Diode, also known as transistor, is the most important device in electronic circuits, and its main functions are current amplification and switching. As the name implies, triode has three electrodes. Diode is composed of a PN junction, while triode is composed of two PN junctions, and the common electrode becomes the base of triode (denoted by the letter B). The other two electrodes become the collector (denoted by the letter C) and the emitter (denoted by the letter E). Due to different combination methods, one is NPN transistor and the other is PNP transistor.