Electronics is a branch of physics and electrical engineering concerned with the emission, behaviour, and effects of electrons and electronic devices. Electronics encompasses an exceptionally wide range of technology. The term was originally applied to the study of the behaviour and motion of electrons, particularly as observed in early electron tubes. It came into use in its broadest sense with advances in knowledge about the fundamental nature of electrons and how the motion of these particles could be used.
Today, many scientific and technical disciplines deal with different aspects of electronics. Research in these fields has led to the development of key devices such as transistors, integrated circuits, lasers, and fibre optics. These, in turn, have made it possible to manufacture a wide range of consumer, industrial, and military electronics. In fact, it can be said that the world is in the midst of an electronic revolution at least as important as the industrial revolution of the 19th century.
Composite Semiconductor Materials
Many semiconductor materials exist besides silicon and germanium, and they have different useful properties. Silicon carbide is a compound semiconductor, the only one formed by two elements from column IV of the periodic table. It is particularly suitable for manufacturing devices for specialized high-temperature applications. Other compounds formed by combining elements from column III of the periodic table, such as aluminium, gallium, and indium, with elements from column V, such as phosphorus, arsenic, and antimony, are of particular interest.
These so-called III-V compounds are used to make semiconductor devices that either emit light efficiently or operate at exceptionally high frequencies. A notable feature of these compounds is that they can, in effect, mix with each other. Gallium arsenide can be produced or replace part of the gallium with aluminium or also replace part of the arsenic with phosphorus. When this is done, the electrical and optical properties of the material continuously change subtly in proportion to the amount of aluminium or phosphor used.
With the exception of silicon carbide, these compounds have the same crystal structure. This makes possible the gradation of the composition and therefore of the properties of the semiconductor material within a continuous crystalline body. Modern materials processing techniques allow these changes in composition to be precisely controlled on an atomic scale. These characteristics are exploited in the manufacture of semiconductor lasers that produce light of any wavelength over a considerable range. Such lasers are used, for example, in compact disc players and as light sources for fibre optic communication.
Computers understand only two numbers, 0 and 1, and perform all of their arithmetic operations in this binary mode. Many electrical and electronic devices have two states: they are off or on. A light switch is a familiar example, as are vacuum tubes and transistors. Because computers have been a major application for integrated circuits since their inception, digital ICs have become commonplace. Therefore, it has become easy to design electronic systems that use digital language to control their functions and communicate with other systems.
A great advantage of using digital methods is that the accuracy of a digital signal stream can be verified and, if necessary, errors can be corrected. Conversely, signals that vary in proportion to the sound of an orchestra, for example, can be corrupted by “noise”, which once present cannot be removed. An example is the sound of a phonograph record, which always contains some strange sound from the groove surface of the recording, even when the record is new. The noise becomes more pronounced with wear. Compare this to the sound of a digital compact disc recording. No sound is heard that was not present in the recording studio. The disc and player contain error correction features that remove incorrect pulses (perhaps from dust on the disc) from the data as it is read from the disc.
As electronic systems become more complex, it is essential to eliminate errors caused by noise; otherwise, systems may malfunction. Many electronic systems are required to operate in electrically noisy environments, such as in a car. The only practical way to ensure noise immunity is to make such a system work digitally. In principle, it is possible to fix any arbitrary number of errors, but in practice, this may not be possible. The amount of additional information that must be handled to correct large error rates reduces the system’s ability to handle the desired information, and therefore compromises are necessary.
One consequence of the veritable explosion in the number and type of electronic systems has been a sharp growth in the level of electrical noise in the environment. Any electrical system generates some noise, and all electronic systems are to some degree susceptible to noise disturbances. Noise can be conducted through cables connected to the system or it can radiate through the air. Care needs to be taken in the design of systems to limit the amount of noise that is generated and to shield the system adequately to protect it from external noise sources.
A new direction in electronics uses photons (packets of light) instead of electrons. By common consent, these new approaches are included in electronics because the functions performed are, for the moment at least, the same as those performed by electronic systems and because these functions are often embedded in a largely electronic environment. This new direction is called optical electronics or optoelectronics.
In 1966 it was proposed on theoretical grounds that glass fibres could be made of such high purity that light could travel great distances through them. These fibres were produced in the early 1970s. They contain a central core in which light travels. The outer cladding is made of glass of a different chemical formulation and has a lower refractive optical index. This difference in the refractive index indicates that light travels faster in the cladding than in the core. Therefore, if the light beam starts to move from the core towards the cladding, its path is diverted to return to the core. Light is restricted within the core even if the fibre is bent in a circle.
The core of early optical fibres had such a diameter (several micrometres [μm], or about one-tenth the diameter of a human hair) that the various light rays in the core could travel on slightly different paths, the shortest being straight down. the axis and other longer paths wandering back and forth through the core. This limited the maximum distance that a pulse of light could travel without unduly propagating by the time it reached the receiving end of the fibre, with the central beam arriving first and the others later. In a digital communications system, successive pulses may overlap each other and be indistinguishable at the receiving end. These fibres are called multimode fibres, referring to the different paths (or modes) that light can follow.