Band theory Chemistry Notes
→ When atomic orbitals of atoms of metals combine with each other then molecular orbitals are formed, whose energies are so close to each other that they looks like a band. Empty space is found between two bands which is called Forbidden zone or Energy Gap. The atomic orbitals of valence shell of metal atoms on combination form molecular orbital and there is very
→ less difference in energy of molecular orbitals. By which these form a band which is called valence band. The higher empty-band near to filled valence band is called conduction band. We can explain conductors, insulators and semiconductors on the basis of band theory.
→ If valence band is partially filled or it overlaps with the higher energy or unoccupied conduction band, then electrons can flow easily under the influence of applied electric field. Therefore, the metals conduct electricity.
→ If there is more difference between filled valence band and unoccupied conduction band then electrons can not jump from the energy gap due to which the conductivity of substances is either very less or behave as insulators.
Semi Conductors :
- Here, there is less energy difference between valence band and conduction band. So, some electrons jump in conduction band and exhibit less conductivity.
- The energy of electron increases with increase in temperature. Thus, more number of electrons can jump in conduction band due to which the conductivity of semi conductors increases with increase in temperature.
Types of semiconductors : These are of two types :
- Intrinsic Semiconductors
- Extrinsic Semiconductors
→ Intrinsic Semiconductors : Some substances are non-conductors at room temperature but becomes semiconductors with increase in temperature. These are called intrinsic semiconductors.
→ Example : Silicon and germanium are non-conductors at room temperature due to presence of covalent bond only. But when temperature increases then some covalent bonds break and release electrons. These free electrons make the substance semiconductor. Such type of semiconductors are called intrinsic semiconductors
→ Extrinsic Semiconductors: A considerable amount of impurities are mixed to increase the conductivity of semiconductors. This method called doping. The semiconductors formed by this method are called extrinsic semiconductors.
Two types of semiconductora are formed by doping:
→ Electron Rich impurities and n-Type Semiconductors: When elements of 15th group like P or As, in which five valence electrons are present are doped with elements of 14th group i.e. Si, Ge, then some lattice points are formed in crystals of 14th group and 4e– out of 5 form covalent bond with neighbouring Si atoms while 5th e– is free. This free e– increases the conductivity. So, it is called n-type semiconductor.
→ Electron Deficient Impurities and p-Type Semiconductors: When elements of 13th group like Al, Ga, In, in which three valence electrons are present when doped with elements of 14th group like Si, Ge then these come on lattice point in crystals of 14th group. Three electrons form covalent bond with neighbouring Si or Ge atoms and one place is vacant. The place where bonding does not occur is called electron hole.
→ Electron from neighbouring atom is fill this hole but due to this, electron hole is formed at that place. Thus, here conductivity is due to filling of electron or movement of hole in opposite direction. So, it is called p-type semiconductor.
Uses of Semiconductors :
- The various combinations of n-type and p-type semiconductors are used in making electronic components.
Diode is a combination of n-type and ptype semiconductors and is used in the form of purifier.
- Transistors are formed by making sandwitch of one layer of a type of semiconductor in between two layers of other types of semiconductors, Transistors of n.p- and p-n-p type are used to detect or amplify radio or audio signals.
- Solar cell is an efficient photo diode which is used to convert light energy into electric energy.
- The compounds of 12-16 and 13-15 groups are formed by mixing of elements of group 12 and group 16 or group 13 and group 15. Their average valency is four similar to Ge or Si. The compounds of group 13-15 are InSb, AIP, GaAs etc.
→ GaAs is very fast responsing semiconductor. Zns, Cds, CdSe, HgTe etc are examples of compounds of group 12-16. In these compounds. bonds are completely covalent and their ionic properties depend on difference of electronegativities of both elements present in them.