When electron-hole pairs are generated in a semiconductor, or when carriers are excited into higher impurity levels from which they fall to their equilibriumstates, light can be given off by the material. Many of the semiconductors are well suited for light emission, particularly the compound semiconductors with direct band gaps. The general property of light emission is called luminescence.
Types of the luminescence:
There is three types of the luminescence:
This overall category can be subdivided according to the excitation mechanism:
If carriers are excited by photon absorption, the radiation resulting from the recombination of the excited carriers is called photoluminescence.
If the excited carriers are created by high-energy electron bombardment of the material, the mechanism is called cathodoluminescence.
If the excitation occurs by the introduction of current into the sample, the resulting luminescence is called electroluminescence.
- There are many ways by which electrical energy can be used to generate photon emission in a solid.
- In LEDs an electric current causes the injection of minority carriers into regions of the crystal where they can recombine with majority carriers, resulting in the emission of recombination radiation.
- The first electroluminescent effect to be observed was the emission of photons by certain phosphors in an alternating electric field (the Destriau effect).
- In this device, a phosphor powder such as ZnS is held in a binder material (often a plastic) of a high dielectric constant.
- When an a-c electric field is applied, light is given off by the phosphor.
- Such cells can be useful as lighting panels, although their efficiency has thus far been too low for most applications and their reliability is poor.