Some of the properties of solids which are exploited in electronic and magnetic devices such as transistor, Computers, and Telephones etc., are described below:
Electrical Properties: Not all solids conduct electricity with the same ease. Depending upon the character and magnitude of conductivity, the solids may be divided into parts. Metals are good conductor of electricity and have conductivity in the order of 10 rest to 4 to 10 rest to 7 S m -1.
The conductivity of solids may be due to the movement of electrons, holes or ions. Ionic solids such as pure alkali halides do not conduct electricity at near room temperature. Thus, ionic solids at room temperature are insulators. However, due to the presence of vacancies and other defects, these solids show slight conductivity, which increases with temperature.
1} Conduction of electricity in Metals:
Metals conduct electricity both in the solid as well as molten state. The conductivity of a metal depends on the number of valence electrons in its atom.In metals, the atomic orbitals form molecular orbitals. These molecular orbitals are so close in energy that they form a band. If the valence band is partially filled or the valence and the conduction bands partially overlap each other, those the electrons can flow easily under an applied field and the metal show conductivity.
In insulators, the energy gap is quite large. So, the electrons of the valance band cannot be promoted to the conduction band. As a result, such materials/substances do not conduct electricity even at higher temperatures.
2} Conduction of Electricity in Semiconductors: In intrinsic semiconductors, the energy gap is very small. A slight increase in temperature or on the application of small voltage, some of the electrons from the valence band get promoted to the conduction band. As a result, such materials are able to conduct electricity through with much lesser ease than the metals (or conductors).
The conductivity of semiconductors increases with a rise in temperature.
Silicon (Si) and Germanium (Ge) are typical intrinsic semiconductors.