Electroluminescence (Page 2 of 2)

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The transition of an electron from the conduction band to the valence band that results in the emission of a photon is known as radiative recombination.  Certain materials, known as direct bandgap semiconductors (DBS), have their radiative recombinations converted primarily into photon emissions, making them efficient for light emission purposes. 

In DBS materials, the momentum of the electrons at the bottom of the conduction band and that of the holes at the top of the valence band are equal.  As such, interband transitions in DBS materials that result in photon emission are highly probable, with the energy of the emitted photons equal to the bandgap energy of the DBS.

Interband transitions are unlikely in indirect bandgap semiconductors (IBS), since their radiative recombination processes involve phonons and other scattering agents. Thus, IBS materials need to be 'altered' in order to enhance their radiative processes. This is usually accomplished by introducing specific impurities (such as nitrogen) into the IBS to form efficient radiative recombination centers within them.

Light-emitting diodes, or LED's, are basically p-n junctions that emit external spontaneous optical radiations (infrared, visible, or ultraviolet) under forward-biased conditions. Visible Light Emitting Diodes, or visible LED's, involve semiconductors with energy bandgaps greater than 1.8 eV, since this is the minimum energy of light to which the human eye is sensitive (λ = 0.7 µm). The color of the light emitted by an LED depends on the semiconductor material used, which is characterized by its own energy bandgap.

Relative eye sensitivity, V(λ), the measure of the effectiveness of light in stimulating the human eye, is a function of wavelength, with maximum stimulation attained for λ = 0.555 µm, for which V(0.555) = 1.0. The value of V(λ) decreases as λ moves away from 0.555 µm to either side, reaching almost zero at λ = 0.39 µm and λ = 0.77 µm.   The luminosity of radiant energy, on the other hand, describes the effectiveness of the available radiant energy on human vision.     

There are three loss mechanisms that decrease photoemission: 1) absorption of the photons within the LED material itself; 2) fresnel loss, which occurs when the photons pass from one medium type to another (e.g., from LED material to air), causing some photons to be reflected back to the media interface; and 3) critical angle loss, caused by the over-all internal reflection of photons incident to the surface at angles greater than a critical value defined by Snell's Law. This critical angle is about 16 degrees for GaAs and 17 degrees for GaP.

Generally, red LED's are fabricated on substrates of GaAs, which is a direct bandgap semiconductor.  LED's of other common colors (orange, yellow, and green) are usually fabricated on substrates of GaP, which is an indirect bandgap semiconductor. Blue LED's can be manufactured using ZnS or SiC.