Extract A

The two semiconductor materials used in the manufacture of semiconductor devices, such as diodes and transistors, are germanium and silicon. Both these materials fall into group IV of the Periodic Table of Elements. An atom of either substance may be represented by a central core having a positive charge and surrounded by orbiting electrons, each having a negative charge.

In its solid state silicon forms crystals of the diamond type, i.e. it forms a cubic lattice in which all the atoms except those at the surface are equidistant from their immediately neighbouring atoms. A study of crystal structure shows that the greatest number of atoms that can be neighbours to a particular atom at an equal distance away from that atom and yet be equidistant from one another is four. Hence each atom in a silicon crystal has four neighbouring atoms. In the crystal lattice each atom employs its four valence electrons to form covalent bonds with its four neighbouring atoms; each bond consists of two electrons, one from each atom. Each pair of electrons orbits around both its parent atom and a neighbouring atom.

Extract B

Semiconductors are soilids whose resistivity lies between those of electrical conductors and insulators. Semiconductors are used in computers, in radio and TV receivers, and in other electronic products.

Semiconductor devices perform many control functions. They may be used as rectifiers, amplifiers, detectors, oscillators and switching elements. Some characteristics, which make the semiconductors such an attractive member of the electronics family are the following:

1. Semiconductors are small and light in weight.

2. Semiconductors are solids. There is therefore little chance that elements will vibrate. Element vibration in vacuum tubes was the cause of microphonics.

3. Semiconductors require little power and radiate less heat than tubes. They do not need warm up time and operate as soon as power is applied.

4. Semiconductors do not undergo the chemical deterioration which occurs in tube cathodes. The deterioration of tube cathodes eventually results in unacceptable tube performance.

Silicon is the material of which most semiconductor devices are presently constructed.

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