Branch : Computer Science and Engineering
Subject : Fundamental of Electronic Devices
Unit : Basic Electronics
Light Emitting Materials
Materials:
- The band gaps of various binary compound semiconductors are illustrate in relative to the spectrum.
- There is a wide variation in band gaps and, therefore, in available photon energies, extending from the ultraviolet (GaN, 3.4 eV) into the infrared (InSb, 0.18 eV).
- In fact, by utilizing ternary and quaternary compounds the number of available energies can be increased significantly.
- A good example of the variation in photon energy obtainable from the compound semiconductors is the ternary alloy gallium arsenide-phosphide, which is illustrated in fig.
- When the percentage of As is reduced and P is increased in this material, the resulting band gap varies from the direct 1.43-eV gap of GaAs (infrared) to the indirect 2.26-eV gap of GaP (green).
- The band gap of GaAs1-xPx varies almost linearly with x until the 0.45 composition is reached, and electronhole recombination is direct over this range.
- The most common alloy compositionn used in LED displays is x = 0.4. For this composition the band gap is direct, since the T minimum (at k = 0) is the lowest part of the conduction band.
- This results in efficient radiative recombination, and the emitted photons (-1.9 eV) are in the red portion of the spectrum.
- For GaAs^P* with P concentrations above 45 percent, the band gap is due to the indirect X minimum.
- Radiative recombination in such indirect materials is generally unlikely, because electrons in the conduction band have different momentum from holes in the valence band.
- However, indirect GaAs1-xPx (including GaP, x = 1) doped with nitrogen can be used in LEDs with light output in the yellow to green portions of the spectrum.
- This is possible because the nitrogen impurity binds an electron very tightly.
- This confinement in real space (Ax) means that the electron momentum is spread out in momentum space Ap by the Heisenberg uncertainty principle
- As a result, the momentum conservation rules, which generally prevent radiative recombination in indirect materials, are circumvented.
- Thus nitrogen doping of GaAs1-xPx is not only useful technologically, but also provides an interesting and practical illustration of the uncertainty principle.
- In many applications light from a laser or an LED need not be visible to the eye.
- Infrared emitters such as GaAs, InP, and mixed alloys of these compounds are particularly well suited to fiber-optical communication systems or TV remote controls.
- For example, a laser or light-emitting diode can be used in conjunction with a photodiode or other photosensitive device to transmit information optically between locations.
- By varying the current through the diode, the light output can be modulated such that analog or digital information appears in the optical signal directed at the detector.
- Alternatively, the information may be introduced between the source and detector.
- For example, a semiconductor laser-photodetector arrangement can be used in a compact disc or DVD system for reading digital information from the spinning disc.