Direct and indirect band gap semiconductors

DIRECT AND INDIRECT BAND GAP SEMICONDUCTORS

Semiconductors are also classified into

(a) Direct band gap semiconductor

(b) Indirect band gap semiconductor.

The electrons and holes in a semiconductor have energy and momentum. The momentum (k) depends on energy (E). A plot of energy versus momentum is as shown in fig. 3.2

The lower curves represent energy and momentum values of holes in valence band of semiconductor. Similarly upper curves denote corresponding values for electrons in conduction band.

In direct band gap semiconductor, the energy maximum of valence band and the energy minimum of the conduction band are having same momentum value.

During the recombination of electron from conduction band with hole in valence band, the momentum of the electrons remains virtually constant. The energy equal to band gap energy is released as light photon.

But, in the case of indirect band gap semiconductor, the maximum energy of valence band and minimum energy of conduction band are having different values of momentum.

During recombination, electron first loses momentum such that it has momentum equal to the momentum corresponding to energy maximum of valence band.

To conserve the momentum, emission of third particle known as a phonon is generated. Thus, in this type of recombination phonon is produced.

Types of Semiconductors

When a suitable impurity is added to a pure semiconductor, its electrical conductivity changes. Based on this property, the semiconductors are classified into two types. They are

(i) Intrinsic semiconductor or pure semiconductor

(ii) Extrinsic semiconductor or impure semiconductor or Doped semiconductor