Text 10. Semiconductors

A transistor is an active semiconductor device with three or more electrodes. By active we mean that the transistor is capable of current gain, voltage, amplification and power gain. A transistor is an electron device in which electronic conduction takes place within a semiconductor.

A semiconductor is an electric conductor with resistivity in the range between metals and insulators, in which the electrical charge carrier concentration increases with increasing temperature over some temperature range.

The resistivities of semiconductors and insulators decrease rapidly with rising temperatures, while those of metals increase relatively slowly. Unlike metals and insulators, the resistivity of semiconductors depends upon the direction of current flow. The direction of easiest current flow or lowest resistivity is called the forward direction, the direction of restricted current flow or highest resistivity is known as the reverse or back direction.

Semiconductors, such as the elements germanium and silicon, possess two types of current carriers, namely, negative electrons and positive holes. A hole is a mobile vacancy in the electronic valence structure of a semiconductor which acts like a positive electronic charge with a positive mass.

TEXT 11. SILICON:THE PROSPECTS OF SEMICONDUCTOR TECHNOLOGY

The discovery and development of semiconductors made possible great advances in electronics. The most important among semiconductors is silicon.

There is a very good reason1 why silicon is used to fabricate electronic components. Next to oxygen, it is the most abundant material on the earth's surface. More important, it is very easy to grow insulating layers on silicon. But the impulse to higher performance — faster and denser devices—demands that the industry go beyond silicon in both logic and memory development. For silicon is to semiconductor technology what iron and steel have been to modern metallurgy.

Already silicon has been superseded not simply in laboratory experiment but for commercial production of memory devices with capacities at least four times higher than anything so far realized with silicon: the magnetic-bubble memory. Other technologies which include the use of gallium arsenide and the superconducting materials of Josephson junctions are on the horizon. These technologies, however, are constrained at present by processing and operating costs. For example, gallium arsenide is highly toxic, and insulating layers are also impossible to grow on it. And Josephson junctions at present must be cooled to near absolute zero to work.

The dominant role of silicon as the material for microelectronics is attributed to the properties of its oxide. Silicon dioxide is a clear glass with a softening point higher than 1,400 degrees C. It plays a major role both in the fabrication of silicon devices and in their operation.