NEET Physics Notes Modern Physics-Electronic Devices-Energy Bands in Solids
Energy Bands in Solids
Energy Bands in Solids
According to band theory of solids, in a crystalline solid due to mutual interaction among valence electrons of neighbouring atoms, instead of sharp energy levels, energy bands are formed.
Energy bands are of the following three types
- Valence band It is the energy band formed by a series of energy levels of valence electrons actually present. Ordinarily, valence band is completely filled and electrons in this band are unable to gain energy from external electric field. The highest energy level in a valence band at 0 K is called fermi energy level.
- Conduction band The energy band having just higher energy than the valence band is called conduction band. Electrons can conduct when present in conduction band.
Electrons in conduction band are commonly called the free electrons. - Forbidden band The energy gap between the valence band and the conduction band of a solid is called the forbidden energy gap Eg or forbidden band. Width of forbidden energy gap depends upon the nature of substance.
In semiconducting solids, the valence band is completely filled, conduction band is completely empty and the energy gap between them is small enough (Eg < 3 eV). At absolute zero temperature, it behaves as an insulator.
A pure semiconductor, in which no impurity of any sort has been mixed, is called intrinsic semiconductor. Germanium (Eg =0.72eV) and Silicon
(Eg = 1.1 eV) are examples of intrinsic semiconductors. Electrical conductivity of pure semiconductor is very small. To increase the conductivity of a pure semiconducting material, it is doped with a controlled quantity (1 in 105 or 106) of suitable impurity. Such a doped semiconductor is called an extrinsic semiconductor.
Superconductors
When few metals are cooled, then below a certain critical temperature, their electrical resistance suddenly becomes zero. In this state, these substances are called superconductors and this phenomena is called superconductivity. Mercury become superconductor at 4.2 K, lead at 7.25 K and niobium at 9.2 K.
Types of Semiconductor
There are two types of semiconductor
- n-type Semiconductor
To prepare an n-type semiconductor a pentavalent impurity e.g. P, As, Sb is used as a dopant with Si or Ge. Such an impurity is called donor impurity because each dopant atom provides one free electron.
In n-type semiconductor 
, i.e. electrons are majority charge carriers and the holes are minority charge carriers, such that 
A n-type semiconductor is electrically neutral and is not negatively charged.
- p-type Semiconductor
To prepare a p-type semiconductor a trivalent impurity, e.g. B, Al, In, Ga etc. is used as a dopant with Si or Ge. Such an impurity is called acceptor impurity as each impurity atom wants to accept an electron from the crystal lattice. Thus, effectively each dopant atom provides a hole.
In p-type semiconductor 
i.e. holes are majority charge carriers and electrons minority charge carriers, such that 
. A p-type semiconductor is electrically neutral, and is not positively charged.
The number of free electrons in a semiconductor varies with temperature as T3/2.