UNIT-III

Optoelectronics

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LED Device structure, Materials, Characteristics

�What is Light Emitting Diode (LED)�

• Light Emitting Diodes (LEDs) are the most widely used semiconductor diodes among all the different types of semiconductor diodes. Light emitting diodes emit either visible light or infrared light when it is forward biased.
• LED (Light Emitting Diode) is an optoelectronic device which works on the principle of electro-luminescence. Electro-luminescence is the property of the material to convert electrical energy into light energy and later it radiates this light energy.

• When Light Emitting Diode (LED) is forward biased, free electrons in the conduction band recombines with the holes in the valence band and releases energy in the form of light.
• The process of emitting light in response to the strong electric field or flow of electric current is called electroluminescence.
• Like the normal p-n junction diodes, LEDs also operates only in forward bias condition. To create an LED, the n-type material should be connected to the negative terminal of the battery and p-type material should be connected to the positive terminal of the battery. In other words, the n-type material should be negatively charged and the p-type material should be positively charged.

Symbol of LED

• The semiconductor material used in LED is Gallium Arsenide (GaAs), Gallium Phosphide (GaP) or Gallium Arsenide Phosphide (GaAsP). Any of the above-mentioned compounds can be used for the construction of LED, but the colour of radiated light changes with the change in material.
• The electrons dissipate energy in the form of heat for silicon and germanium diodes. But in Galium- Arsenide-phosphorous (GaAsP) and Galium-phosphorous (GaP) semiconductors, the electrons dissipate energy by emitting photons.

Principle

• A Light emitting diode (LED) is specially designed a p-n junction diode that gives off light when it is forward biased. When carriers are injected across a forward-biased junction, it emits incoherent light.
• The wavelength (color) of the light emitted, depends on the energy gap of the materials forming the p-n junction. The emitted photon energy is approximately equal to the energy gap of the semiconductor.
• The following equation relates the wavelength and the energy gap.
• hν = Eg
• hc/λ = Eg λ = hc/ Eg
• Where h is Plank’s constant, c is the speed of the light and Eg is the energy band gap.

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Construction of LED�

• The recombination of the charge carrier occurs in the P-type material, and hence P-material is the surface of the LED. For the maximum emission of light, the anode is deposited at the edge of the P-type material. The cathode is made of gold film, and it is usually placed at the bottom of the N-region. This gold layer of cathode helps in reflecting the light to the surface.

Working of LED

• The LED is connected in the forward biased, which allows the current to flows in the forward direction. The flow of current is because of the movement of electrons in the opposite direction. The recombination shows that the electrons move from the conduction band to valence band and they emits electromagnetic energy in the form of photons. The energy of photons is equal to the gap between the valence and the conduction band.

Continued…

• The electrons are majority carriers in N-type and holes are majority carriers in P-type. The electrons of N-type are in the conduction band and holes of P-type are in the valence band. The energy level of the Conduction band is higher than the energy level of the Valence band. Thus, if electrons tend to recombine with holes they have to lose some part of the energy to fall in lower energy band.
• The electrons can lose their energy either in the form of heat or light. The electrons in Silicon and Germanium lose their energy in the form of heat. Thus, they are not used for LEDs as we want semiconductor in which electrons lose their energy in the form of light.
• Thus, semiconductor compounds such as Gallium Phosphide (Gap), Gallium Arsenide (GaAs), Gallium Arsenide Phosphide (GaAsP) etc. emit light when electrons-holes recombine. The electrons in these compounds lose their energy by emission of photons.

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• The actual colour of a light emitting diode is determined by the wavelength of the light emitted, which in turn is determined by the actual semiconductor compound used in forming the PN junction during manufacture.

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V - I Characteristics of LEDs

Light Emitting Diode (LED) Schematic symbol and I-V Characteristics Curves�showing the different colours available.

Applications of Light Emitting Diodes�

• There are many applications of the LED and some of them are explained below.
• As indicators and light source in fiber optics communication.
• LED is used as a bulb in the homes and industries
• The light emitting diodes are used in the motorcycles and cars
• A number of LED‘s may be grouped together to form a display
• At the traffic light signals LED‘s are used

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Advantages of LED’s

• O/P is bright and intensity is controlled by varying current.
• They can operate at low voltages.
• Very fast response time in the order of ns.
• Available in different colors.
• Has long life time (10⁵ hours)
• The cost of LED’s is less and they are tiny.
• The intensity of the LED differs with the help of the microcontroller.

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