ELECTRIC LAMPS – PART – 10 – LIGHT EMITTING DIODE (LED)

INVENTOR OF LED
The early years of the 1960s witnessed of a 'race' in the field of semiconductors. LED was 'discovered' in the year 1961 by James R. Biard and Gary Pittman. In 1962 – Nick Holonyak, Jr. a consulting scientist for General Electric invented first visible-spectrum LED.
At that time the production cost of one LEDs was $200. These LEDs used a semiconductor combining gallium, arsenic and phosphorus - GaAsP. This type produced red light, but the efficiency of the devices was very low. In 1987 the Hewlett Packard (HP) being produced Aluminium Gallium Arsenide (AlGaAs) diodes which were bright enough for the first applications within lighting. In 1998 Aluminium Indium Gallium Phosphide (AlInGaP) diodes were manufactured by HP which are superior to Aluminium Gallium Arsenide (AlGaAs) diodes, giving double the light output. In 1993 HP started to use Gallium Phosphide (GaP) to provide high output green LEDs and HP also developed high output orange lamps.

WORKING PRINCIPLE
LED work on the principle of Electroluminescence (EL).
Electroluminescence is an optical phenomenon and electrical phenomenon, in which a material emits light in response to the passage of an electric current or to a strong electric field.

CONSTRUCTION
1. LED is basically a specialized type of PN junction diode, made
    up of a very thin layer of moderately doped semiconductor
    material.
2. LEDs are made from exotic (exotic means strikingly strange or
    unusual) semiconductor compounds such as Gallium Arsenide
    (GaAs), Gallium Phosphide (GaP), Gallium Arsenide Phosphide
    (GaAsP), Silicon Carbide (SiC), Gallium Indium Nitride
    (GaInN) which are together at different ratios to produce a
    distinct wavelength of colour.
3. Different LED compounds emit light in specific regions of the
    visible light spectrum and therefore produce different
    intensity levels.
4. The PN junction of an LED is surrounded by a transparent,
    hard plastic epoxy resin hemispherical shaped shell or body
    which protects the LED from both vibration and shock.
5. The epoxy resin body is constructed in a such way that the
    Photons of light emitted are focused upwards through the
    domed top of the LED.
6. The epoxy resin acts like a lens concentrating the amount of
    light.
7. All LEDs are not made with a hemispherical shaped dome for
    their epoxy shell.
8. Some LEDs have a rectangular or cylindrical shaped
    construction with a flat surface on top or their body which is
    shaped into a bar or like an arrow.
9. The most common colours of LEDs are RED, AMBER, YELLOW 
    and GREEN and are highly used as visual indicators and as
    moving light displays.
10. Blue and white coloured LEDs are also available but they are
      more expensive than standard colour LEDs.

OPERATION

1.Under forward biased condition, the LED emit light due   to  the

   recombination of holes and electrons within the device,    

   releasing    energy as photons which are called as photon

   electrons.

2.The electrons dissipate energy in the form of heat for ordinary

   diodes. But in LED the electrons dissipate energy by emitting

   photons. 

3.The charge carriers recombine in a forward P-N junction as

   the electrons cross from the N-region and recombine with the

   holes existing in the P-region.

4.Free electrons are in the conduction band of energy levels,

   while holes are in the valence energy band.

5.Thus the energy level of the holes will be lesser than the

   energy levels of the electrons.

6.Some part of the energy must be dissipated in order to

   recombine the electrons and  the holes.

7.This energy is emitted in the form of heat and light.

8.The majority of the light is produced from the area of the

   junction nearer to the P-type region.

9.If the semiconductor is translucent, the junction becomes the

   source of light as it is emitted, thus becoming a light emitting

   diode (LED).

10.LED will not emit light when it is reverse biased and at the

    same time it also get damaged.

TYPES OF LEDS AND ITS COLOURS

1. GaAs            - infra-red

2. GaAsP          - red to infra-red, orange

3. AlGaAsP       - orange-red, orange, and yellow

4. GaP              - red, yellow and green

5. AlGaP           - green

6. GaN              - green, emerald green

7. GaInN           - near ultraviolet, bluish-green and blue

8. SiC               - blue as a substrate

9. ZnSe            - blue

10. AlGaN         - ultraviolet

ADVANTAGES

1. Energy efficient source of light for short distances and small

    areas.

2. Miniature in size and hence light weight.

3. Low voltage and current are enough to drive the LED and

    require only 30-60 milliwatts to operate.

4. Durable and shockproof unlike glass bulb lamp types.

5. The response time is very less – only about 10 nanoseconds.

6. It can withstand shock and vibrations because it is rugged.

7. Lumens per watt: 28 - 150 (depends on environment)

8. Lamp life: 25,000 - 100,000 hours [more than 20 years]

9. Available in 0.01 - 3 W

10. LEDs long life, rich color, and easily-controlled features with integrated electronics offer a scalable lighting solution. As technology continues to bring rapid improvements in luminous efficiency and as cost compression persists, applications expand rapidly.

DISADVANTAGES

1. A slight excess in voltage or current can damage the device.

2. Unreliable in outside applications with great variations in

    summer/winter temperatures.

3. Reduced lumen output over time.

4. Much wider bandwidth compared to the laser.

5. The temperature affects LEDs radiant output power and

    wavelength.

APPLICATIONS

1. One mobile phone takes two LED backlight sources and six

    SMD (Surface Mount Diode) LED key lights. As a result, mobile

    phones create a demand for 3.2 billion LEDs per year.

2. Interior usage of automobiles include indicator lights on    

    dashboard gauges, audio status lights, security status lights

    and warning signals and exterior usage includes third brake

    lights, left and right rear lamps, turn signals, etc.

3. The LED screen has become a new display medium for

    advertising and information.

4. Today, LEDs have been integrated as warning lights and

    indicators on most electronic applications.

5. LEDs are being used in advertising billboards, illumination of

    commercial building exteriors, landmark buildings, bridges,

    roads, town centers, airports, subways, hotels, shopping

    centers and landscape lighting because of numerous advantages

    they offer.