Led and ordinary diode is composed of a PN junction, also has one-way conductivity. When a positive voltage is applied to the led, the holes in the n-zone are injected from the p-zone and the electrons in the p-zone are injected from the n-zone. The electrons in the n-zone and the holes in the p-zone are compounded with the electrons in the n-zone and the holes in the p-zone within a few microns around the PN junction, respectively, to produce spontaneous emission fluorescence. The energy states of electrons and holes in different semiconductor materials are different. The more energy is released, the shorter the wavelength of the light. Diodes that emit red, green, or yellow light are commonly used. The reverse breakdown voltage of led is greater than 5 v. Its forward volt-ampere characteristic curve is very steep and must be used in series of current limiting resistors to control the current through the diode.