LED is a heavily doped p-n junction diode which converts electrical energy into light energy. The diode can emits light under forward bias condition. The process of emitting light in response to the strong electric field or flow of electric current is called Electroluminescence. The LED’s are made up of a special type of semiconductor materials such as Gallium Arsenide (Ga As), Gallium Arsenide Phosphide (Ga As P), Gallium phosphide (Ga P).
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The light emitting diode is a specially doped p-n junction diode which emits light when electric current passes through it in forward direction. In the LED, the recombination of charge carrier takes place. The electron from the N-side and the hole from the P-side are combined and gives the energy in the form of heat and light.
The LED’s are made up of a special type of semiconductor materials such as Gallium Arsenide (Ga As), Gallium Arsenide Phosphide (Ga As P), Gallium phosphide (Ga P).
Silicon and Germanium are not used for LED’s because they are heat producing materials and are very poor in producing light.
The forward voltage ratings of LEDs is from 1V to 3 V and forward current ratings is from 10 to 80 mA. The switching speed of LED is very high 1ns.
Symbol Of LED
The circuit symbol of LED is shown in figure given below. LED symbol is very similar to a normal p-n junction diode symbol with two small arrows that indicate the emission of light, thus it is called light-emitting diode (LED). The LED having two terminals namely anode (+) and cathode (-).
Basics Structure Of LED
The Structure of a LED is very different from that of a normal P-N junction diode. The PN junction semiconductor chip ( LED chip ) of an LED is surrounded by a transparent, hard plastic epoxy resin hemispherical shaped shell . Which protects the PN junction of an LED from vibration and shock.
The outer cover of LED is constructed in such a way that the light emitted by the junction are reflected away from the surrounding substrate base to which the diode is attached and are focused upwards through the domed top of the LED, which itself acts like a lens concentrating the amount of light. Because of this emitted light appears to be brightest at the top of the LED.
There are five major structure of LED (Light emitting diode) these are-
- Dome LED
- Planer LED
- Surface Emitter LED
- Edge Emitter LED
- Supper luminescent LED
Out of these five structures of LED, first two structures (dome LED and Planer LED structure) are type of homojunction LED’s. That produce invisible light which is mostly used in T.V remote control and industrial counting. And last three structures ( Surface Emitter LED, Edge Emitter LED, Supper luminescent LED ) are heterojunction LED used as a light source in Optical Fiber Communication System.
The main part of LED is a LED chip. LED chip is nothing but a p-n junction semiconductor chip. It consist of a p-type semiconductor and a n-type semiconductor. When p-n type semiconductor is join a junction is form called p-n junction and a depletion region is form near the junction due to concentration gradient of electron and hole in p-n type semiconductor. And a potential barrier is develop across the junction. This potential barrier opposes the flow of electrons from n-type semiconductor and flow of holes from p-type semiconductor.
To overcome the potential barrier, a external voltage is apply which is greater than the potential barrier of depletion layer. If the applied voltage is greater than the potential barrier of the depletion layer, the electric current starts flowing.
Working principle of LED (Light Emitting Diode)
The working of Light Emitting Diode (LED) is very similar to normal p-n junction diode. When LED is forward bias, the majority carrier electron of the n- type and the majority carrier hole of the p-type move across the junction. As a result, electron-hole recombination take place. Due to this recombination energy release in the form of light. This recombination of electron and hole takes place in depletion region as well as in p-type and n-type semiconductor.
The LED’s are made up of a special type of semiconductor materials such as Gallium Arsenide (Ga As), Gallium Arsenide Phosphide (Ga As P), Gallium phosphide (Ga P) they are light producing materials. Where as normal p-n junction diode is made up of silicon and germanium they are heat producing materials and are very poor in producing light.
Principle of Electroluminescence
LED ( light emitting diode ) work on the principle of electroluminescence. Electroluminescence is a process of converting electrical energy into (nonthermal) light.
When the LED is forward biased, the electrons in the n-region will cross the junction and recombine with the hole in the p-type material.
These free electrons reside in the conduction band and hence at a higher energy level than the holes in the valance band. The electrons in the higher energy level (conduction band) will not stay for long time. So, after a short time period, these electron return back to the lower energy level (valance band) and while ,returning back to the valance band energy releases in the form of a light or photon. This process is called electroluminescence. In this way LED emits light.
The semiconductor material with large forbidden energy gap emits high intensity light whereas the semiconductor material with small forbidden energy gap emits low intensity light. So we can say that, the intensity of the emitted light is depends on the material used for constructing LED and forward current flow through the LED.
LED biasing
The Voltage drop across a conducting LED is the range of 1 V to 3 V, depending on the material used. This voltage is much higher then that of conventional diode. The current rating of the LED is 10 to 80 mA.
When we applied forward voltage across LED in between 1 V to 3 V, LED start conduct and emitted light. However, if the applied forward voltage across LED is increased greater than 3 volts. The depletion region in the LED breaks down and suddenly large electric current flow through the device. This sudden large current may destroy the device.
To protect the LED from damage a resistance Rs is connected in series with LED, this resistance is called current limiting resistance. This resistor restricts the flow of extra current through the LED which may destroy the LED.
Output characteristics of LED
The output characteristics curve of LED is linear. It means the amount of emitted by the LED is directly proportional to the applied forward current across the LED. More the applied forward current , the greater is the emitted output light.
Colour of the emitted light
The colour of the emitted light is decided by the material use to construct an LED. We know that, every colour having different wavelength. The wavelength of light or colour depends on the value of forbidden gap (Eg). Forbidden gap (Eg) value is depend on the type of material used in LED.
- Gallium Arsenide (GaAs) – infra-red
- Gallium Arsenide Phosphide (GaAsP) – red to infra-red, orange
- Aluminium Gallium Arsenide Phosphide (AlGaAsP) – high-brightness red, orange-red, orange, and yellow
- Gallium Phosphide (GaP) – red, yellow and green
- Aluminium Gallium Phosphide (AlGaP) – green
- Gallium Nitride (GaN) – green, emerald green
- Gallium Indium Nitride (GaInN) – near ultraviolet, bluish-green and blue
- Silicon Carbide (SiC) – blue as a substrate
- Zinc Selenide (ZnSe) – blue
- Aluminium Gallium Nitride (AlGaN) – ultraviolet
Silicon and Germanium are not used for LEDs because their forbidden gap do not allow the light emission in visible spectrum.
The most common available LEDs emit green, red, yellow, orange and blue colour.
Some LEDs are capable of emitting infrared light which is not visible to human eyes.
Advantages Of LED
- LED are small in size and light weight. Therefore it is possible to pack large number of LED in a small space while manufacturing a display.
- They are available in different spectral colours.
- They have longer life as compared to the lamp.
- The light emitted by an LED is proportional to the amount of current flowing through it. Hence we can control the current flowing through LED to vary their brightness as per the requirement of the application.
- The switching speed of LED is high less than 1ns. So it is suitable at high operating speed applications.
- LEDs are very economical and easily available.
- The quantum efficiency of LED is high.
- It has better linearity. The input output characteristics curve of LED is linear.
- LED is less temperature dependent.
Disadvantages of LED
- Output power is affected by changes in temperature.
- LED need large power for their operation as compare to normal p-n junction diode .
- Luminous efficiency of LED is low.
- Overcurrent can damage LED easily.
Application Of LED
- LED work as a indicators in various electronic circuits.
- LED used in infrared remote control.
- LED used in traffic signal.
- LED used in digital watches.
- LED used in automotive heat lamps
- LED used in Camera flashes.
- LED used in seven segment and alphanumeric displays.
Difference between a normal Diode and a LED
| Normal Diode | LED |
| 1. The semiconductor device like a diode conducts simply in one direction. | The LED is one type of diode, used to generate light. |
| 2. The designing of the diode can be done with a semiconductor material & the flow of electrons in this material can give their energy the heat form. | The LED is designed with the gallium phosphide & gallium arsenide whose electrons can generate light while transmitting the energy. |
| 3. The diode changes the AC into the DC | The LED changes the voltage into light |
| 4. It has a high reverse breakdown voltage | It has a low-reverse breakdown voltage. |
| 5. The on-state voltage of the diode is 0.7v for silicon whereas, for germanium, it is 0.3v | The on-state voltage of LED approximately ranges from 1.2 to 2.0 V. |
| 6. The diode is used in voltage rectifiers, clipping & clamping circuits, voltage multipliers. | The applications of LED are traffic signals, automotive headlamps, in medical devices, camera flashes, etc. |