Basic Semiconductor Theory

In years before the Second World War solid state electronic devices were unknown. Vacuum tubes, magnetic amplifiers and relays were the only devices which were used in the processes of electrical signal generation, amplification, transmission, wave shaping and switching. After the invention of the transistor in - 1944 and subsequent development and improvement of the transistor and other solid state electronic devices, vacuum tubes are being replaced in many fields of applications, except in high-power
applications, by these solid state devices A solid state electronic
device consists of a semiconducting material and the development of a wide variety of semiconductor devices is due to wide range of electrical properties which a semiconductor acquires by minor chemical additions of some elements.
In this chapter we give a brief account of simple atomic theory of semiconductors.
11. Semiconductors.
The electrical resistivity of a good conductor like copper or
silver is very small, while that of an insulator is very large. F'or
example, the resistivity of silver is 1.6 x10-8 ohm-m and that of quartz is about 1012 ohm-rrt. - A semiconductor is a solid material whose electrical resistivity is higher than that of a conductor and lower than that of an insulator. Typical values of the resistivity of a semiconductor lie between 10-2 and 1 ohm m at room temperatures. This is not the only essential characteristic of a semiconductor. Other essential characteristics of a semiconductor are : -
(i) The electrical resistance of a semiconductor decreases with increase in temperature over a particular temperature-range which is charaCteristic of the semiconductor.