According to the band theory of solids, semiconductors are defined as substances that have the forbidden energy gap in the region of 1.1 eV (electron-volt). They have conductivity value lying between that of an insulator and a conductor. This means they are neither good conductors nor good insulators. Semiconductors are widely used in manufacturing electronic devices.
Semiconductor devices are electronic devices that use the properties of a semiconductor in their operations. Semiconductor devices have replaced vacuum tubes in most of the devices. They give better performance than a vacuum tube. This is because semiconductor devices conduct electricity in a solid-state rather than a thermionic state, as is the case in a vacuum tube.
Semiconductors can be used to make single devices or integrated circuit (IC) chips. These chips could be made up of as many as a billion different interconnected devices. The chips are manufactured on a single semiconductor wafer, which is also called the substrate.
The performance of a semiconductor device can be improved by means of a process called doping. Doping refers to the introduction of certain impurities in a semiconductor. These impurities help in the generation of ‘holes’ in the semiconductor. Holes are vacancies in the valence band. Holes are responsible for the flow of electricity in a semiconductor. The conductivity of a semiconductor can be controlled by the introduction of an electric or magnetic field. This can also be achieved by exposing the semiconductor to light or heat. This helps in stabilizing the flow of current in a semiconductor.
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Semiconductors were used in the electronics field well before the invention of the transistor. They were then used as detectors in radios. In the early part of the twentieth century, Jagdish Chandra Bose invented a device called ‘Cat’s Whisker’. It was a kind of detector. The working of this device proved to be very problematic. The device worked by moving a small tungsten filament (called ‘Cat’s Whisker’) around the surface of galena or carborundum until the whisker picked up signals. Then after a few hours or a few days, the detector would lose the signal. Then the process would have to be repeated again. The device made by Bose is an early example of a Schottky diode.
When World War II began, the radar receivers were still using vacuum tubes. The war pushed the radar receivers to work with higher and higher frequencies. At that time, Cat’s Whisker was used again as a receiver and worked much better than the vacuum tubes. This led to renewed research in the field of semiconductors. Furthermore, led to the invention of the first diode.
A diode is a simple device made up of two semiconductors separated by a thin strip of a conductor. Semiconductors on both sides are doped. This allows for the current to pass only in one direction. This simple device has found many uses in electronics. The unidirectional nature of the current is due to the formation of a depletion region near the thin conductor strip.