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At a Glance of LED

In today’s blog, we will learn about the LED and special properties of LED. It emits light and what kind of semiconducting materials are used for construction and the characteristics of LED.

Following Block Diagram will provide you a roadmap so that you can understand the Blog Effictively.

In today's blog, we will learn about the LED and special properties of LED. It emits light and what kind the semiconducting materials are used for construction, and the characteristics of LED. — Roadmap of Blog – LED

What is LED?

LED stands for the light-emitting diode.
The Light Emitting Diode (LED) is one type of semiconductor diode that emits a particular color light whenever current flows through it in the forward direction.
It is a special-purpose diode that emits light when forward biased and opposes current when reverse biased like PN junction diode.
LED is a current-dependent device; output light intensity is directly proportional to the forward current passing through it.
The LED symbol is the same as the PN junction diode, except it has two arrows pointing outwards, which represents the emission of light.

Construction of LED

The LED is formed by a P-N junction having the terminals anode (positive terminal) and cathode (negative terminal). This PN junction is made from compound semiconducting material such as gallium arsenide, gallium phosphide.

The main reason they are not made from silicon is that silicon is less temperature-sensitive, and it releases energy in the form of heat. And since the light emission is based on the semiconductor material used for construction and doping concentration, we avoid the use of silicon to construct LED.

While the above compound material releases energy in the form of light, so most suited for LED construction.
The lead frame is connected to the cathode terminal, which is also called an anvil.
And the reflecting cap holds the semiconducting material.

The N – region from the PN junction is at the bottom, connected to the cathode terminal, as cathode represents negative terminal and N – region has electrons as a majority charge carrier.
The P – region from the PN junction is at the top and connected through the wire that connects it to the post lead frame,
which is connected to the Anode terminal since the anode represents a positive terminal. The P – region has the holes as a majority charge carrier.

The reason behind placing the P – region at the top is that majority of light produced by LED is from the area of junction nearer to the P – region, so it is placed close to the surface for more light output.
This P-N junction is surrounded by the transparent hard plastic epoxy resin hemispherical shape shell, which protects the diode from external shocks.

How LED emits light

When the supply is connected to the PN junction positive terminal is connected to the P – region. The negative region is connected to the N – region as LED works only in forward bias conditions.

As the P – region is connected to the positive terminal, it injects holes, and N – region is connected to the negative terminal it injects electrons, due to which recombination of empty holes and free electrons occurs, causing the reduction in the depletion region during this process energy is created in the form of light.

But it is difficult to imagine this, so let me give you a ride through band structure in semiconductors.
As I explained earlier in my blog semiconductor has two bands separated with energy gap namely valance band and covalent band.

The conduction band contains free electrons. Due to their absence in the valance band, holes are created when we connect LED to supply this free electron from conduction band jumps towards valance band for recombination with holes.
While recombination of free electrons and empty holes, they exchange energy levels during this process, and electrons release energy in the form of a photon. Because the conduction band is at higher potential and the valance band is at lower potential. This released energy is proportional to the energy bandgap between the valance band and covalent band.

This same process happens in the PN junction diode, but instead of emission of light, it emits heat due to the semiconducting material used and doping concentration.
But the photons have different wavelength based on their energy or energy based on their wavelength,

Colour Schema in LED and How to achieve it?

The relationship between the energy of a photon and its wavelength is given by
E= hc/λ
Where E= energy of a photon
h= plank constant
c= speed of light
λ= wavelength

To achieve the desired color, one has to figure out how much energy an electron passing from one material to another needs to be loose to emit the photon of the correct wavelength to get the desired color.
From the above equation, we can see that energy is inversely proportional to wavelength, so we can achieve different colors by changing wavelength.

Materials used in construction of LED

The colour of light emitted by LED is not determined by the colour of the plastic body enclosing LED.

the below table represents semiconducting material used in construction of LED to emit different colours of light.

semiconductor material	wavelength (NM)	voltage Drop (V)	color
Gallium Arsenide Aluminium Gallium Arsenide	>760	<1.9	Infrared
Aluminium Gallium Arsenide Gallium Arsenide Phosphide allium Phosphide Aluminum Gallium Indium Phosphide	610-760	1.6- 2.0	Red
Gallium Arsenide Phosphide Gallium Phosphide Aluminum Gallium Indium Phosphide	590-610	2.0-2.1	orange
Indium Gallium Phosphide Gallium Indium Phosphide Aluminum Gallium Indium Phosphide Aluminum Gallium Phosphide	500-570	1.9-4.0	Green
Zinc Selenide Indium allium Nitride Silicon Carbide Silicon	450-500	2.5-3.7	Blue
Indium Gallium Nitride	400-450	2.8-4.0	Voilet
Dual Blue/ Red LEDs Blue with Red Phosphor White with Purple Plastic	Multiple types	2.4-3.7	Purple
Aluminum Gallium Nitride Boron Nitride Aluminum Nitride Diamond Aluminum Gallium Nitride Aluminum Gallium Indium Nitride	<400	3.1-4.4	Ultravoilet
Blue with Phosphor Yellow with Red, Orange, or Pink Phosphor White With Pink Pigment	Multiple types	3.3	Pink
Blue/UV diode with Yellow Phospher	Broad Spectrum	3.5	White

table for radiation pattern

Characteristics

The electrical characteristics of LED are similar to the PN junction diode, and the V-I characteristics of LED are very similar to the rectifier diode.
But for the LED, the forward voltage drop is larger than the PN junction diode. So for the PN junction diode, the forward voltage drop is in the range of 0.6 to 0.7-volt nut for the LED depending upon the emitting diode depending upon the emitted color, the forward voltage drop can vary from the 1.8v to 3.5v.
So from the infrared light to blue light, the forward voltage drop of the LED will increase for any LED forward bias condition when the applied input voltage is more than the forward voltage drop.

The LED emits light of a particular color, but without any kind of series resistor, the current flowing through the LED will be very high, and due to that, LED may get damage. So to restrict the current, series resistor should always be connected with the LED

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