Insulators, conductors and semiconductors

Insulators are materials which prevent the flow of heat (thermal insulators) or electrical current (electrical insulators). The opposite of electrical insulators are conductors and semiconductors, which permit the flow of current (A semiconductor is strictly speaking also an insulator, since it prevents the flow of electrical current at low temperatures, unless it is doped with atoms that release extra charges to carry the current).

For the purpose of electronics and electrical engineering, materials are classified according to their electrical resistance, which describes how readily they allow electric current to pass when a voltage is applied. Apart from conductors, materials are classed as insulators (very poor conductors), semi-conductors (materials whose ability to conduct electricity can be controlled), and superconductors which (below a critical temperature, usually cryogenic) offer no significant electrical resistance, allowing circular currents, once established, to flow indefinitely.

All conductors contain movable electric charges which will move when an electric potential difference (measured in volts) is applied across separate points on a wire (etc) made from the material. This flow of charge (measured in amperes) is what is meant by electric current. In most materials, the amount of current is proportional to the voltage (Ohm's law) provided the temperature remains constant and the material remains in the same shape and state. The ratio between the voltage and the current is called the resistance (measured in ohms) of the object between the points where the voltage was applied. The resistance across a standard mass (and shape) of a material at a given temperature is called the resistivity of the material. The inverse of resistance and resistivity is conductance and conductivity.

Most familiar conductors are metallic. Copper is the most common material for electrical wiring, and gold for high-quality surface-to-surface contacts. However, there are also many non-metallic conductors, including graphite, solutions of salts, and all plasmas. See electrical conduction for more information on the physical mechanism for charge flow in materials.

Non-conducting materials lack mobile charges, and so resist the flow of electric current, generating heat. In fact, all materials offer some resistance and warm up when a current flows. Thus, proper design of an electrical conductor takes into account the temperature that the conductor needs to be able to endure without damage, as well as the quantity of electrical current. The motion of charges also creates an electromagnetic field around the conductor that exerts a mechanical radial squeezing force on the conductor. Since all conductors have some resistance, and all insulators will carry some current, there is no theoretical dividing line between conductors and insulators. However, there is a large gap between the conductance of materials that will carry a useful current at working voltages and those that will carry a negligible current for the purpose in hand, so the categories of insulator and conductor do have practical utility.

A semiconductor is a solid material that has electrical conductivity in between that of a conductor and that of an insulator;it can vary over that wide range either permanently or dynamically. Semiconductors are tremendously important in technology. Semiconductor devices, electronic components made of semiconductor materials,are essential in modern electrical devices.Examples range from computers to cellular phones to digital audio players. Silicon is used to create most semiconductors commercially, but dozens of other materials are used as well.

Semiconductors are very similar to insulators. The two categories of solids differ primarily in that insulators have larger band gaps — energies that electrons must acquire to be free to move from atom to atom. In semiconductors at room temperature, just as in insulators, very few electrons gain enough thermal energy to leap the band gap from the valence band to the conduction band, which is necessary for electrons to be available for electric current conduction. For this reason, pure semiconductors and insulators in the absence of applied electric fields, have roughly similar resistance. The smaller bandgaps of semiconductors, however, allow for other means besides temperature to control their electrical properties.

The resistance of semiconductors is normally modified dynamically by applying electric fields. The ability to control resistance/conductivity in regions of semiconductor material dynamically through the application of electric fields is the feature that makes semiconductors useful. It has led to the development of a broad range of semiconductor devices, like transistors and diodes. Semiconductor devices that have dynamically controllable conductivity, such as transistors, are the building blocks of integrated circuits devices like the microprocessor. These "active" semiconductor devices (transistors) are combined with passive components implemented from semiconductor material such as capacitors and resistors, to produce complete electronic circuits.

In most semiconductors, when electrons lose enough energy to fall from the conduction band to the valence band (the energy levels above and below the band gap), they often emit light, a quantum of energy in the visible electromagnetic spectrum. This photoemission process underlies the light-emitting diode (LED) and the semiconductor laser, both of which are very important commercially. Conversely, semiconductor absorption of light in photodetectors excites electrons to move from the valence band to the higher energy conduction band, thus facilitating detection of light and vary with its intensity. This is useful for fiber optic communications, and providing the basis for energy from solar cells.

Semiconductors may be elemental materials such as silicon and germanium, or compound semiconductors such as gallium arsenide and indium phosphide, or alloys such as silicon germanium or aluminium gallium arsenide.

IV. Ответьте на следующие вопросы, пользуясь информацией из текста

1. Why is a semiconductor often referred to as an insulator?

2. What are conductors in science and engineering?

3. How can materials be classified?

4. Under what condition will electric charge move?

5. What is the resistivity of the material?

6. Why do non-conducting materials resist the flow of electric current?

7. Transistors have dynamically controllable conductivity , don’t they?

8. What is the resistance of semiconductors modified by?