A type of diode that emits light when a current is passed through it. So an LED is a special type of diode that converts electrical energy into light energy. A simple PN junction diode that emits light in the forward direction. The junction is covered with transparent epoxy that directs the light emitted from the junction in all directions.
Features of LED
- Light generated by LED is directional
- LED can generate different light color
- The temperature will affect LED efficiency
- Low energy consumption
- Long life
Symbol of LED
Construction of LED
The construction of LEDs is similar to that of regular p-n junction diodes, except that gallium, phosphorous, and arsenic materials are used in the construction instead of silicon or germanium materials.
Silicon is most commonly used for normal PN junction diodes because it is less sensitive to temperature. Moreover, the current flows efficiently without damage. In some cases, germanium is used to build diodes.
However, silicon or germanium diodes do not emit energy in the form of light. Instead, it releases energy in the form of heat. Therefore, no silicon or germanium is used in building LEDs.
VI Characteristics of LED
The VI characteristic shows the relationship between voltage and current through the device. The horizontal or x-axis represents voltage and the vertical or y-axis represents current.
LEDs basically work like normal diodes. In other words, forward current is allowed, so the graph only shows forward voltage and forward current. The forward voltage of an LED is higher than that of a normal diode due to the presence of the active layer. First, the LED conducts no current and produces no light until the forward voltage exceeds the knee voltage. Different LED colors indicate different knee voltage. As the LED begins to conduct, the current increases exponentially. It is directly proportional to the intensity of the emitted light.
Working of LED
Light-emitting diodes (LEDs) work only in forward bias. When a light-emitting diode (LED) is forward-biased, free electrons are pushed out from the n-side towards the junction, and holes are pushed out from the p-side.
When the free electrons reach the transition region or depletion region, some of the free electrons recombine with the +ve ion holes. Positive ions are known to have fewer electrons than protons. They are therefore ready to accept electrons. Therefore, free electrons recombine with holes in the depletion region. Similarly, holes from the p-side recombine with electrons in the depletion region.
The recombination of free electrons and holes in the depletion region reduces the width of the depletion region. As a result, more charge carriers cross the PN junction.
Some of the carriers from the p-side and n-side cross the PN junction before recombining in the depletion region. For example, some of the free electrons from the n-type semiconductor pass through the PN junction and recombine with holes in the p-type semiconductor. Similarly, holes from the p-type semiconductor cross the PN junction and recombine with free electrons in the n-type semiconductor.
Types of LED
- Miniature LEDs
- High-Power LEDs
- Flash LED
- Strip LED
- RGB LEDs
- Addressable LED
- Built-in Resistor LED
- Ultraviolet LED
- Infrared LED
Advantage of LED
- Temperature range: Operable over a wide temperature range of 0°C to 700°C
- Switching time: LED switching time is on the order of 1 ns. It is therefore useful for dynamic operations involving large numbers of arrays.
- Low power consumption: LEDs consume less power and can be used even with low input DC power.
- Better control: The radiated power of an LED depends on the current through it. In this way, the light intensity of the LED can be easily controlled.
- Economical and Reliable: LEDs are inexpensive and highly reliable.
- Small size and portability: They are small and can be stacked to form an alphanumeric display.
- High Efficiency: LEDs are 10-50 times more efficient than tungsten lamps in converting electricity into light energy. LEDs have a response time of 0.1 seconds, whereas tungsten lamps have tens and hundreds of milliseconds.
Disadvantages of LED
- Over-voltage or over-current: LEDs can be damaged if the current increases beyond certain limits.
- Overheating due to radiant power: Too much radiant power will cause overheating. This can damage the LED.
Applications of LED
- Indicator in AC circuit
- Display Panel Indicator
- Digital Watches, Calculators & Multimeters
- Remote Control Systems
- Burglar alarm Systems