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Conductors, semiconductors and insulators
Materials are classified as conductors, insulators, or semiconductors according to their electric conductivity. The classifications can be understood in atomic terms. Electrons in an atom can have only certain well-defined energies, and, depending on their energies, the electrons are said to occupy particular energy levels. In a typical atom with many electrons, the lower energy levels are filled, each with the number of electrons allowed by a quantum mechanical rule known as the Pauli exclusion principle. Depending on the element, the highest energy level to have electrons may or may not be completely full. If two atoms of some element are brought close enough together so that they interact, the two-atom system has two closely spaced levels for each level of the single atom. If 10 atoms interact, the 10-atom system will have a cluster of 10 levels corresponding to each single level of an individual atom. In a solid, the number of atoms and hence the number of levels is extremely large; most of the higher energy levels overlap in a continuous fashion except for certain energies in which there are no levels at all. Energy regions with levels are called energy bands, and regions that have no levels are referred to as band gaps.
Conductors
Conductors are the materials or substances which allow electricity to flow through them. They are able to conduct electricity because they allow electrons to flow inside them very easily. Conductors have this property of allowing the transition of heat or light from one source to another. 
Metals, humans, earth, and animal bodies are all conductors. This is the reason we get electric shocks! The main reason is that being a good conductor, our human body allows a resistance-free path for the current to flow from wire to our body. Conductors have free electrons on its surface which allows current to pass through. This is the reason why conductors are able to conduct electricity.
Examples of Conductors
- Silver is the best conductor of electricity. However, it is costly and so, we don’t use silver in industries and transmission of electricity.
- Copper, Brass, Steel, Gold, and Aluminium are good conductors of electricity. We use them mostly in electric circuits and systems in form of wires.
- Mercury is an excellent liquid conductor that finds use in many instruments.
- Gases are not good conductors of electricity as the particles of matter are quite far away and thus, they are unable to conduct electrons.
Semiconductors
Semiconductor, any of a class of crystalline solids intermediate in electrical conductivity between a conductor and an insulator. Semiconductors are employed in the manufacture of various kinds of electronic devices, including diodes, transistors, and integrated circuits. Such devices have found wide application because of their compactness, reliability, power efficiency, and low cost. As discrete components, they have found use in power devices, optical sensors, and light emitters, including solid-state lasers. They have a wide range of current- and voltage-handling capabilities and, more important, lend themselves to integration into complex but readily manufacturable microelectronic circuits. They are, and will be in the foreseeable future, the key elements for the majority of electronic systems, serving communications, signal processing, computing, and control applications in both the consumer and industrial markets.
Solid-state materials are commonly grouped into three classes: insulators, semiconductors, and conductors. (At low temperatures some conductors, semiconductors, and insulators may become superconductors.) The figure shows the conductivities ? (and the corresponding resistivities ? = 1/?) that are associated with some important materials in each of the three classes. Insulators, such as fused quartz and glass, have very low conductivities, on the order of 10?18 to 10?10 siemens per centimetre; and conductors, such as aluminum, have high conductivities, typically from 104 to 106 siemens per centimetre. The conductivities of semiconductors are between these extremes and are generally sensitive to temperature, illumination, magnetic fields, and minute amounts of impurity atoms. For example, the addition of about 10 atoms of boron (known as a dopant) per million atoms of silicon can increase its electrical conductivity a thousand fold.
The study of semiconductor materials began in the early 19th century. The elemental semiconductors are those composed of single species of atoms, such as silicon (Si), germanium (Ge), and tin (Sn) in column IV and selenium (Se) and tellurium (Te) in column VI of the periodic table. There are, however, numerous compound semiconductors, which are composed of two or more elements. Gallium arsenide (GaAs), for example, is a binary III-V compound, which is a combination of gallium (Ga) from column III and arsenic (As) from column V. Ternary compounds can be formed by elements from three different columns—for instance, mercury indium telluride (HgIn2Te4), a II-III-VI compound. They also can be formed by elements from two columns, such as aluminum gallium arsenide (AlxGa1 ? xAs), which is a ternary III-V compound, where both Al and Ga are from column III and the subscript x is related to the composition of the two elements from 100 percent Al (x = 1) to 100 percent Ga (x = 0). Pure silicon is the most important material for integrated circuit applications, and III-V binary and ternary compounds are most significant for light emission.
Insulators
Insulators are the materials or substances which resist or don’t allow the current to flow through them. They are mostly solid in nature and are finding use in a variety of systems. They do not allow the flow of heat as well. The property which makes insulators different from conductors is its resistivity. Wood, cloth, glass, mica, and quartz are some good examples of insulators. Insulators are also protectors as they give protection against heat, sound and of course passage of electricity. Insulators don’t have any free electrons and it is the main reason why they don’t conduct electricity.
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