Light-emitting diodes (LEDs) use compound semiconductor materials such as gallium arsenide or indium phosphide. The relevant symbol is illustrated in Figure 5.4b. When forward current passes, light is emitted from the junction. The colour of the light depends on the semiconductor material used for the diode and the brightness is approximately proportional to the size of forward current. LEDs have higher forward voltages when conducting; around 1.6 V to 2.2 V as compared to the 0.5 V to 0.8 V of a silicon junction. The maximum permitted reverse voltages are very low, typically only 3 V, so a silicon diode must be connected across the LED as shown in Figure 5.5 if there is any likelihood of reverse voltage (or an AC signal) being applied to the diode. A series resistor must always be used to limit the forward current unless pulsed operation is used.

Figure 5.5: Protecting an LED from reverse voltage.

In Table 5.1 is shown some of the current range of LEDs with output colour and forward voltage drop. Note that the infra-red types emit little or no visible light; typical applications include remote controls and short-range signalling. In addition to the types noted in the table, all-white outputs can be achieved by combination structures either (1) using a combination of red, green and blue or (2) combining a blue/UV diode with a white phosphor coating (notably from Marl Optosource Ltd.).