What is LED?
A light-emitting diode (LED) is a semiconductor device that emits light when an electric current flows through it. When current passes through an LED, the electrons recombine with holes emitting light in the process. LEDs allow the current to flow in the forward direction and blocks the current in the reverse direction.
LED Symbol
The LED symbol is the standard symbol for a diode, with the addition of two small arrows denoting the emission of light.
Simple LED Circuit
The figure below shows a simple LED circuit.
How does an LED work?
When the diode is forward biased, the minority electrons are sent from p → n while the minority holes are sent from n → p. At the junction boundary, the concentration of minority carriers increases. The excess minority carriers at the junction recombine with the majority charges carriers.
The energy is released in the form of photons on recombination. In standard diodes, the energy is released in the form of heat. But in light-emitting diodes, the energy is released in the form of photons. We call this phenomenon electroluminescence. Electroluminescence is an optical phenomenon, and electrical phenomenon where a material emits light in response to an electric current passed through it. As the forward voltage increases, the intensity of the light increases and reaches a maximum.
When the diode is forward biased, the minority electrons are sent from p → n while the minority holes are sent from n → p. At the junction boundary, the concentration of minority carriers increases. The excess minority carriers at the junction recombine with the majority charges carriers.
What determines the colour of an LED?
The colour of an LED is determined by the material used in the semiconducting element. The two primary materials used in LEDs are aluminium gallium indium phosphide alloys and indium gallium nitride alloys. Aluminium alloys are used to obtain red, orange and yellow light, and indium alloys are used to get green, blue and white light. Slight changes in the composition of these alloys change the colour of the emitted light.
The colour of an LED is determined by the material used in the semiconducting element. The two primary materials used in LEDs are aluminium gallium indium phosphide alloys and indium gallium nitride alloys. Aluminium alloys are used to obtain red, orange and yellow light, and indium alloys are used to get green, blue and white light. Slight changes in the composition of these alloys change the colour of the emitted light.
Advantages and Disadvantages of LEDs
Advantages of LEDs
- LEDs are small-sized and can be assembled to form numeric and alphabet displays.
- LEDs are environmentally and economically friendly.
- LEDs are rugged and can-, therefore, withstand shocks and vibrations.
- LEDs can be operated in a wide range of temperatures.
- The switching time of LED (both on and off) is less than one. So, they are very suitable for the dynamic operation of many arrays.
- LEDs are available in different colours like red, yellow, green, and blue.
- They need moderate power. They are used where low dc power is available.
- They occupy a small area.
- LED devices can be driven by transistor-transistor logic (TTL), whereas gas discharge devices need intermediate transistor stages to be driven by 5V TTL.
Disadvantages of LEDs
- The drawbacks of LEDs are that they get damaged by overvoltage or overcurrent.
- They have wide optical bandwidth compared to LASER (≅ 10nm).
- Their temperature depends on the radiant output power and wavelength.
- LEDs are not suited for large-area displays, primarily because of their high cost. For larger displays, devices using gas-filled are used.
- Protecting against reverse bias. The LEDs have low reverse voltage ratings. For example, a typical LED may have a maximum reverse voltage rating of 3V. Therefore, the LED may get destroyed if a reverse voltage greater than 3V is applied to the LED. Therefore, we must be careful not to use LEDs with a high level of reverse bias.
Advantages of LEDs
- LEDs are small-sized and can be assembled to form numeric and alphabet displays.
- LEDs are environmentally and economically friendly.
- LEDs are rugged and can-, therefore, withstand shocks and vibrations.
- LEDs can be operated in a wide range of temperatures.
- The switching time of LED (both on and off) is less than one. So, they are very suitable for the dynamic operation of many arrays.
- LEDs are available in different colours like red, yellow, green, and blue.
- They need moderate power. They are used where low dc power is available.
- They occupy a small area.
- LED devices can be driven by transistor-transistor logic (TTL), whereas gas discharge devices need intermediate transistor stages to be driven by 5V TTL.
Disadvantages of LEDs
- The drawbacks of LEDs are that they get damaged by overvoltage or overcurrent.
- They have wide optical bandwidth compared to LASER (≅ 10nm).
- Their temperature depends on the radiant output power and wavelength.
- LEDs are not suited for large-area displays, primarily because of their high cost. For larger displays, devices using gas-filled are used.
- Protecting against reverse bias. The LEDs have low reverse voltage ratings. For example, a typical LED may have a maximum reverse voltage rating of 3V. Therefore, the LED may get destroyed if a reverse voltage greater than 3V is applied to the LED. Therefore, we must be careful not to use LEDs with a high level of reverse bias.
Applications of LEDs
LED have broad uses due to their small size, less consumption of energy, extended lifetime, and adjustability in terms of use in various applications. Some uses of LEDs are:
- Seven Segment Display: Seven segment displays are the best alternative to a dot matrix display and are easier to display decimal numerals. We have seen this display in our digital watches, meters, boards, etc. The small size of the LED makes it the best fit for this display.
- LED used in TV Remote: TV remote controls work on the principle of sending and receiving signals but use a type of light called infrared (or IR for short). The remote control has an LED light that flashes quickly to emit a message picked up by the TV. The remote is the transmitter, and the TV is the receiver.
- Picturephone: LEDs are used in image sensing circuits for ‘picturephones.’
- Computers: LEDs supply power to LASERs for entering information into optical computer memories.
LED have broad uses due to their small size, less consumption of energy, extended lifetime, and adjustability in terms of use in various applications. Some uses of LEDs are:
- Seven Segment Display: Seven segment displays are the best alternative to a dot matrix display and are easier to display decimal numerals. We have seen this display in our digital watches, meters, boards, etc. The small size of the LED makes it the best fit for this display.
- LED used in TV Remote: TV remote controls work on the principle of sending and receiving signals but use a type of light called infrared (or IR for short). The remote control has an LED light that flashes quickly to emit a message picked up by the TV. The remote is the transmitter, and the TV is the receiver.
- Picturephone: LEDs are used in image sensing circuits for ‘picturephones.’
- Computers: LEDs supply power to LASERs for entering information into optical computer memories.
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