Insight - How LED works

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.

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

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

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

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

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

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

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

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.