Light Emitting Diode (LED)

LEDs are produced in a variety of shapes and sizes.

An LED is a diode that converts electrical current directly into light and therefore it is more efficient than other sources of light which convert some energy into heat.

An LED has a p-type semiconductor layer with free electrons, and an n-type semiconductor layer with holes. The two layers are separated by a depletion region where no mobile charge carriers, free electrons or holes, are present. The depletion region opposes the flow of electrons from the n-type semiconductor to the p-type semiconductor.

When a forward bias between 1 volt and 3 volts, depending upon the material used to construct the LED, is applied, the depletion layer is overcome, and free electrons from the n-type layer and holes from the p-type layer move into the depletion layer. Electrons from the n-type layer recombine with holes from the p-type layer the electric current starts flowing.

When electrons recombine with holes in the depletion region they release energy in the form of light photons. In germanium and silicon diodes, the amount of energy released is too small to create light. However, in semiconductors like gallium phosphide and gallium nitride, the amount of energy released is sufficient to produce visible light.

The forward bias voltage required for a germanium diode to conduct is 0.3 volts, for a silicon diode is 0.7 volts. The safe bias voltage range of LEDs is from 1 volt to 3 volts, with forward current ratings from 200 mA to 100 mA. If the forward voltage applied to an LED is increased to greater than 3 volts, the depletion region breaks down and current rises substantially. This increase in current can destroy the device. To protect the LED, a current limiting resistor should be placed in series with the LED.

To calculate the resistance of the series resistor required, use ohms law R = V/I. You need the typical forward voltage (VF) and typical forward current (IF) from the LED's datasheet. Subtract VF from the supply voltage VS. Lets say the VS is 9 volts and VF is 2 volts. That makes V = 7 volts. Lets say IF is .020 Amps, then RS = 7/.02 = 350 ohms.

The color of light emitted form an LED is determined by the semiconductor material of which it is constructed. Gallium phosphide LEDs emit red, yellow and green light. Gallium nitride LEDs emit bright blue light. Aluminum gallium phosphide LEDs emit green light.

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