Light Emitting Diode (LED)|What is Light Emitting Diode (LED) ?

Light Emitting Diode (LED) : As the name indicates, it is a forward-biased P-N junction which emits visible light when energised. As discussed earlier, charge carrier re- combination takes place when electrons from the N-side cross the junction and recombine with the holes on the P-side.

Light Emitting Diode (LED)
Fig. 1 Light Emitting Diode (LED)

Light Emitting Diode (LED) :

(a) Theory

Light Emitting Diode (LED)
Light Emitting Diode (LED)

Now, electrons are in the higher conduction band on the N-side whereas holes are in the lower ring recombination, some of the energy difference is given up in the form of heat and light (i.e. 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 galliumarsenide (GaAs), gallium Phosphide (GaP) and gallium-arsenide-phosphide (GaAsP), a greater percent- age 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.e. a light-emitting diode (LED) as shown schematically in Fig. 1. The colour of the emitted light depends on the type of material used as valence band on the P-side. given on the next page.

  1. GaAs infrared radiation (invisible).
  2. GaP red or green light.
  3. GAASP red or yellow (amber) light.
Light Emitting Diode (LED)
Light Emitting Diode (LED)

LEDs that emit blue light are also available but red is the most common. LEDs emit no light when reverse biased. In fact, operating LEDSLEDs in reverse direction will quickly destroy them. Fig. 3 shows a picture of LEDS that emits different colours of light.

(b) Construction

Broadly speaking, the LED structures can be divided into two categories:

  1. Surface-emitting LEDS : These LEDS emit light in a direction perpendicular to the PN junction plane.
  2. Edge-emitting LEDS : These LEDS emit light in a direction parallel to the PN junction plane.
Light Emitting Diode (LED)
Fig. 2 Light Emitting Diode (LED)

Fig. 2 shows the construction of a surface-emit- ting LEDS. As seen from this figure, an N-type layer is grown on a substrate and a P-type layer is deposited on it by diffusion. Since carrier recombination takes place in the P-layer, it is kept upper most. The metal anode connections are made at the outer edges of the P-layer so as to allow more central surface area for the light to escape. LEDs are manufactured with domed lenses in order to lessen the reabsorption problem.

A metal (gold) film is applied to the bottom of the substrate for reflecting as much light as possible to the surface of the device and also to provide cathode connection. LEDs are always encased in order to protect their delicate wires.

Being made of semiconductor material, it is rugged and has a life of more than 10,000 hours.

(c) Working

The forward voltage across an LED is considerably greater than for a silicon PN junction diode. Typically the maximum forward voltage for LED is between 1.2 V and 3.2 V depending on the device. Reverse breakdown voltage for an LED is of the order of 3 V to 10 V. Fig. 3 (a) shows a simple circuit to illustrate the working of an LED. The LED emits light in response to a sufficient forward current. The amount of power output translated into light is directly proportional to the forward current as shown in Fig. 3 (b). It is evident from this figure that greater the forward current, the greater the light output.

Light Emitting Diode (LED)
Fig. 3 Light Emitting Diode (LED)

(d) Applications

To chose 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, tum-on and turn-off time, mounting arrangement, light intensity and brightness etc.

Since LEDs operate at voltage levels from 1.5 V to 3.3 V, they are highly compatible with solid state circuitry.

Their uses include the following: 1.

  1. LEDs are used in burglar-alarm systems.
  2. for solid-state video displays which are rapidly replacing cathode-ray tubes (CRT).
  3. in image sensing circuits used for “picturephone’.
  4. in the field of optical fibre communication systems where high-radiance GaAs diodes are matched into the silica-fibre optical cable.
  5. in data links and remote controllers.
  6. in arrays of different types for displaying alphanumeric (letters and numbers) or supplying input power to lasers or for entering information into optical eomputer memories.
  7. for numeric displays in hand-held or pocket calculators.
Light Emitting Diode (LED)
Fig. 4

As shown in Fig. 4 (a) a seven-segment display consists of seven rectangular LEDs which can form the digits 0 to 9. The seven LED segments are labelled ta’ to g’. Each of this segments is 5. 6. controlled through one of the display LEDs. Seven segment displays come in two types, common-cathode and common-anode type. In the common-cathode type, all the cathodes of the diodes are tied together as shown in Fig. 4 (b). This makes it possible to light any segment by forward-biasing that particular LED. For example, to light number 5, segments a, f. g. c and d must be forward biased. Since the cathodes are tied to ground, only 5 volt is to be applied to the anode of these segments to light them.

The common-anode seven-segment display has all its anodes tied together to +5 volt and ground is used to light the individual segments. Fig. 4(c) shows a picture of a seven-segment display.

(e) Multicoloured LEDs

Light Emitting Diode (LED)
Fig. 5

LEDs are available which gives out light in either two or three colours. There are also blinking LEDs. A two colour LED is a three-terminal device as shown in Fig. 5. The longest lead is the cathode and the remaining two leads are the anodes. When leads R and C are forward-biased, the LED emits red light and when leads G and C are forward-biased, LED emits green light. The tri-colour LED looks simi- Jar to the ordinary LED but emits, red, green or yellow light depending on operating conditions. It has two leads and each of these acts as both anode and cathode. When dc current flows through it in one direction, LED emits red light but when current flows in the opposite direction, LED emits green light. However, with ac current, yellow light is given out.

The blinking LED is a combination of an oscillator LED in one package. Since it has an anode and a cathode lead, it looks like an ordinary LED. The blinking frequency is usually 3 Hz when the diode forward bias is 5 V. It conducts about 20 mA of current when ON and 0.9 mA when OFF.

Use of LEDs in Facsimile Machines

Fig. 6 shows a simplified schematic diagram of a facsimile (or fax) machine. As seen, the light from the LED array is focussed on the document paper. The light reflected at the paper is focussed on a charge-coupled device (CCD) by a combination of mirror and a lens. This causes the optical information to be converted into electrical information. The electrical information is then sent through the data-processing unit to its destination via telephone line.

Light Emitting Diode (LED)
Fig. 6

What is Light Emitting Diode (LED) ?

As the name indicates, it is a forward-biased P-N junction which emits visible light when energised. As discussed earlier, charge carrier re- combination takes place when electrons from the N-side cross the junction and recombine with the holes on the P-side.
Read more

What is the theory of Light Emitting Diode (LED) ?

electrons are in the higher conduction band on the N-side whereas holes are in the lower ring recombination, some of the energy difference is given up in the form of heat and light (i.e. photons).
Read more

What is the construction of Light Emitting Diode (LED) ?

Broadly speaking, the LED structures can be divided into two categories:
1. Surface-emitting LEDS : These LEDS emit light in a direction perpendicular to the PN junction plane.
2. Edge-emitting LEDS : These LEDS emit light in a direction parallel to the PN junction plane.
Read more

What is the working process of Light Emitting Diode (LED) ?

The forward voltage across an LED is considerably greater than for a silicon PN junction diode. Typically the maximum forward voltage for LED is between 1.2 V and 3.2 V depending on the device. Reverse breakdown voltage for an LED is of the order of 3 V to 10 V.
Read more

What is the application of Light Emitting Diode (LED) ?

To chose 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, tum-on and turn-off time, mounting arrangement, light intensity and brightness etc.
Read more

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