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LEDs are simple and reliable. They are constructed of semiconductor material. When a free electron from a semiconductor drops into a semiconductor hole, energy is given off. This is true in all semiconductor materials. However, the energy released when this happens in certain materials is in the frequency range of visible light. Figure 11-41  is a table that illustrates common LED colors and the semiconductor material that is used in the construction of the diode. 

NOTE: When the diode is reversed biased, no light is given off. When the diode is forward biased, the energy given off is visible in the color characteristic for the material being used. Figure 11-42  illustrates the anatomy of a single LED, the symbol of an LED, and a graphic depiction of the LED process. 

## Basic Analog Circuits 

The solid-state semiconductor devices described in the previous section of this chapter can be found in both analog 

and digital electronic circuits. As digital electronics evolve, analog circuitry is being replaced. However, many aircraft still make use of analog electronics in radio and navigation equipment, as well as in other aircraft systems. A brief look at some of the basic analog circuits follows. 

## Rectifiers 

Rectifier circuits change AC voltage into DC voltage and are one of the most commonly used type of circuits in aircraft electronics. [Figure 11-43]  The resulting DC waveform output is also shown. The circuit has a single semiconductor diode and a load resistor. When the AC voltage cycles below zero, the diode shuts off and does not allow current flow until the AC cycles through zero voltage again. The result is pronounced pulsating DC. While this can be useful, half of the original AC voltage is not being used. 

A full wave rectifier creates pulsating DC from AC while using the full AC cycle. One way to do this is to 

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