Light emitting diode (LED)

LIGHT EMITTING DIODE (LED)

It is a p-n junction diode which emits light when it is forward biased.

Principle

The injection of electrons into the p - region from n- region makes a direct transition from the conduction band to valence band. Then, the electrons recombine with holes and emits photons of energy E_g'

The forbidden energy gap is given by

E_g = hv …. (1)

where h - Planck's constant

v - Frequency of the emitted radiation.

But v = c/λ … (2)

where c - velocity of the light

λ - wave length of the light.

E_g = hc/ λ … (3)

Hence, the wavelength of the emitted photon is given by the relation

λ = hc/ E_g ... (4)

The wave length of the light emitted purely depends on the band gap energy.

Note: Bandgap energy E_g (generally in eV) should be converted into joule unit. [1 eV = 1.6 × 10^-19 J]

Construction:

Fig. 4.19 shows cross sectional view of a LED.

A 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 deposited on the top.

For maximum light emission, a metal film anode is deposited at the outer edges of the p-type layer. The bottom of the substrate is coated with a metal (gold) film. It reflects most of the light to the surface of the device and also provides cathode connection. Fig. 4.20 shows circuit and symbol of LED.

Working

When the p-n junction diode is forward biased, the barrier width is reduced, raising the potential energy on the n- side and lowering that of the p-side.

The free electrons and holes have sufficient energy to move into the junction region. If a free electron meets a hole, it recombines with each other resulting in the release of a light photon.

Thus, light radiation from LED is caused by the recombination of holes and electrons that are injected into the junction by a forward bias voltage. (Fig. 4.21)

Advantages of LEDs

• LEDs are smaller in size. A number of LEDs can be stacked together in a small space to form numerical display.

• LED's can be turned ON and OFF in less than 1 nano second (10^-9 second). So, they are known as fast devices.

• Variety of LEDs are available which emit light in different colours like red, green, yellow etc.

• Light modulation can be achieved with pulse supply.

• It has long life time.

• It has low drive voltage and low noise.

• It is easily interfaced to digital logic circuits.

• It can be operated over a wide range of temperatures.

Disadvantages of LEDs

• They require high power.

• Their preparation cost is high when compared to LCD.

Applications and uses of LEDs

• Because of their miniature size, they are widely used in numeric and alphanumeric display devices. (Fig. 4.22)

• They are used as indicator lamps.

• They are used as light sources in fiber-optic communication system.

• Infrared LEDs are used in burglar alarms.

• They are used in image sensing circuits used for picture phone.

• They are used as a pilot light.

• Infra red LEDs are widely used with photo diodes or photo transistors to enable short range wire-less communication.