A LED gives off green light when manufactured using Gallium Phosphide and also gives off red light when manufactured using Gallium Arsenide.
When LED is forward biased, the electrons from the n - type material cross the pn junction and recombine with the holes in the p - type material.
Thus, free electrons are in the conduction band and at a higher energy level than the holes in the valence band.
When recombination takes place the electrons emit energy in the form of light and heat.
In Silicon and Germanium, during recombination most of the energy is given off in the form of heat and the energy emitted in the form of light is insufficient that is why Silicon and Germanium materials are not used in the production of light emitting diodes.
However, materials like Gallium Arsenide and Gallium Phosphide during recombination releases a sufficient amount of light energy to produce a quite intense visible light.
LED
LED Symbol
LED has a safe value of operation and beyond that safe value it destroys.
Therefore, in order that current through the LED does not exceed the safe value, a resistor Rs is connected in series to the LED.
LED connected to resistor Rs
Safety Equations of LED:
VD is the voltage across the LED
Vs is the input voltage
Voltage across Rs = Vs - VD
Circuit Current, If = (Vs - VD) / Rs
Advantages of LED:
It requires a low voltage.
It has a longer life.
It exhibits fast ON - OFF switching
LED has a low reverse voltage rating.
Therefore, one must be careful not to use LEDs with a high level of reverse bias.
One way to protect a LED is to connect a rectifier diode in parallel with LED. If reverse voltage greater than the reverse voltage rating of LED is accidentally applied, the rectifier diode will be turned ON. This protects the LED from damage.
Advantages of LED:
It requires a low voltage.
It has a longer life.
It exhibits fast ON - OFF switching
LED has a low reverse voltage rating.
Therefore, one must be careful not to use LEDs with a high level of reverse bias.
One way to protect a LED is to connect a rectifier diode in parallel with LED. If reverse voltage greater than the reverse voltage rating of LED is accidentally applied, the rectifier diode will be turned ON. This protects the LED from damage.
LED connected to a Rectifier Diode
Multicolour LED
A LED that emits one colour when forward biased and another colour when reverse biased is know ans a multicolour LED.
Multicolor LEDs actually contain two pn junctions that are connected in reverse - parallel that is they are in parallel with the anode of one connected to the cathode of the other.
Multicolour LED
If a positive potential is applied at the terminal by the left, the pn junction on top is forward biased and light emits from the LED but the pn junction below is reverse biased and current does not pass through.
If the potential is reverse, the pn junction at the bottom is now forward biased and light emits from the LED but the pn junction on top is reverse biased and current does not pass through.
Note that LED is useful as an indicator and not for illumination.
Two Primary Applications of LED:
1. Power Indicator
2. Seven Segment Display
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