Definitions [9]
What are free electrons?
FREE ELECTRONS. Electrons of the outermost orbit of a solid, which can easily leave the atom and become free to move inside the solid are called FREE ELECTRONS
What do you mean by conservation of charges?
According to the Law of conservation of charge
“When two different bodies are rubbed together, both bodies get charged equally but with charges of the opposite kind.” Thus, the total charge of two bodies before and after rubbing remains the same.
Example: When an ebonite rod is rubbed with fur, the electrons from the fur are transferred to the ebonite rod and therefore the ebonite rod becomes negatively charged (due to gain of electrons), while the fur becomes equally positively charged (due to deficit of a same number of electrons). As the same no. of electrons are, exchanged by the ebonite rod and fur, the magnitude of charges main same but with opposite sign.
The material with electrical conductivity between that of a conductor and an insulator, whose number of charge carriers can be controlled as per requirement, is called a semiconductor. (e.g. Silicon, Germanium)
The different energy levels with continuous energy variation are called energy bands.
The range of energies possessed by valence electrons is called valence band.
The range of energies possessed by conduction electrons is called conduction band.
The energy difference between the valence band and the conduction band is called forbidden energy gap.
The solids which have a large number of free electrons are called conductors. (e.g. Iron, Aluminium)
The solids which have very small number of free electrons are called insulators. (e.g. Glass, Wood)
Key Points
- Conductors → Eg = 0 - bands overlap, electrons flow freely.
- Semiconductors → Eg < 3 eV — small gap, conducts at room temperature.
- Insulators → Eg > 5 eV — large gap, no conduction.
- Ge = 0.72 eV, Si = 1.1 eV — both semiconductors.
- Metal conductivity decreases with temp. Semiconductor conductivity increases with temp.
