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Insight - How LED works

Written By: 

Ashok Sharma

 

LED (Light Emitting Diode) is a special diode that emits light when an electric voltage is applied to it. It is common electronic component that is being used in devices like TV, computer, etc. generally for indicating purpose. They are available in various colors like red, yellow, green etc.
Post and Anvil of LED
 
Fig. 1: "Post" and "Anvil" of LED
 
There are two leads of an LED that are used to supply input voltage. The longer lead is positive and known as ‘Post’, and the smaller is negative known as ‘Anvil’ as shown in the image above.

Front View

Image of LED Structure
 
Fig. 2: Image of LED Structure
 
The above image shows the structure of an LED. A metal cup is placed on the negative lead (Anvil) which holds a semiconductor die. The semiconductor die is a combination of two semiconductor materials – N type and P type and an active region (known as P-N junction) between them. The outer body is formed of an epoxy glass that provides directionality to the light as well as protection to the die and leads.

Structure

Close View of LED Components
 
Fig. 3: Close View of LED Components
 
A closer look shows the structure of various components clearly. A cone shape cup, which holds the semiconductor die. The conical shape plays an important role to reflect the light emitted from the semiconductor die. Both leads are connected with the die by a wire bond.

Outer Glass Casing

Image of Epoxy Glass Case of LED
 
Fig. 3: Image of Epoxy Glass Case of LED
 
The entire assembly is encapsulated in a epoxy glass case. The shape of the glass gives directionality to the light emitted from semiconductor die.

Internal View

Semiconductor Die and Wires as seen from Top View of LED
 
Fig. 4: Semiconductor Die and Wires as seen from Top View of LED
 
This image is the top view of the LED after removing some section of the epoxy glass case. You can see how the semiconductor die is placed in the cone. The wires are also visible on a closer look.

Leads

Image Showing Structure of Post and Anvil
 
Fig. 5: Image Showing Structure of Post and Anvil
 
After removing the glass completely, the above image shows the front view of the leads. It showcases the structure of both the leads – Anvil and Post, which are connected to the N layer and P layer of the semiconductor die respectively by wire bond.

Cavity

Structure and Components of Anvil
 
Fig. 6: Structure and Components of Anvil
 
The negative terminal (Anvil) has a cavity where the semiconductor die is placed. This cavity is shaped so as to reflect the produced light into upward direction.

Semiconductor Die

A Close up View of Semiconductor Die
 
Fig. 7: A Close up View of Semiconductor Die
 
The semiconductor die which is the heart of the LED, placed on the cavity located on negative terminal (Anvil).
Working:
The P type semiconductor is connected with the positive end of battery and N type with the negative end of the battery. When a current is passed through the LED, free electrons from the N layer moves to the P layer in the active region (P-N Junction). This involves a drop from the conduction band to the lower orbit and electrons release some energy in the form of photons. The frequency and wavelength of the light produced photons is dependent on the material and doping level of the semiconductor. In case of LED, the material used and doping levels are set so as to produce photons with wavelength in the range of desired color of light in the visible spectrum.

Comments

amazing. turns out how little we know about the most commonly used electronic component.

gr8 work EngineersGarage!

That made my concept clear. Thank you Engineersgarage! How ever, I would like to know more about the last point mentioned in this article."The material used and doping levels are set so as to produce photons with wavelength in the range of desired color of light in the visible spectrum". What is this doping level? How is it selected?

good work..

 

The doping level is adjusted my the amount of doping material used for making the semiconductor..
And on the basis of the amount of doping material the amount of photons is calculated..
Which let us know the colour of LED..
I think this may clear the doubt...

Really good work. I wonder how you took that cavity out of the glass casing without any damage.

nice work dhawal likes it