Light Emitting Diode (LED)

LIGHT EMITTING DIODE (LED)

Definition

LED is a semiconductor p-n junction diode which converts electrical energy to light energy under forward biasing. It emits light in both visible and IR region.

Types of LED

There are two types of LED. viz.,

(i) Planar (or) Surface emitting LED and

(ii) Dome shaped LED.

Let us discuss in detail about the planar (or) surface emitting LED.

Principle

Injection luminescence is the principle used in both the LED's. When LED is forward biased, the majority charge carriers moves from p to n and similarly from n to p region and becomes excess minority carriers. Then these excess minority charge carriers diffuses through the junction and recombines with the majority charge carriers in n and p region respectively to produce light.

Construction

The surface emitting LED is as shown in Fig. 4.20. Here the p-n junction is formed by diffusion or epitaxial techniques. The p-n junction is made by doping silicon with GaAs crystal. Since silicon can act both as donar (when it replaces Gallium) and acceptor (when it replaces arsenide) it is used as the impurity atom (or) dopent. Therefore a shallow p-n junction is formed on GaAs substrate such that p-layer is formed by diffusion on 'n' layer.

In order to increase the probability of radiative recombination, the thickness of the 'n' layer is taken higher than that of the thickness of the 'p' layer.

Ohmic contacts are made with the help of aluminium in such a way that top layer of the 'p' material is left uncovered, for the emission of light. Proper biasing can be applied at the ohmic contacts. The whole p-n junction is surrounded by plastic material so that the losses due to reflection can be minimised.

Working

The diode is forward biased. Due to forward bias, the majority charge carriers from 'n' and 'p' regions cross the junction and become minority charge carriers in the other junction (i.e) Electrons, which are majority charge carriers in 'n' region cross the junction and go to 'p' region and become minority charge carriers in p-region as shown in Fig. 4.21.

Similarly, holes which are majority charge carries in 'p' region cross the junction and go to 'n' region and become minority charge carriers in 'n' region.

By the similar process, excess of minority carriers (shaded electrons and holes) are injected in both 'p' and 'n' regions as shown in Fig. 4.21 and this phenomenon is called minority carrier injection.

Now if the biasing voltage is further increased, these excess minority carriers diffuse away from the junction and they directly recombine with the majority carriers. (i.e.) the electrons, which are excess minority carriers in p-region recombine with the holes which are the majority carriers in 'p' region and emit light. Similarly, the holes which are excess minority carriers in 'n' region recombine with the electrons which are majority carriers in 'n' region and emit light.

Therefore electron-hole recombination process occurs more and more and thereby light (photons) is emitted through the top layer of the p-material which is left uncovered as shown in Fig. 4.20.

Advantages

(i) They are smaller in size.

(ii) Its cost is very low.

(iii) It has long life time.

(iv) LED's are available in different colours at low cost.

(v) It operates even at very low voltage.

(vi) Response time of LED is very fast, in the order of 10^-9 seconds.

(vii) Its intensity can be controlled easily.

(viii) It can be operated at a wide range of temperatures (0 - 70°C).

(ix) Dome shaped LED has less scattering losses.

Disadvantages

(i) Power output is low.

(ii) Intensity is less than laser.

(iii) The light cannot travel through longer distance.

(iv) The light output is incoherent and are not in phase.

(v) The light will not have directionality.