LED Full Form
LED stands for light emitting diode. LED is a device which converts electrical energy into light energy. Just like a general purpose diode we apply a voltage across p-type and n-type terminals. But output of LED is light.
As we have learned during the lecture on electronics materials when we provide energy to an atom it releases its valence electron. This electron leaves a hole in the valence band.
In the light emitting diode (LED) the process is reverse. Here the free electron in the conduction band combines with the hole in the valence band. During this process it releases its energy (See Fig-1). This LED band gap energy could be in the visible spectrum or invisible spectrum.
Fig-1: Process in LED
The most important thing in LED is the band gap between conduction band and valence band (Read lecture on Electronic materials to understand theory of band gap). The LED diode can produce visible light energy if band gap is from 1.8eV to around 3.0 eV (Electron-volt). The band gap of an ordinary Silicon diode is 1.1 electron volt. Which is not sufficient to produce energy equivalent to light energy.
So we need special materials with large band gap. I collected information about some of LED materials with LED band gap requirement for you. You can see the table below for more understanding.
Table: Some LED Materials
The construction of LED is similar to pn-junction diode. For example in case of GaAsP we use Ga as doping material to create p-type region and we use AsP as doping materials to create n-type region. (See Fig-2)
Fig-2: LED Working
When we forward bias LED, depletion region starts shrinking. As a result free electrons of conduction band in n-type region starts combining with holes of p-type region. These free electrons must release the energy because conduction band has higher energy level and valence band has low energy level. Free electrons can’t combine with holes without releasing energy.
The forward bias voltage of an LED is greater than Si diode. This is between 1.2V to 3.2V. The reverse bias voltage is low typically between 5-10V. That’s why it is not suitable for many applications.
The LED symbol is similar to general purpose diode except to tilted arrows shows light emission. (See Fig-3)
Fig-3: LED symbol
- Consumes low power
- Five times more efficiency as compared to incandescent bulb
- Very Small in size
- Long life time