Light Emitting Diode (LED)

Light Emitting Diode  ( LED )

You can see light coming out of certain diodes when they are forward biased . Such diodes are called light emitting diodes .

An interesting fact about light emitting diode is that they are made up of elements like phosphorous , gallium & arsenic . They are not made of the conventional materials like germanium and silicon . I will explain you later , why germanium or silicon is not used .

Different elements are used in different quantities to produce lights of different colors . For example :

An LED which is made of gallium phosphide will give green light

An LED which is made of gallium arsenide wil give red light

Theory of Light Emitting Diode ( LED )

When you forward bias a light emitting diode , the electrons present in the n-type material cross the pn junction . After crossing, these electrons combine with the holes present in the p-type material . It is important to note here that the energy level of these free electrons is higher than that of the holes .


Energy level of free electrons (present in conduction band)>Energy level of holes (present in valence band)

So, when these electrons combine with the holes, the electrons (as they are at a higher energy level) release energy in the form of light & heat . And that is the reason why silicon or germanium is not used . Because when the diode is made of germanium or silicon the energy released is entirely in the form of heat . The light component is almost negligible . Hence we do not see any visible light when germanium or silicon is used. However, when we use materials like gallium arsenide , a very intense visible light is generated.

Schematic Diagram of a Light Emitting Diode ( LED )

The schematic diagram of an LED is shown below :

LED Symbol

In the diagram we see that arrows are pointing away from the diode . This indicates that the diode is emitting light when it is forward biased.

Effect of Forward Current on Intensity of Light

You can see in the graph below that the intensity of emitted light increases with the increase in forward current.

Intensity of current in LED

LED Circuit Diagram

You can see the LED circuit diagram below :

LED Circuit Diagram

Some important things to note are :

  • Forward voltage ratings are generally from 1V to 3V
  • Forward current ratings are from 20mV to 100mV

In the diagram, you can see that a resistor Rs is connected in series with the diode . The purpose of Rs is to ensure that current through the light emitting diode does not exceed the safe value .

  • Vs = Input voltage
  • VD = Voltage across VD

Hence ,

14. LED

Advantages of Light Emitting Diode ( LED )

  • LEDs need low voltage
  • The life of LEDs is longer . It is more than 20 years.
  • Fast on-off switching.

Multicolor LED

A multicolor LED is an LED that emits a different color when it is forward biased and yet another color when it is reverse biased .

The schematic symbol of a multicolor LED is shown below:

 Schematic Symbol of Multicolor LED

In the above diagram, you can see that there are two diode. Both these diodes are connected in reverse-parallel . By reverse-parallel, I mean to say that the anode of one is connected to the cathode of another.

In figure (i), when positive voltage is applied, the left diode will emit light, because that one gets forward biased.

In figure (ii), when negative voltage is applied, the right diode will emit light, because that one gets forward biased.

Applications of LED

LEDs are basically used as:

  • Power Indicator
  • Seven-segment Display

Application of LED as a Power Indicator

The circuit diagram below shows LED as a power indicator .

LED as power indicator

Application of LED as  Seven – segment Display

The circuit diagram below shows LED as a seven – segment display .

LED as seven segment display