At the end of this module, you should be able to:
Identify major light-emitting devices and describe their operation and applications.
Draw and label the schematic symbols associated with light-emitting devices
Identify packages used for light-emitting devices.
Going through this module can be both a fun and a meaningful learning experience. All you need to do is make use of your time and resources efficiently. To do this, here are some
tips for you:
1. Take time in reading and understanding each lesson. It is better to be slow but sure than to hurry finishing the module only to find out that you missed the concepts you are supposed to learn.
2. Do not jump from one chapter to another. Usually, the lessons are arranged such that one is built upon another, hence an understanding of the first is essential in comprehending the succeeding lessons.
3. Be honest. When answering the test items, do not turn to the key to correction page unless you are done. Likewise, when performing experiments, record only what you have really observed.
4. Safety first. Perform the experiments with extra precaution. Wear safety gears whenever necessary.
5. Don’t hesitate to ask. If you need to clarify something, approach your teacher or any knowledgeable person.
Light-emitting diodes are used as indicators and transmitters and in optoisolators.
Light-emitting devices produce light when subject to a current flow, converting electrical energy to light energy. A light-emitting diode (LED) is the most common semiconductor light-emitting device.
Being a semiconductor component, it has an unlimited life span due to the absence of a filament, a cause of trouble in conventional lamps.
Any PN junction diode can emit light when subject to an electrical current flow. The light is produced when the free electrons combine with holes and the extra energy is released in the form of light. The frequency of the light emitted is determined by the type of semiconductor material used in construction of the diode. Regular diodes do not emit light because of the opaque material used in their packaging.
An LED is simply a PN junction diode that emits light when a current flows through its junction. The light is visible because the LED is packaged in semitransparent material. The frequency of the light emitted depends on the material used in the construction of the LED. Gallium arsenide (GaAs) produces light in the infrared region, which is invisible to the human eye. Gallium arsenide phosphide (GaAsP) emits a visible red light. By changing the phosphide content, different frequencies of light can be produced.
The main semiconductor materials used to manufacture LEDs are:
Indium gallium nitride (InGaN): blue, green and ultraviolet high-brightness LEDs
Aluminum gallium indium phosphide (AlGaInP): yellow, orange and red high-brightness LEDs
Aluminum gallium arsenide (AlGaAs): red and infrared LEDs
Gallium phosphide (GaP): yellow and green LEDs
The P layer is made thin so that light energy near the PN junction only needs to travel a short distance through it. After the LED is formed, it is mounted in a package designed for optimum emission of light.
Most LEDs contain a lens that gathers and intensifies the light. The case may act as a colored filter to enhance the natural light emitted.
In a circuit, an LED is connected with forward bias to emit light. The forward bias must exceed 1.2 volts before a forward current can flow. Because LEDs can be easily damaged by an excessive amount of voltage or current, a series resistor is connected to limit current flow.
