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LED LCD Photo Diode - electronics notes
Bsc. Computer science (BCS2B02)
University of Calicut
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LIGHT EMITTING DIODE (LED)
As the name indicates, it is a forward-biased P-N junction which emits visible light when energized. charge carrier recombination takes place when electrons from the N-side cross the junction and recombine with the holes on the P-side. Now, electrons are in the higher conduction band on the N-side whereas holes are in the lower valence band on the P-side. During recombination, some of the energy difference is given up in the form of heat and light (i. photons). For Si and Ge junctions, greater percentage of this energy is given up in the form of heat so that the amount emitted as light is insignificant. But in the case of other semiconductor materials like gallium arsenide (GaAs), gallium phosphide (GaP) and gallium- arsenide-phosphide (GaAsP), a greater percentage of energy released during recombination is given out in the form of light. If the semiconductor material is translucent, light is emitted and the junction becomes a light source i. a light-emitting diode (LED) as shown schematically in Fig. The colour of the emitted light depends on the type of material used. ❖ GaAs — infrared radiation (invisible). ❖ GaP — red or green light. ❖ GaAsP — red or yellow (amber) light. LEDs that emit blue light are also available but red is the most common. LEDs emit no light when reverse-biased. In fact, operating LEDs in reverse direction will quickly destroy them. Fig. 53 shows a picture of LEDs that emits different colours of light. LED structures can be divided into two categories : 1-emitting LEDs : These LEDs emit light in a direction perpendicular to the PN junction plane.
2-emitting LEDs : These LEDs emit light in a direction parallel to the PN junction plane. Applications To choose emitting diodes for a particular application, one or more of the following points have to be considered: ❖ wavelength of light emitted ❖ input power required, ❖ output power, ❖ efficiency, ❖ turn-on and turn-off time, ❖ mounting arrangement, ❖ light intensity and brightness etc. Since LEDs operate at voltage levels from 1 V to 3 V, they are highly compatible with solid-state circuitry. Their uses include the following: 1 are used in burglar-alarm systems; 2 solid-state video displays which are rapidly replacing cathode-ray tubes (CRT); 3 image sensing circuits used for ‘picturephone’; 4 the field of optical fibre communication systems where high-radiance GaAs diodes are matched into the silica-fibre optical cable; 5 data links and remote controllers; 6 arrays of different types for displaying alphanumeric (letters and numbers) or supplying input power to lasers or for entering information into optical computer memories; 7 numeric displays in hand-held or pocket calculators.
ADVANTAGES
An LCD has the distinct advantage of extremely low power requirement (about 10-15 μW per 7 - segment display as compared to a few mW for a LED). It is due to the fact that it does not itself generate any illumination but depends on external illumination for its visual effect (colour depending on the incident light). They have a life-time of about 50,000 hours Uses ❖ Field-effect LCDs are normally used in watches and portable instruments where source of energy is a prime consideration. ❖ Thousands of tiny LCDs are used to form the picture elements (pixels) of the screen in one type of B & W pocket TV receiver. ❖ Recent desk top LCD monitors. ❖ Note book computer display ❖ Cellular phone display.
PHOTODIODE
It is a two-terminal junction device which is operated by first reverse-biasing the junction and then illuminating it. A reverse-biased P-N junction has a small amount of reverse saturation current Is (or I 0 ) due to thermally-generated electron-hole pairs. In silicon, Is is the range of nanoamperes. The number of these minority carriers depends on the intensity of light incident on the junction. When the diode is in glass package, light can reach the junction and thus change the reverse current. The basic biasing arrangement, construction and symbols of a photodiode are shown in Fig. A lens has been used in the cap of the unit to focus maximum light on the reverse-biased junction. The active diameter of these devices is about 2 mm but they are mounted with a window to allow maximum incident light.
The characteristics of Fig. shows that for a given reverse voltage, Iλ (or Is) increases with increase in the level of illumination. The dark current refers to the current that flows when no light is incident. By changing the illumination level, reverse current can be changed. A photodioe can turn its current ON and OFF in nanoseconds. Hence, it is one of the fastest photo detectors. It is used where it is required to switch light ON and OFF at a maximum rate. Applications of a photodiode include ❖ Detection, both visible and invisible; ❖ Demodulation; ❖ Switching; ❖ Logic circuit that requires stability and high speed; ❖ Character recognition; ❖ Optical communication equipment, Encoders etc.
LED LCD Photo Diode - electronics notes
Course: Bsc. Computer science (BCS2B02)
University: University of Calicut
