Light Emitting Diode

Light emitting diodes, or LEDs, are widely used as a standard source of light in electrical equipment. It has a wide array of applications ranging from your mobile phone to large advertising billboards. They are extensively made for the use in devices for showing what the time is and for displaying different types of data. In this post, the main focus would be on learning a lot about LED such as its operations and functions.

What is a LED?

Light Emitting DiodeA light releasing diode is an electric component that emits light when the electric current flows through it. It is a light source based on semiconductors. When current passes through the LED, the electrons recombine with holes emitting light in the process. It is a specialised type of diode having similar characteristics as the p-n junction diode. This, in turn, means that an LED allows the flow of current in its forward direction while it blocks the flow in the reverse direction. Light emitting diodes are made from a very thin layer of heavily doped semiconductor material. Based on the semiconductor material used and the amount of doping, an LED will emit a coloured light at a particular spectral wavelength when forward biased.

The figure demonstrates the LED symbol: this would be somewhat similar to p-n junction diode where the only change would be that the two arrows would indicate that the diode would emit the light.

Given in the table below are the links related to LED:

Light Emitting Diode Circuit

The LEDs also have a specific drop in voltage forward in cases where it is used normal circuits like a conventional diode. The drop in voltage depends on the current of LED, the color of the produced light, etc. There are different values in the drop of voltage that would vary from 1.5V to 2.5 V current for 10 to 50 mA current.

LED Basic Operation

The holes lie in the valence band and the free electrons are placed in the conduction band. When there is a forward bias in the p-n junction, the electron which is a part of the n-type semiconductor material would overrun the p-n junction and join with the holes in the p-type semiconductor material. Therefore regarding the holes, the free electrons would be at the higher energy bands.

The figure demonstrates the elementary process principle.

When there is a recombination of free electrons at the hole,  there would be a change regarding the energy level concerned from a value that is higher to the value that is lower and it drops from the conduction band to the valance band.  There is a release of energy due to the motion of the electron. In standard diodes, the release of energy in the manner of heat. But in LED the release of energy in the form of photons that would emit the light energy. The entire process is termed as electroluminescence and the diodes are termed as a light emitting diode.

In LED, energy discharged in light form hinges on the forbidden energy gap. One could manipulate the wavelength of the light produced. Therefore, the light color and its visibility or not could be controlled from its wavelength. The color and wavelength of the light emitted can be determined by doping it with several impurities.

Practise This Question

Two cars are moving on two perpendicular roads towards a crossing with uniform speeds of 72 km/h and 36 km/h. If second car blows horn of frequency 280 Hz, then the frequency of horn heard by the driver of first car when the line joining the cars makes angle of 45 with the roads, will be (velocity of sound is 330 m/s)