LectureNotes_9 - The Light-Emitting Diode(LED The...

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The Light-Emitting Diode (LED) The increasing use of digital displays in calculators, watches and all forms of instrumentation has contributed to the current extensive interest in structures that will emit light when properly biased. The two types in common use today to perform this function are the LED and the liquid-crystal display (LCD). The LED is a diode that will give off visible light when it is energized. In any forward-biased pn junction there is, within the structure and primarily close to the junction, a recombination of holes and electrons. Recall that these free electrons are in the conduction band and at a higher energy level than the holes in the valence band. The recombination requires that the energy possessed by the unbound free electron be transferred to another state When recombination takes place, the recombining electrons release energy in the form of heat and light. A large exposed surface area on one layer of the semiconductor material permits the photons to be emitted as visible light. The process of giving off light by applying an electrical source of energy is called electroluminescence . Various impurities are added during the doping process to establish the wavelength of the emitted light. The wavelength determines the colour of the light and if it is visible or invisible (infrared). In some semiconductors the transition of an electron from the conduction band to the valence band can take place simply with the emission of a photon. This is so because the minimum energy of the conduction band lies at the same momentum value as the maximum of the valence band. These semiconductors are direct bandgap materials (e.g. GaAs, AlGaAs)
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In indirect bandgap materials such as Ge and Si, the energy transition must be accompanied by a momentum change and this requires that some energy be given up as heat to the crystal lattice. LEDs are made of gallium arsenide (GaAs), gallium arsenide phosphide (GaAsP), or gallium phosphide (GaP). Neither silicon nor germanium are used because they are essentially heat- producing materials and are poor at producing light. GaAs LEDs emit infrared radiation, which is not visible. GaAsP produces either red or yellow visible light. GaP emits red or green visible light. LEDs that emit blue light are also available. Red is the most common. LED Biasing The forward voltage across an LED is considerably greater than for a silicon diode. Typically the maximum V F for LEDs is in the range 1.2 V to 3.2 V depending on the device. Reverse breakdown for an LED is much less than for a silicon rectifier diode (3 V to 10 V is typical) The LED emits light in response to a sufficient forward current. V
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This note was uploaded on 02/23/2011 for the course ELETRONICS P24L taught by Professor Learymyers during the Spring '11 term at University of the West Indies at Mona.

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LectureNotes_9 - The Light-Emitting Diode(LED The...

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