Classification of Materials
Solids are classified into insulators, metals, and semiconductors.
The valence band and the conductor band are separated from each other by an enormous energy gap. The energy gap between the valence band and the conductor is approximately 6eV.
The energy gap is enormous, and even high voltage or electric field or change in temperature, does make conduction. The electrical conduction is not possible as there are no free electrons. These materials are called insulators and their range of resistivity is 1011 – 1019 Ωm.
In valence band and conduction band overlaps in metals. The electrons can flow freely into the conduction band which results in a large number of free electrons in the conduction band.
Thereby, even at low-temperature conduction is possible. The resistivity range between 10-12 to 10-8 Ωm.
Semiconductors, a thing is forbidden energy gap (Eg < 3eV) endures among the valence band and the conductor band. At a limited temperature, thermal disturbance in the solid can destroy the convent bond between the atoms.
That covalent bond is developed due to the distribution of electrons to attain a steady electronic configuration. This loosens some electrons from the valence band to the conduction band.
Since free electrons are little in quantity, the conductivity of the semiconductors is not as big as the conductors. The resistivity ranges of the semiconductor 10-5 to 106 Ωm. In semiconductors, electrons in the valence band are bound electrons that cannot move and contribute to conduction.
When the temperature is raised more, a larger number of electrons is forwarded to the conduction band and the conduction is increased. Thus, the electrical conduction increases with the increase in temperature in the semiconductors.
Hence, “semiconductors have a negative temperature coefficient of resistance”. Examples of semiconductors are Silicon (Si) and Germanium (Ge). The energy gap for Silicon and Germanium at room temperature are 1.1 eV and 0.7 eV.