Construction and Working of Light Emitting Diode

A PN junction diode, which emits light when forward biased, is known as a light emitting diode (abbreviated as LED). The emitted light may be visible or invisible. The amount of light output is directly proportional to the forward current. Thus, higher the forward current, higher is the light output.

The schematic symbol of a light emitting diode is shown in Fig. 1 (a). The arrows, pointing away from the diode symbol represent the light, which is being transmitted away from the junction.

Fig. 1 (b) shows the basic structure of a light emitting diode. Here, an N-type layer is grown on a P-type substrate (not indicated in Figure) by a diffusion process. Then a thin P-type layer is grown on the N-type layer. The metal connections to both the layers make anode and cathode terminals as indicated. The light energy is released at the junction, when the recombination of electrons with holes take place.

After passing through the P-region, the light is emitted through the window provided at the top of the surface.

It will be interesting to know that when the LED is forward biased, the electrons and holes move towards the junction and the recombination takes place. After recombination, the electrons, lying in the conduction bands of N-region, fall into the holes lying in the valence band of a P-region.

The difference of energy between the conduction band and valence band is radiated in the form of light energy. In ordinary diodes, this energy is radiated in the form of heat.

The semiconducting materials used for manufacturing light emitting diodes are gallium arsenide, gallium arsenide phosphide. The silicon and germanium is not used for manufacturing light emitting diodes because these are heat producing materials. Moreover, these materials are very poor in emitting light radiations.

The LED’s radiate light in different colours such as red, green, yellow, blue, orange etc. Some of the LED’s emit infrared (i.e., invisible) light also. The colour, of the emitted light, depends upon the type of the semiconductor used. Thus gallium arsenide emits infrared radiations, gallium arsenide phosphide produces either red or yellow light, gallium phosphide emits red or green light and gallium nitrite produces blue light.

Since LED’s have clear (or semiclear) cases, there is normally no label on the cased to identify the leads. The two leads of a LED are identified using one of the several schemes as discussed below.

• The leads may have different lengths as shown in Fig. 2 (a), when the schemes is used, the longer of the two leads is usually the anode.
• One of the leads may be flattened as shown in Fig. 2 (b). The flattened lead is usually the anode.
• One side of the case is flattened as shown in Fig. 2 (c). The lead closest to the flattened side is usually the anode.

Advantages of LED

Thought the LED has a number of advantages, yet some of them are given below:

1. Efficiency: LEDs emit more light per watt than incandescent bulbs Their efficiency is not affected by shape and size, unlike Fluorescent light bulbs or tubes.

2. Color: LEDs can emit light of an intended color without using any color filters as traditional lighting methods need. This is more efficient and can lower initial costs.

3. Size: LEDs can be very small (smaller than 2 mm) and are easily populated onto printed circuit boards.

4. On/Off time: LEDs light up very quickly. A typical red indicator LED will achieve full brightness in under a microsecond. LEDs used in communications devices can have even faster response times.

5. Lifetime: LEDs can have a relatively long useful life. One report estimates 35,000 to 50,000 hours of useful life, though time to complete failure may be longer. Fluorescent tubes typically are rated at about 10,000 to 15,000 hours, depending partly on the conditions of use, and incandescent light bulbs at 1,000 – 2,000 hours.

6. Shock resistance: LEDs, being solid state components, are difficult to damage with external shock, unlike fluorescent and incandescent bulbs which are fragile.

7. Focus: The solid package of the LED can be designed to focus its light. 8. Low toxicity: LEDs do not contain mercury, unlike fluorescent lamps.

Disadvantages of LED

LED has some disadvantages also, they are given below:

1. Temperature dependence: LED performance largely depends on the ambient temperature of the operating environment.

2. Voltage sensitivity: LEDs must be supplied with the voltage above the threshold and a current below the rating. This can involve series resistors or current-regulated power supplies.

LED Applications

The LED’s operate at low voltages i.e., from 1.5 V to 2.5 V. They have a long life of about 10000 hours and can be switched ‘ON’ and ‘OFF’ at a very fast speed. These features make; LED’s very important electronic device. Following are the important applications of the LED’s:

• In 7-segment, 16-segment and dot matrix displays. Such displays are used to indicate alphanumeric characters and symbols in various systems such as digital clocks, microwave ovens, stereo tuners, calculators electronic d.c. power supplies etc.
• For indicating power ON/OFF conditions, power level indicators in stereo amplifiers.
• In optical switching applications.
• For solid state video displays.
• In the field of optical communication, where LED’s are used to transfer (or couple) energy from one circuit to another. They are also used to send light energy to fiber optical cable, which transmits energy by means of total internal reflection. The fiber optical cable is of light weight, flexible, often transparent and as small a 0.043 mm in diameter.
• For image sensing circuits in picture phone.
• In burglar alarm systems. In such applications, LED’s radiating infrared light are preferred.

Multicolour LEDs

The LEDs, which emit one colour of light when forward biased and another when reverse biased, are called multicoloured LEDs. One commonly used symbol for a multicolour LED is as shown in Fig. 3.

Multicolour LEDs actually contain two PN junction that are connected in reverse-parallel, i.e., they are in parallel, with the anode of one being connected to the cathode of the other. Multicolour LEDs are typically red when biased in one direction and green when biased in the other direction.

Incidentally if a multicolour LED is switched fast, the LED will produced a third colour. For example, a red/green LED will produce a yellow light when rapidly switched back and forth between biasing polarities.