of insulation. They are class 1insulation and class 2 insulation.
Class 1 insulation mandates that the metal body and other exposed metal parts should be connected to the earth via a grounding wire.
Class 2 insulation means double insulation. It is used in some appliances like shavers, dryers etc: all energized internal components are enclosed inside an insulating body and should not come in contact with live parts.