LED (Light Emitting Diode) is a special diode that emits light when electric voltage is applied to it. It is a common electronic equipment used in many devices for indication purpose.
So what is a LED ? and how actually does it work ? Lets see….
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’
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.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.
The entire assembly is encapsulated in a epoxy glass case. The shape of the glass gives directionality to the light emitted from semiconductor die.
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.The semiconductor die which is the heart of the LED, placed on the cavity located on negative terminal (Anvil).
How it glows? :-
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.
Details from engineersgarage.com