Luminescence- Property of Light Emission!!

Introduction:
When carriers are excited to higher impurity levels from which they fall to their respective equilibriumstates or when electron-hole pairs are produced in a semiconductor,the material gives off light. Many semiconductors are ideal for light emission phenomenon, for instance, the compoundsemiconductors which have direct band gaps. Generally, this property of light emission is referred to as luminescence.

Types of the luminescence:
There are three main types of the luminescence:

• Photoluminescence
• Cathodoluminescence
• Electroluminescence

Overall, these can be categorized or subdivided on the basis of their excitation mechanism:
Photoluminescence:
If carriers absorb photons to get excited, the resulting radiationfrom such recombination of excited carriers is referred to as photoluminescence.
Cathodoluminescence:
If bombardment of the material with high-energy electron generates the excited carriers, the mechanism is referred to ascathodoluminescence.
Electroluminescence:
If current is introduced in the sample, excitation occurs which generates luminescence that is referred to as electroluminescence.

• Electrical energy can be made use of in several ways in order to stimulate photon emission in a solid.

• In LED’s, an electric current stimulates the injection of minority carriers into those areas of crystal where they can recombine with majority carriers, resulting in the emission of recombination radiation.

• The electroluminescent effect was first observed in the form of emission of photons by some phosphors in an alternating electric field (i.e. the Destriau effect).

• In this device, ZnSi.e. a phosphor powder is held in a binding material (i.e. often a plastic) that has high dielectric constant.

• On applying a-c electric field, phosphor gives off light.

• Despite of the fact that their efficiency for most of the applications and reliability is poor, such cells can be useful as lighting panels.