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. 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. 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. A full wave rectifier creates pulsating DC from AC  while using the full AC cycle. One way to do this is to  The solid-state semiconductor devices described in the  previous section of this chapter can be found in both analog  11-19 Figure 11-41.  LED colors and the materials used to construct them as well as their wavelength and voltages.  Rectifiers Basic Analog Circuits Infrared Red Orange Yellow Green Blue Violet Purple Ultraviolet White ∆V < 1.9 1.63 < ∆V < 2.03 2.03 < ∆V < 2.10 2.10 < ∆V < 2.18 1.9[32] < ∆V < 4.0 2.48 < ∆V < 3.7 2.76 < ∆V < 4.0 2.48 < ∆V < 3.7 3.1 < ∆V < 4.4 ∆V = 3.5 Gallium arsenide (GaAs) Aluminium gallium arsenide (AlGaAs) Aluminium gallium arsenide (AlGaAs) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Gallium arsenide phosphide (GaAsP) Aluminium gallium indium phosphide (AlGaInP) Gallium(III) phosphide (GaP) Indium gallium nitride (InGaN) / Gallium(III) nitride (GaN) Gallium(III) phosphide (GaP) Aluminium gallium indium phosphide (AlGaInP) Aluminium gallium phosphide (AlGaP) Zinc selenide (ZnSe) Indium gallium nitride (InGaN) Silicon carbide (SiC) as substrate Silicon (Si) as substrate — (under development) Indium gallium nitride (InGaN) Dual blue/red LEDs, blue with red phosphor, or white with  purple plastic Diamond (235 nm)[33] Boron nitride (215 nm)[34][35] Aluminium nitride (AlN) (210 nm)[36] Aluminium gallium nitride (AlGaN) Aluminium gallium indium nitride (AlGaInN) — (down to 210 nm)[37] Blue/UV diode with yellow phosphor Color Wavelength (nm) Voltage (V) Semiconductor Material